ARM Linux 3.x的設備樹(Device Tree) .

ARM 182瀏覽

1.?? ?ARM Device Tree起源

Linus Torvalds在2011年3月17日的ARM Linux郵件列表宣稱“this whole ARM thing is a f*cking pain in the ass”,引發ARM Linux社區的地震,隨后ARM社區進行了一系列的重大修正。在過去的ARM Linux中,arch/arm/plat-xxx和arch/arm/mach-xxx中充斥著大量的垃圾代碼,相當多數的代碼只是在描述板級細節,而這些板級細節對于內核來講,不過是垃圾,如板上的platform設備、resource、i2c_board_info、spi_board_info以及各種硬件的platform_data。讀者有興趣可以統計下常見的s3c2410、s3c6410等板級目錄,代碼量在數萬行。
社區必須改變這種局面,于是PowerPC等其他體系架構下已經使用的Flattened Device Tree(FDT)進入ARM社區的視野。Device Tree是一種描述硬件的數據結構,它起源于 OpenFirmware (OF)。在Linux 2.6中,ARM架構的板極硬件細節過多地被硬編碼在arch/arm/plat-xxx和arch/arm/mach-xxx,采用Device Tree后,許多硬件的細節可以直接透過它傳遞給Linux,而不再需要在kernel中進行大量的冗余編碼。
Device Tree由一系列被命名的結點(node)和屬性(property)組成,而結點本身可包含子結點。所謂屬性,其實就是成對出現的name和value。在Device Tree中,可描述的信息包括(原先這些信息大多被hard code到kernel中):

  • CPU的數量和類別
  • 內存基地址和大小
  • 總線和橋
  • 外設連接
  • 中斷控制器和中斷使用情況
  • GPIO控制器和GPIO使用情況
  • Clock控制器和Clock使用情況

它基本上就是畫一棵電路板上CPU、總線、設備組成的樹,Bootloader會將這棵樹傳遞給內核,然后內核可以識別這棵樹,并根據它展開出Linux內核中的platform_device、i2c_client、spi_device等設備,而這些設備用到的內存、IRQ等資源,也被傳遞給了內核,內核會將這些資源綁定給展開的相應的設備。

2.?? ?Device Tree組成和結構

整個Device Tree牽涉面比較廣,即增加了新的用于描述設備硬件信息的文本格式,又增加了編譯這一文本的工具,同時Bootloader也需要支持將編譯后的Device Tree傳遞給Linux內核。

DTS (device tree source)

.dts文件是一種ASCII 文本格式的Device Tree描述,此文本格式非常人性化,適合人類的閱讀習慣。基本上,在ARM Linux在,一個.dts文件對應一個ARM的machine,一般放置在內核的arch/arm/boot/dts/目錄。由于一個SoC可能對應多個machine(一個SoC可以對應多個產品和電路板),勢必這些.dts文件需包含許多共同的部分,Linux內核為了簡化,把SoC公用的部分或者多個machine共同的部分一般提煉為.dtsi,類似于C語言的頭文件。其他的machine對應的.dts就include這個.dtsi。譬如,對于VEXPRESS而言,vexpress-v2m.dtsi就被vexpress-v2p-ca9.dts所引用,
vexpress-v2p-ca9.dts有如下一行:
/include/ "vexpress-v2m.dtsi"
當然,和C語言的頭文件類似,.dtsi也可以include其他的.dtsi,譬如幾乎所有的ARM SoC的.dtsi都引用了skeleton.dtsi。
.dts(或者其include的.dtsi)基本元素即為前文所述的結點和屬性:

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  1. /?{??
  2. ????node1?{??
  3. ????????a-string-property?=?"A?string";??
  4. ????????a-string-list-property?=?"first?string",?"second?string";??
  5. ????????a-byte-data-property?=?[0x01?0x23?0x34?0x56];??
  6. ????????child-node1?{??
  7. ????????????first-child-property;??
  8. ????????????second-child-property?=?<1>;??
  9. ????????????a-string-property?=?"Hello,?world";??
  10. ????????};??
  11. ????????child-node2?{??
  12. ????????};??
  13. ????};??
  14. ????node2?{??
  15. ????????an-empty-property;??
  16. ????????a-cell-property?=?<1?2?3?4>;?/*?each?number?(cell)?is?a?uint32?*/??
  17. ????????child-node1?{??
  18. ????????};??
  19. ????};??
  20. };??
/ {
    node1 {
        a-string-property = "A string";
        a-string-list-property = "first string", "second string";
        a-byte-data-property = [0x01 0x23 0x34 0x56];
        child-node1 {
            first-child-property;
            second-child-property = <1>;
            a-string-property = "Hello, world";
        };
        child-node2 {
        };
    };
    node2 {
        an-empty-property;
        a-cell-property = <1 2 3 4>; /* each number (cell) is a uint32 */
        child-node1 {
        };
    };
};

上述.dts文件并沒有什么真實的用途,但它基本表征了一個Device Tree源文件的結構:
1個root結點"/";
root結點下面含一系列子結點,本例中為"node1" 和 "node2";
結點"node1"下又含有一系列子結點,本例中為"child-node1" 和 "child-node2";
各結點都有一系列屬性。這些屬性可能為空,如" an-empty-property";可能為字符串,如"a-string-property";可能為字符串數組,如"a-string-list-property";可能為Cells(由u32整數組成),如"second-child-property",可能為二進制數,如"a-byte-data-property"。
下面以一個最簡單的machine為例來看如何寫一個.dts文件。假設此machine的配置如下:
1個雙核ARM Cortex-A9 32位處理器;
ARM的local bus上的內存映射區域分布了2個串口(分別位于0x101F1000 和 0x101F2000)、GPIO控制器(位于0x101F3000)、SPI控制器(位于0x10170000)、中斷控制器(位于0x10140000)和一個external bus橋;
External bus橋上又連接了SMC SMC91111 Ethernet(位于0x10100000)、I2C控制器(位于0x10160000)、64MB NOR Flash(位于0x30000000);
External bus橋上連接的I2C控制器所對應的I2C總線上又連接了Maxim DS1338實時鐘(I2C地址為0x58)。
其對應的.dts文件為:

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  1. /?{??
  2. ????compatible?=?"acme,coyotes-revenge";??
  3. ????#address-cells?=?<1>;??
  4. ????#size-cells?=?<1>;??
  5. ????interrupt-parent?=?<&intc>;??
  6. ??
  7. ????cpus?{??
  8. ????????#address-cells?=?<1>;??
  9. ????????#size-cells?=?<0>;??
  10. [email protected]?{??
  11. ????????????compatible?=?"arm,cortex-a9";??
  12. ????????????reg?=?<0>;??
  13. ????????};??
  14. [email protected]?{??
  15. ????????????compatible?=?"arm,cortex-a9";??
  16. ????????????reg?=?<1>;??
  17. ????????};??
  18. ????};??
  19. ??
  20. [email protected]?{??
  21. ????????compatible?=?"arm,pl011";??
  22. ????????reg?=?<0x101f0000?0x1000?>;??
  23. ????????interrupts?=?<?1?0?>;??
  24. ????};??
  25. ??
  26. [email protected]?{??
  27. ????????compatible?=?"arm,pl011";??
  28. ????????reg?=?<0x101f2000?0x1000?>;??
  29. ????????interrupts?=?<?2?0?>;??
  30. ????};??
  31. ??
  32. [email protected]?{??
  33. ????????compatible?=?"arm,pl061";??
  34. ????????reg?=?<0x101f3000?0x1000??
  35. ???????????????0x101f4000?0x0010>;??
  36. ????????interrupts?=?<?3?0?>;??
  37. ????};??
  38. ??
  39. ????intc:[email protected]?{??
  40. ????????compatible?=?"arm,pl190";??
  41. ????????reg?=?<0x10140000?0x1000?>;??
  42. ????????interrupt-controller;??
  43. ????????#interrupt-cells?=?<2>;??
  44. ????};??
  45. ??
  46. [email protected]?{??
  47. ????????compatible?=?"arm,pl022";??
  48. ????????reg?=?<0x10115000?0x1000?>;??
  49. ????????interrupts?=?<?4?0?>;??
  50. ????};??
  51. ??
  52. ????external-bus?{??
  53. ????????#address-cells?=?<2>??
  54. ????????#size-cells?=?<1>;??
  55. ????????ranges?=?<0?0??0x10100000???0x10000?????//?Chipselect?1,?Ethernet??
  56. ??????????????????1?0??0x10160000???0x10000?????//?Chipselect?2,?i2c?controller??
  57. ??????????????????2?0??0x30000000???0x1000000>;?//?Chipselect?3,?NOR?Flash??
  58. ??
  59. [email protected],0?{??
  60. ????????????compatible?=?"smc,smc91c111";??
  61. ????????????reg?=?<0?0?0x1000>;??
  62. ????????????interrupts?=?<?5?2?>;??
  63. ????????};??
  64. ??
  65. [email protected],0?{??
  66. ????????????compatible?=?"acme,a1234-i2c-bus";??
  67. ????????????#address-cells?=?<1>;??
  68. ????????????#size-cells?=?<0>;??
  69. ????????????reg?=?<1?0?0x1000>;??
  70. ????????????interrupts?=?<?6?2?>;??
  71. [email protected]?{??
  72. ????????????????compatible?=?"maxim,ds1338";??
  73. ????????????????reg?=?<58>;??
  74. ????????????????interrupts?=?<?7?3?>;??
  75. ????????????};??
  76. ????????};??
  77. ??
  78. [email protected],0?{??
  79. ????????????compatible?=?"samsung,k8f1315ebm",?"cfi-flash";??
  80. ????????????reg?=?<2?0?0x4000000>;??
  81. ????????};??
  82. ????};??
  83. };??
/ {
    compatible = "acme,coyotes-revenge";
    #address-cells = <1>;
    #size-cells = <1>;
    interrupt-parent = <&intc>;

    cpus {
        #address-cells = <1>;
        #size-cells = <0>;
        [email protected] {
            compatible = "arm,cortex-a9";
            reg = <0>;
        };
        [email protected] {
            compatible = "arm,cortex-a9";
            reg = <1>;
        };
    };

    [email protected] {
        compatible = "arm,pl011";
        reg = <0x101f0000 0x1000 >;
        interrupts = < 1 0 >;
    };

    [email protected] {
        compatible = "arm,pl011";
        reg = <0x101f2000 0x1000 >;
        interrupts = < 2 0 >;
    };

    [email protected] {
        compatible = "arm,pl061";
        reg = <0x101f3000 0x1000
               0x101f4000 0x0010>;
        interrupts = < 3 0 >;
    };

    intc: [email protected] {
        compatible = "arm,pl190";
        reg = <0x10140000 0x1000 >;
        interrupt-controller;
        #interrupt-cells = <2>;
    };

    [email protected] {
        compatible = "arm,pl022";
        reg = <0x10115000 0x1000 >;
        interrupts = < 4 0 >;
    };

    external-bus {
        #address-cells = <2>
        #size-cells = <1>;
        ranges = <0 0  0x10100000   0x10000     // Chipselect 1, Ethernet
                  1 0  0x10160000   0x10000     // Chipselect 2, i2c controller
                  2 0  0x30000000   0x1000000>; // Chipselect 3, NOR Flash

        [email protected],0 {
            compatible = "smc,smc91c111";
            reg = <0 0 0x1000>;
            interrupts = < 5 2 >;
        };

        [email protected],0 {
            compatible = "acme,a1234-i2c-bus";
            #address-cells = <1>;
            #size-cells = <0>;
            reg = <1 0 0x1000>;
            interrupts = < 6 2 >;
            [email protected] {
                compatible = "maxim,ds1338";
                reg = <58>;
                interrupts = < 7 3 >;
            };
        };

        [email protected],0 {
            compatible = "samsung,k8f1315ebm", "cfi-flash";
            reg = <2 0 0x4000000>;
        };
    };
};

上述.dts文件中,root結點"/"的compatible 屬性compatible = "acme,coyotes-revenge";定義了系統的名稱,它的組織形式為:<manufacturer>,<model>。Linux內核透過root結點"/"的compatible 屬性即可判斷它啟動的是什么machine。
在.dts文件的每個設備,都有一個compatible 屬性,compatible屬性用戶驅動和設備的綁定。compatible 屬性是一個字符串的列表,列表中的第一個字符串表征了結點代表的確切設備,形式為"<manufacturer>,<model>",其后的字符串表征可兼容的其他設備。可以說前面的是特指,后面的則涵蓋更廣的范圍。如在arch/arm/boot/dts/vexpress-v2m.dtsi中的Flash結點:

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  1. [email protected],00000000?{??
  2. ?????compatible?=?"arm,vexpress-flash",?"cfi-flash";??
  3. ?????reg?=?<0?0x00000000?0x04000000>,??
  4. ?????<1?0x00000000?0x04000000>;??
  5. ?????bank-width?=?<4>;??
  6. ?};??
[email protected],00000000 {
     compatible = "arm,vexpress-flash", "cfi-flash";
     reg = <0 0x00000000 0x04000000>,
     <1 0x00000000 0x04000000>;
     bank-width = <4>;
 };

compatible屬性的第2個字符串"cfi-flash"明顯比第1個字符串"arm,vexpress-flash"涵蓋的范圍更廣。
再比如,Freescale MPC8349 SoC含一個串口設備,它實現了國家半導體(National Semiconductor)的ns16550 寄存器接口。則MPC8349串口設備的compatible屬性為compatible = "fsl,mpc8349-uart", "ns16550"。其中,fsl,mpc8349-uart指代了確切的設備, ns16550代表該設備與National Semiconductor 的16550 UART保持了寄存器兼容。
接下來root結點"/"的cpus子結點下面又包含2個cpu子結點,描述了此machine上的2個CPU,并且二者的compatible 屬性為"arm,cortex-a9"。
注意cpus和cpus的2個cpu子結點的命名,它們遵循的組織形式為:<name>[@<unit-address>],<>中的內容是必選項,[]中的則為可選項。name是一個ASCII字符串,用于描述結點對應的設備類型,如3com Ethernet適配器對應的結點name宜為ethernet,而不是3com509。如果一個結點描述的設備有地址,則應該給出@unit-address。多個相同類型設備結點的name可以一樣,只要unit-address不同即可,如本例中含有[email protected][email protected]以及[email protected][email protected]這樣的同名結點。設備的unit-address地址也經常在其對應結點的reg屬性中給出。ePAPR標準給出了結點命名的規范。
可尋址的設備使用如下信息來在Device Tree中編碼地址信息:

  • ??? reg
  • ??? #address-cells
  • ??? #size-cells

其中reg的組織形式為reg = <address1 length1 [address2 length2] [address3 length3] ... >,其中的每一組address length表明了設備使用的一個地址范圍。address為1個或多個32位的整型(即cell),而length則為cell的列表或者為空(若#size-cells = 0)。address 和 length 字段是可變長的,父結點的#address-cells和#size-cells分別決定了子結點的reg屬性的address和length字段的長度。在本例中,root結點的#address-cells
= <1>;和#size-cells = <1>;決定了serial、gpio、spi等結點的address和length字段的長度分別為1。cpus 結點的#address-cells = <1>;和#size-cells = <0>;決定了2個cpu子結點的address為1,而length為空,于是形成了2個cpu的reg = <0>;和reg = <1>;。external-bus結點的#address-cells = <2>和#size-cells = <1>;決定了其下的ethernet、i2c、flash的reg字段形如reg
= <0 0 0x1000>;、reg = <1 0 0x1000>;和reg = <2 0 0x4000000>;。其中,address字段長度為0,開始的第一個cell(0、1、2)是對應的片選,第2個cell(0,0,0)是相對該片選的基地址,第3個cell(0x1000、0x1000、0x4000000)為length。特別要留意的是i2c結點中定義的 #address-cells = <1>;和#size-cells = <0>;又作用到了I2C總線上連接的RTC,它的address字段為0x58,是設備的I2C地址。
root結點的子結點描述的是CPU的視圖,因此root子結點的address區域就直接位于CPU的memory區域。但是,經過總線橋后的address往往需要經過轉換才能對應的CPU的memory映射。external-bus的ranges屬性定義了經過external-bus橋后的地址范圍如何映射到CPU的memory區域。

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  1. ranges?=?<0?0??0x10100000???0x10000?????//?Chipselect?1,?Ethernet??
  2. ??????????1?0??0x10160000???0x10000?????//?Chipselect?2,?i2c?controller??
  3. ??????????2?0??0x30000000???0x1000000>;?//?Chipselect?3,?NOR?Flash??
        ranges = <0 0  0x10100000   0x10000     // Chipselect 1, Ethernet
                  1 0  0x10160000   0x10000     // Chipselect 2, i2c controller
                  2 0  0x30000000   0x1000000>; // Chipselect 3, NOR Flash

ranges是地址轉換表,其中的每個項目是一個子地址、父地址以及在子地址空間的大小的映射。映射表中的子地址、父地址分別采用子地址空間的#address-cells和父地址空間的#address-cells大小。對于本例而言,子地址空間的#address-cells為2,父地址空間的#address-cells值為1,因此0 0? 0x10100000?? 0x10000的前2個cell為external-bus后片選0上偏移0,第3個cell表示external-bus后片選0上偏移0的地址空間被映射到CPU的0x10100000位置,第4個cell表示映射的大小為0x10000。ranges的后面2個項目的含義可以類推。
Device Tree中還可以中斷連接信息,對于中斷控制器而言,它提供如下屬性:
interrupt-controller – 這個屬性為空,中斷控制器應該加上此屬性表明自己的身份;
#interrupt-cells – 與#address-cells 和 #size-cells相似,它表明連接此中斷控制器的設備的interrupts屬性的cell大小。
在整個Device Tree中,與中斷相關的屬性還包括:
interrupt-parent – 設備結點透過它來指定它所依附的中斷控制器的phandle,當結點沒有指定interrupt-parent 時,則從父級結點繼承。對于本例而言,root結點指定了interrupt-parent = <&intc>;其對應于intc: [email protected],而root結點的子結點并未指定interrupt-parent,因此它們都繼承了intc,即位于0x10140000的中斷控制器。
interrupts – 用到了中斷的設備結點透過它指定中斷號、觸發方法等,具體這個屬性含有多少個cell,由它依附的中斷控制器結點的#interrupt-cells屬性決定。而具體每個cell又是什么含義,一般由驅動的實現決定,而且也會在Device Tree的binding文檔中說明。譬如,對于ARM GIC中斷控制器而言,#interrupt-cells為3,它3個cell的具體含義Documentation/devicetree/bindings/arm/gic.txt就有如下文字說明:

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  1. 01???The?1st?cell?is?the?interrupt?type;?0?for?SPI?interrupts,?1?for?PPI??
  2. 02???interrupts.??
  3. 03??
  4. 04???The?2nd?cell?contains?the?interrupt?number?for?the?interrupt?type.??
  5. 05???SPI?interrupts?are?in?the?range?[0-987].??PPI?interrupts?are?in?the??
  6. 06???range?[0-15].??
  7. 07??
  8. 08???The?3rd?cell?is?the?flags,?encoded?as?follows:??
  9. 09?????????bits[3:0]?trigger?type?and?level?flags.??
  10. 10?????????????????1?=?low-to-high?edge?triggered??
  11. 11?????????????????2?=?high-to-low?edge?triggered??
  12. 12?????????????????4?=?active?high?level-sensitive??
  13. 13?????????????????8?=?active?low?level-sensitive??
  14. 14?????????bits[15:8]?PPI?interrupt?cpu?mask.??Each?bit?corresponds?to?each?of??
  15. 15?????????the?8?possible?cpus?attached?to?the?GIC.??A?bit?set?to?'1'?indicated??
  16. 16?????????the?interrupt?is?wired?to?that?CPU.??Only?valid?for?PPI?interrupts.??
01   The 1st cell is the interrupt type; 0 for SPI interrupts, 1 for PPI
02   interrupts.
03
04   The 2nd cell contains the interrupt number for the interrupt type.
05   SPI interrupts are in the range [0-987].  PPI interrupts are in the
06   range [0-15].
07
08   The 3rd cell is the flags, encoded as follows:
09         bits[3:0] trigger type and level flags.
10                 1 = low-to-high edge triggered
11                 2 = high-to-low edge triggered
12                 4 = active high level-sensitive
13                 8 = active low level-sensitive
14         bits[15:8] PPI interrupt cpu mask.  Each bit corresponds to each of
15         the 8 possible cpus attached to the GIC.  A bit set to '1' indicated
16         the interrupt is wired to that CPU.  Only valid for PPI interrupts.

另外,值得注意的是,一個設備還可能用到多個中斷號。對于ARM GIC而言,若某設備使用了SPI的168、169號2個中斷,而言都是高電平觸發,則該設備結點的interrupts屬性可定義為:interrupts = <0 168 4>, <0 169 4>;
除了中斷以外,在ARM Linux中clock、GPIO、pinmux都可以透過.dts中的結點和屬性進行描述。

DTC (device tree compiler)

將.dts編譯為.dtb的工具。DTC的源代碼位于內核的scripts/dtc目錄,在Linux內核使能了Device Tree的情況下,編譯內核的時候主機工具dtc會被編譯出來,對應scripts/dtc/Makefile中的“hostprogs-y := dtc”這一hostprogs編譯target。
在Linux內核的arch/arm/boot/dts/Makefile中,描述了當某種SoC被選中后,哪些.dtb文件會被編譯出來,如與VEXPRESS對應的.dtb包括:

[plain]
view plaincopyprint?
  1. dtb-$(CONFIG_ARCH_VEXPRESS)?+=?vexpress-v2p-ca5s.dtb???
  2. ????????vexpress-v2p-ca9.dtb???
  3. ????????vexpress-v2p-ca15-tc1.dtb???
  4. ????????vexpress-v2p-ca15_a7.dtb???
  5. ????????xenvm-4.2.dtb??
dtb-$(CONFIG_ARCH_VEXPRESS) += vexpress-v2p-ca5s.dtb 
        vexpress-v2p-ca9.dtb 
        vexpress-v2p-ca15-tc1.dtb 
        vexpress-v2p-ca15_a7.dtb 
        xenvm-4.2.dtb

在Linux下,我們可以單獨編譯Device Tree文件。當我們在Linux內核下運行make dtbs時,若我們之前選擇了ARCH_VEXPRESS,上述.dtb都會由對應的.dts編譯出來。因為arch/arm/Makefile中含有一個dtbs編譯target項目。

Device Tree Blob (.dtb)

.dtb是.dts被DTC編譯后的二進制格式的Device Tree描述,可由Linux內核解析。通常在我們為電路板制作NAND、SD啟動image時,會為.dtb文件單獨留下一個很小的區域以存放之,之后bootloader在引導kernel的過程中,會先讀取該.dtb到內存。

Binding

對于Device Tree中的結點和屬性具體是如何來描述設備的硬件細節的,一般需要文檔來進行講解,文檔的后綴名一般為.txt。這些文檔位于內核的Documentation/devicetree/bindings目錄,其下又分為很多子目錄。

Bootloader

Uboot mainline 從 v1.1.3開始支持Device Tree,其對ARM的支持則是和ARM內核支持Device Tree同期完成。
為了使能Device Tree,需要編譯Uboot的時候在config文件中加入
#define CONFIG_OF_LIBFDT
在Uboot中,可以從NAND、SD或者TFTP等任意介質將.dtb讀入內存,假設.dtb放入的內存地址為0x71000000,之后可在Uboot運行命令fdt addr命令設置.dtb的地址,如:
U-Boot> fdt addr 0x71000000
fdt的其他命令就變地可以使用,如fdt resize、fdt print等。
對于ARM來講,可以透過bootz kernel_addr initrd_address dtb_address的命令來啟動內核,即dtb_address作為bootz或者bootm的最后一次參數,第一個參數為內核映像的地址,第二個參數為initrd的地址,若不存在initrd,可以用 -代替。

3.?? ?Device Tree引發的BSP和驅動變更

有了Device Tree后,大量的板級信息都不再需要,譬如過去經常在arch/arm/plat-xxx和arch/arm/mach-xxx實施的如下事情:
1.?? ?注冊platform_device,綁定resource,即內存、IRQ等板級信息。

透過Device Tree后,形如

[cpp]
view plaincopyprint?
  1. 90?static?struct?resource?xxx_resources[]?=?{??
  2. 91?????????[0]?=?{??
  3. 92?????????????????.start??=?…,??
  4. 93?????????????????.end????=?…,??
  5. 94?????????????????.flags??=?IORESOURCE_MEM,??
  6. 95?????????},??
  7. 96?????????[1]?=?{??
  8. 97?????????????????.start??=?…,??
  9. 98?????????????????.end????=?…,??
  10. 99?????????????????.flags??=?IORESOURCE_IRQ,??
  11. 100?????????},??
  12. 101?};??
  13. 102??
  14. 103?static?struct?platform_device?xxx_device?=?{??
  15. 104?????????.name???????????=?"xxx",??
  16. 105?????????.id?????????????=?-1,??
  17. 106?????????.dev????????????=?{??
  18. 107?????????????????????????????????.platform_data??????????=?&xxx_data,??
  19. 108?????????},??
  20. 109?????????.resource???????=?xxx_resources,??
  21. 110?????????.num_resources??=?ARRAY_SIZE(xxx_resources),??
  22. 111?};??
90 static struct resource xxx_resources[] = {
91         [0] = {
92                 .start  = …,
93                 .end    = …,
94                 .flags  = IORESOURCE_MEM,
95         },
96         [1] = {
97                 .start  = …,
98                 .end    = …,
99                 .flags  = IORESOURCE_IRQ,
100         },
101 };
102
103 static struct platform_device xxx_device = {
104         .name           = "xxx",
105         .id             = -1,
106         .dev            = {
107                                 .platform_data          = &xxx_data,
108         },
109         .resource       = xxx_resources,
110         .num_resources  = ARRAY_SIZE(xxx_resources),
111 };

之類的platform_device代碼都不再需要,其中platform_device會由kernel自動展開。而這些resource實際來源于.dts中設備結點的reg、interrupts屬性。典型地,大多數總線都與“simple_bus”兼容,而在SoC對應的machine的.init_machine成員函數中,調用of_platform_bus_probe(NULL, xxx_of_bus_ids, NULL);即可自動展開所有的platform_device。譬如,假設我們有個XXX
SoC,則可在arch/arm/mach-xxx/的板文件中透過如下方式展開.dts中的設備結點對應的platform_device:

[cpp]
view plaincopyprint?
  1. 18?static?struct?of_device_id?xxx_of_bus_ids[]?__initdata?=?{??
  2. 19?????????{?.compatible?=?"simple-bus",?},??
  3. 20?????????{},??
  4. 21?};??
  5. 22??
  6. 23?void?__init?xxx_mach_init(void)??
  7. 24?{??
  8. 25?????????of_platform_bus_probe(NULL,?xxx_of_bus_ids,?NULL);??
  9. 26?}??
  10. 32??
  11. 33?#ifdef?CONFIG_ARCH_XXX??
  12. 38??
  13. 39?DT_MACHINE_START(XXX_DT,?"Generic?XXX?(Flattened?Device?Tree)")??
  14. 41?????????…??
  15. 45?????????.init_machine???=?xxx_mach_init,??
  16. 46?????????…??
  17. 49?MACHINE_END??
  18. 50?#endif??
18 static struct of_device_id xxx_of_bus_ids[] __initdata = {
19         { .compatible = "simple-bus", },
20         {},
21 };
22
23 void __init xxx_mach_init(void)
24 {
25         of_platform_bus_probe(NULL, xxx_of_bus_ids, NULL);
26 }
32
33 #ifdef CONFIG_ARCH_XXX
38
39 DT_MACHINE_START(XXX_DT, "Generic XXX (Flattened Device Tree)")
41         …
45         .init_machine   = xxx_mach_init,
46         …
49 MACHINE_END
50 #endif

2.?? ?注冊i2c_board_info,指定IRQ等板級信息。

形如

[cpp]
view plaincopyprint?
  1. 145?static?struct?i2c_board_info?__initdata?afeb9260_i2c_devices[]?=?{??
  2. 146?????????{??
  3. 147?????????????????I2C_BOARD_INFO("tlv320aic23",?0x1a),??
  4. 148?????????},?{??
  5. 149?????????????????I2C_BOARD_INFO("fm3130",?0x68),??
  6. 150?????????},?{??
  7. 151?????????????????I2C_BOARD_INFO("24c64",?0x50),??
  8. 152?????????},??
  9. 153?};??
145 static struct i2c_board_info __initdata afeb9260_i2c_devices[] = {
146         {
147                 I2C_BOARD_INFO("tlv320aic23", 0x1a),
148         }, {
149                 I2C_BOARD_INFO("fm3130", 0x68),
150         }, {
151                 I2C_BOARD_INFO("24c64", 0x50),
152         },
153 };

之類的i2c_board_info代碼,目前不再需要出現,現在只需要把tlv320aic23、fm3130、24c64這些設備結點填充作為相應的I2C controller結點的子結點即可,類似于前面的

[cpp]
view plaincopyprint?
  1. [email protected],0?{??
  2. ??????compatible?=?"acme,a1234-i2c-bus";??
  3. ??????…??
  4. [email protected]?{??
  5. ??????????compatible?=?"maxim,ds1338";??
  6. ??????????reg?=?<58>;??
  7. ??????????interrupts?=?<?7?3?>;??
  8. ??????};??
  9. ??};??
      [email protected],0 {
            compatible = "acme,a1234-i2c-bus";
            …
            [email protected] {
                compatible = "maxim,ds1338";
                reg = <58>;
                interrupts = < 7 3 >;
            };
        };

Device Tree中的I2C client會透過I2C host驅動的probe()函數中調用of_i2c_register_devices(&i2c_dev->adapter);被自動展開。

3.?? ?注冊spi_board_info,指定IRQ等板級信息。

形如

[cpp]
view plaincopyprint?
  1. 79?static?struct?spi_board_info?afeb9260_spi_devices[]?=?{??
  2. 80?????????{???????/*?DataFlash?chip?*/??
  3. 81?????????????????.modalias???????=?"mtd_dataflash",??
  4. 82?????????????????.chip_select????=?1,??
  5. 83?????????????????.max_speed_hz???=?15?*?1000?*?1000,??
  6. 84?????????????????.bus_num????????=?0,??
  7. 85?????????},??
  8. 86?};??
79 static struct spi_board_info afeb9260_spi_devices[] = {
80         {       /* DataFlash chip */
81                 .modalias       = "mtd_dataflash",
82                 .chip_select    = 1,
83                 .max_speed_hz   = 15 * 1000 * 1000,
84                 .bus_num        = 0,
85         },
86 };

之類的spi_board_info代碼,目前不再需要出現,與I2C類似,現在只需要把mtd_dataflash之類的結點,作為SPI控制器的子結點即可,SPI host驅動的probe函數透過spi_register_master()注冊master的時候,會自動展開依附于它的slave。

4.?? ?多個針對不同電路板的machine,以及相關的callback。

過去,ARM Linux針對不同的電路板會建立由MACHINE_START和MACHINE_END包圍起來的針對這個machine的一系列callback,譬如:

[cpp]
view plaincopyprint?
  1. 373?MACHINE_START(VEXPRESS,?"ARM-Versatile?Express")??
  2. 374?????????.atag_offset????=?0x100,??
  3. 375?????????.smp????????????=?smp_ops(vexpress_smp_ops),??
  4. 376?????????.map_io?????????=?v2m_map_io,??
  5. 377?????????.init_early?????=?v2m_init_early,??
  6. 378?????????.init_irq???????=?v2m_init_irq,??
  7. 379?????????.timer??????????=?&v2m_timer,??
  8. 380?????????.handle_irq?????=?gic_handle_irq,??
  9. 381?????????.init_machine???=?v2m_init,??
  10. 382?????????.restart????????=?vexpress_restart,??
  11. 383?MACHINE_END??
373 MACHINE_START(VEXPRESS, "ARM-Versatile Express")
374         .atag_offset    = 0x100,
375         .smp            = smp_ops(vexpress_smp_ops),
376         .map_io         = v2m_map_io,
377         .init_early     = v2m_init_early,
378         .init_irq       = v2m_init_irq,
379         .timer          = &v2m_timer,
380         .handle_irq     = gic_handle_irq,
381         .init_machine   = v2m_init,
382         .restart        = vexpress_restart,
383 MACHINE_END

這些不同的machine會有不同的MACHINE ID,Uboot在啟動Linux內核時會將MACHINE ID存放在r1寄存器,Linux啟動時會匹配Bootloader傳遞的MACHINE ID和MACHINE_START聲明的MACHINE ID,然后執行相應machine的一系列初始化函數。

引入Device Tree之后,MACHINE_START變更為DT_MACHINE_START,其中含有一個.dt_compat成員,用于表明相關的machine與.dts中root結點的compatible屬性兼容關系。如果Bootloader傳遞給內核的Device Tree中root結點的compatible屬性出現在某machine的.dt_compat表中,相關的machine就與對應的Device Tree匹配,從而引發這一machine的一系列初始化函數被執行。

[cpp]
view plaincopyprint?
  1. 489?static?const?char?*?const?v2m_dt_match[]?__initconst?=?{??
  2. 490?????????"arm,vexpress",??
  3. 491?????????"xen,xenvm",??
  4. 492?????????NULL,??
  5. 493?};??
  6. 495?DT_MACHINE_START(VEXPRESS_DT,?"ARM-Versatile?Express")??
  7. 496?????????.dt_compat??????=?v2m_dt_match,??
  8. 497?????????.smp????????????=?smp_ops(vexpress_smp_ops),??
  9. 498?????????.map_io?????????=?v2m_dt_map_io,??
  10. 499?????????.init_early?????=?v2m_dt_init_early,??
  11. 500?????????.init_irq???????=?v2m_dt_init_irq,??
  12. 501?????????.timer??????????=?&v2m_dt_timer,??
  13. 502?????????.init_machine???=?v2m_dt_init,??
  14. 503?????????.handle_irq?????=?gic_handle_irq,??
  15. 504?????????.restart????????=?vexpress_restart,??
  16. 505?MACHINE_END??
489 static const char * const v2m_dt_match[] __initconst = {
490         "arm,vexpress",
491         "xen,xenvm",
492         NULL,
493 };
495 DT_MACHINE_START(VEXPRESS_DT, "ARM-Versatile Express")
496         .dt_compat      = v2m_dt_match,
497         .smp            = smp_ops(vexpress_smp_ops),
498         .map_io         = v2m_dt_map_io,
499         .init_early     = v2m_dt_init_early,
500         .init_irq       = v2m_dt_init_irq,
501         .timer          = &v2m_dt_timer,
502         .init_machine   = v2m_dt_init,
503         .handle_irq     = gic_handle_irq,
504         .restart        = vexpress_restart,
505 MACHINE_END

Linux倡導針對多個SoC、多個電路板的通用DT machine,即一個DT machine的.dt_compat表含多個電路板.dts文件的root結點compatible屬性字符串。之后,如果的電路板的初始化序列不一樣,可以透過int of_machine_is_compatible(const char *compat) API判斷具體的電路板是什么。

?? ?譬如arch/arm/mach-exynos/mach-exynos5-dt.c的EXYNOS5_DT machine同時兼容"samsung,exynos5250"和"samsung,exynos5440":

[cpp]
view plaincopyprint?
  1. 158?static?char?const?*exynos5_dt_compat[]?__initdata?=?{??
  2. 159?????????"samsung,exynos5250",??
  3. 160?????????"samsung,exynos5440",??
  4. 161?????????NULL??
  5. 162?};??
  6. 163??
  7. 177?DT_MACHINE_START(EXYNOS5_DT,?"SAMSUNG?EXYNOS5?(Flattened?Device?Tree)")??
  8. 178?????????/*?Maintainer:?Kukjin?Kim?<[email protected]>?*/??
  9. 179?????????.init_irq???????=?exynos5_init_irq,??
  10. 180?????????.smp????????????=?smp_ops(exynos_smp_ops),??
  11. 181?????????.map_io?????????=?exynos5_dt_map_io,??
  12. 182?????????.handle_irq?????=?gic_handle_irq,??
  13. 183?????????.init_machine???=?exynos5_dt_machine_init,??
  14. 184?????????.init_late??????=?exynos_init_late,??
  15. 185?????????.timer??????????=?&exynos4_timer,??
  16. 186?????????.dt_compat??????=?exynos5_dt_compat,??
  17. 187?????????.restart????????=?exynos5_restart,??
  18. 188?????????.reserve????????=?exynos5_reserve,??
  19. 189?MACHINE_END??
158 static char const *exynos5_dt_compat[] __initdata = {
159         "samsung,exynos5250",
160         "samsung,exynos5440",
161         NULL
162 };
163
177 DT_MACHINE_START(EXYNOS5_DT, "SAMSUNG EXYNOS5 (Flattened Device Tree)")
178         /* Maintainer: Kukjin Kim <[email protected]> */
179         .init_irq       = exynos5_init_irq,
180         .smp            = smp_ops(exynos_smp_ops),
181         .map_io         = exynos5_dt_map_io,
182         .handle_irq     = gic_handle_irq,
183         .init_machine   = exynos5_dt_machine_init,
184         .init_late      = exynos_init_late,
185         .timer          = &exynos4_timer,
186         .dt_compat      = exynos5_dt_compat,
187         .restart        = exynos5_restart,
188         .reserve        = exynos5_reserve,
189 MACHINE_END

???? 它的.init_machine成員函數就針對不同的machine進行了不同的分支處理:

[cpp]
view plaincopyprint?
  1. 126?static?void?__init?exynos5_dt_machine_init(void)??
  2. 127?{??
  3. 128?????????…??
  4. 149??
  5. 150?????????if?(of_machine_is_compatible("samsung,exynos5250"))??
  6. 151?????????????????of_platform_populate(NULL,?of_default_bus_match_table,??
  7. 152??????????????????????????????????????exynos5250_auxdata_lookup,?NULL);??
  8. 153?????????else?if?(of_machine_is_compatible("samsung,exynos5440"))??
  9. 154?????????????????of_platform_populate(NULL,?of_default_bus_match_table,??
  10. 155??????????????????????????????????????exynos5440_auxdata_lookup,?NULL);??
  11. 156?}??
126 static void __init exynos5_dt_machine_init(void)
127 {
128         …
149
150         if (of_machine_is_compatible("samsung,exynos5250"))
151                 of_platform_populate(NULL, of_default_bus_match_table,
152                                      exynos5250_auxdata_lookup, NULL);
153         else if (of_machine_is_compatible("samsung,exynos5440"))
154                 of_platform_populate(NULL, of_default_bus_match_table,
155                                      exynos5440_auxdata_lookup, NULL);
156 }

使用Device Tree后,驅動需要與.dts中描述的設備結點進行匹配,從而引發驅動的probe()函數執行。對于platform_driver而言,需要添加一個OF匹配表,如前文的.dts文件的"acme,a1234-i2c-bus"兼容I2C控制器結點的OF匹配表可以是:

[cpp]
view plaincopyprint?
  1. 436?static?const?struct?of_device_id?a1234_i2c_of_match[]?=?{??
  2. 437?????????{?.compatible?=?"acme,a1234-i2c-bus?",?},??
  3. 438?????????{},??
  4. 439?};??
  5. 440?MODULE_DEVICE_TABLE(of,?a1234_i2c_of_match);??
  6. 441??
  7. 442?static?struct?platform_driver?i2c_a1234_driver?=?{??
  8. 443?????????.driver?=?{??
  9. 444?????????????????.name?=?"a1234-i2c-bus?",??
  10. 445?????????????????.owner?=?THIS_MODULE,??
  11. 449?????????????????.of_match_table?=?a1234_i2c_of_match,??
  12. 450?????????},??
  13. 451?????????.probe?=?i2c_a1234_probe,??
  14. 452?????????.remove?=?i2c_a1234_remove,??
  15. 453?};??
  16. 454?module_platform_driver(i2c_a1234_driver);??
436 static const struct of_device_id a1234_i2c_of_match[] = {
437         { .compatible = "acme,a1234-i2c-bus ", },
438         {},
439 };
440 MODULE_DEVICE_TABLE(of, a1234_i2c_of_match);
441
442 static struct platform_driver i2c_a1234_driver = {
443         .driver = {
444                 .name = "a1234-i2c-bus ",
445                 .owner = THIS_MODULE,
449                 .of_match_table = a1234_i2c_of_match,
450         },
451         .probe = i2c_a1234_probe,
452         .remove = i2c_a1234_remove,
453 };
454 module_platform_driver(i2c_a1234_driver);

對于I2C和SPI從設備而言,同樣也可以透過of_match_table添加匹配的.dts中的相關結點的compatible屬性,如sound/soc/codecs/wm8753.c中的:

[cpp]
view plaincopyprint?
  1. 1533?static?const?struct?of_device_id?wm8753_of_match[]?=?{??
  2. 1534?????????{?.compatible?=?"wlf,wm8753",?},??
  3. 1535?????????{?}??
  4. 1536?};??
  5. 1537?MODULE_DEVICE_TABLE(of,?wm8753_of_match);??
  6. 1587?static?struct?spi_driver?wm8753_spi_driver?=?{??
  7. 1588?????????.driver?=?{??
  8. 1589?????????????????.name???=?"wm8753",??
  9. 1590?????????????????.owner??=?THIS_MODULE,??
  10. 1591?????????????????.of_match_table?=?wm8753_of_match,??
  11. 1592?????????},??
  12. 1593?????????.probe??????????=?wm8753_spi_probe,??
  13. 1594?????????.remove?????????=?wm8753_spi_remove,??
  14. 1595?};??
  15. 1640?static?struct?i2c_driver?wm8753_i2c_driver?=?{??
  16. 1641?????????.driver?=?{??
  17. 1642?????????????????.name?=?"wm8753",??
  18. 1643?????????????????.owner?=?THIS_MODULE,??
  19. 1644?????????????????.of_match_table?=?wm8753_of_match,??
  20. 1645?????????},??
  21. 1646?????????.probe?=????wm8753_i2c_probe,??
  22. 1647?????????.remove?=???wm8753_i2c_remove,??
  23. 1648?????????.id_table?=?wm8753_i2c_id,??
  24. 1649?};??
1533 static const struct of_device_id wm8753_of_match[] = {
1534         { .compatible = "wlf,wm8753", },
1535         { }
1536 };
1537 MODULE_DEVICE_TABLE(of, wm8753_of_match);
1587 static struct spi_driver wm8753_spi_driver = {
1588         .driver = {
1589                 .name   = "wm8753",
1590                 .owner  = THIS_MODULE,
1591                 .of_match_table = wm8753_of_match,
1592         },
1593         .probe          = wm8753_spi_probe,
1594         .remove         = wm8753_spi_remove,
1595 };
1640 static struct i2c_driver wm8753_i2c_driver = {
1641         .driver = {
1642                 .name = "wm8753",
1643                 .owner = THIS_MODULE,
1644                 .of_match_table = wm8753_of_match,
1645         },
1646         .probe =    wm8753_i2c_probe,
1647         .remove =   wm8753_i2c_remove,
1648         .id_table = wm8753_i2c_id,
1649 };

不過這邊有一點需要提醒的是,I2C和SPI外設驅動和Device Tree中設備結點的compatible 屬性還有一種弱式匹配方法,就是別名匹配。compatible 屬性的組織形式為<manufacturer>,<model>,別名其實就是去掉compatible 屬性中逗號前的manufacturer前綴。關于這一點,可查看drivers/spi/spi.c的源代碼,函數spi_match_device()暴露了更多的細節,如果別名出現在設備spi_driver的id_table里面,或者別名與spi_driver的name字段相同,SPI設備和驅動都可以匹配上:

[cpp]
view plaincopyprint?
  1. 90?static?int?spi_match_device(struct?device?*dev,?struct?device_driver?*drv)??
  2. 91?{??
  3. 92?????????const?struct?spi_device?*spi?=?to_spi_device(dev);??
  4. 93?????????const?struct?spi_driver?*sdrv?=?to_spi_driver(drv);??
  5. 94??
  6. 95?????????/*?Attempt?an?OF?style?match?*/??
  7. 96?????????if?(of_driver_match_device(dev,?drv))??
  8. 97?????????????????return?1;??
  9. 98??
  10. 99?????????/*?Then?try?ACPI?*/??
  11. 100?????????if?(acpi_driver_match_device(dev,?drv))??
  12. 101?????????????????return?1;??
  13. 102??
  14. 103?????????if?(sdrv->id_table)??
  15. 104?????????????????return?!!spi_match_id(sdrv->id_table,?spi);??
  16. 105??
  17. 106?????????return?strcmp(spi->modalias,?drv->name)?==?0;??
  18. 107?}??
  19. 71?static?const?struct?spi_device_id?*spi_match_id(const?struct?spi_device_id?*id,??
  20. 72?????????????????????????????????????????????????const?struct?spi_device?*sdev)??
  21. 73?{??
  22. 74?????????while?(id->name[0])?{??
  23. 75?????????????????if?(!strcmp(sdev->modalias,?id->name))??
  24. 76?????????????????????????return?id;??
  25. 77?????????????????id++;??
  26. 78?????????}??
  27. 79?????????return?NULL;??
  28. 80?}??
90 static int spi_match_device(struct device *dev, struct device_driver *drv)
91 {
92         const struct spi_device *spi = to_spi_device(dev);
93         const struct spi_driver *sdrv = to_spi_driver(drv);
94
95         /* Attempt an OF style match */
96         if (of_driver_match_device(dev, drv))
97                 return 1;
98
99         /* Then try ACPI */
100         if (acpi_driver_match_device(dev, drv))
101                 return 1;
102
103         if (sdrv->id_table)
104                 return !!spi_match_id(sdrv->id_table, spi);
105
106         return strcmp(spi->modalias, drv->name) == 0;
107 }
71 static const struct spi_device_id *spi_match_id(const struct spi_device_id *id,
72                                                 const struct spi_device *sdev)
73 {
74         while (id->name[0]) {
75                 if (!strcmp(sdev->modalias, id->name))
76                         return id;
77                 id++;
78         }
79         return NULL;
80 }

4.?? ?常用OF API

在Linux的BSP和驅動代碼中,還經常會使用到Linux中一組Device Tree的API,這些API通常被冠以of_前綴,它們的實現代碼位于內核的drivers/of目錄。這些常用的API包括:

int of_device_is_compatible(const struct device_node *device,const char *compat);

判斷設備結點的compatible 屬性是否包含compat指定的字符串。當一個驅動支持2個或多個設備的時候,這些不同.dts文件中設備的compatible 屬性都會進入驅動 OF匹配表。因此驅動可以透過Bootloader傳遞給內核的Device Tree中的真正結點的compatible 屬性以確定究竟是哪一種設備,從而根據不同的設備類型進行不同的處理。如drivers/pinctrl/pinctrl-sirf.c即兼容于"sirf,prima2-pinctrl",又兼容于"sirf,prima2-pinctrl",在驅動中就有相應分支處理:

[cpp]
view plaincopyprint?
  1. 1682?if?(of_device_is_compatible(np,?"sirf,marco-pinctrl"))??
  2. 1683??????is_marco?=?1;??
1682 if (of_device_is_compatible(np, "sirf,marco-pinctrl"))
1683      is_marco = 1;

struct device_node *of_find_compatible_node(struct device_node *from,

???????? const char *type, const char *compatible);

根據compatible屬性,獲得設備結點。遍歷Device Tree中所有的設備結點,看看哪個結點的類型、compatible屬性與本函數的輸入參數匹配,大多數情況下,from、type為NULL。

int of_property_read_u8_array(const struct device_node *np,

???????????????????? const char *propname, u8 *out_values, size_t sz);

int of_property_read_u16_array(const struct device_node *np,

????????????????????? const char *propname, u16 *out_values, size_t sz);

int of_property_read_u32_array(const struct device_node *np,

????????????????????? const char *propname, u32 *out_values, size_t sz);

int of_property_read_u64(const struct device_node *np, const char

*propname, u64 *out_value);

讀取設備結點np的屬性名為propname,類型為8、16、32、64位整型數組的屬性。對于32位處理器來講,最常用的是of_property_read_u32_array()。如在arch/arm/mm/cache-l2x0.c中,透過如下語句讀取L2 cache的"arm,data-latency"屬性:

[cpp]
view plaincopyprint?
  1. 534?????????of_property_read_u32_array(np,?"arm,data-latency",??
  2. 535????????????????????????????????????data,?ARRAY_SIZE(data));??
534         of_property_read_u32_array(np, "arm,data-latency",
535                                    data, ARRAY_SIZE(data));

在arch/arm/boot/dts/vexpress-v2p-ca9.dts中,含有"arm,data-latency"屬性的L2 cache結點如下:

[cpp]
view plaincopyprint?
  1. 137?????????L2:[email protected]?{??
  2. 138?????????????????compatible?=?"arm,pl310-cache";??
  3. 139?????????????????reg?=?<0x1e00a000?0x1000>;??
  4. 140?????????????????interrupts?=?<0?43?4>;??
  5. 141?????????????????cache-level?=?<2>;??
  6. 142?????????????????arm,data-latency?=?<1?1?1>;??
  7. 143?????????????????arm,tag-latency?=?<1?1?1>;??
  8. 144?????????}??
137         L2: [email protected] {
138                 compatible = "arm,pl310-cache";
139                 reg = <0x1e00a000 0x1000>;
140                 interrupts = <0 43 4>;
141                 cache-level = <2>;
142                 arm,data-latency = <1 1 1>;
143                 arm,tag-latency = <1 1 1>;
144         }

有些情況下,整形屬性的長度可能為1,于是內核為了方便調用者,又在上述API的基礎上封裝出了更加簡單的讀單一整形屬性的API,它們為int of_property_read_u8()、of_property_read_u16()等,實現于include/linux/of.h:

[cpp]
view plaincopyprint?
  1. 513?static?inline?int?of_property_read_u8(const?struct?device_node?*np,??
  2. 514????????????????????????????????????????const?char?*propname,??
  3. 515????????????????????????????????????????u8?*out_value)??
  4. 516?{??
  5. 517?????????return?of_property_read_u8_array(np,?propname,?out_value,?1);??
  6. 518?}??
  7. 519??
  8. 520?static?inline?int?of_property_read_u16(const?struct?device_node?*np,??
  9. 521????????????????????????????????????????const?char?*propname,??
  10. 522????????????????????????????????????????u16?*out_value)??
  11. 523?{??
  12. 524?????????return?of_property_read_u16_array(np,?propname,?out_value,?1);??
  13. 525?}??
  14. 526??
  15. 527?static?inline?int?of_property_read_u32(const?struct?device_node?*np,??
  16. 528????????????????????????????????????????const?char?*propname,??
  17. 529????????????????????????????????????????u32?*out_value)??
  18. 530?{??
  19. 531?????????return?of_property_read_u32_array(np,?propname,?out_value,?1);??
  20. 532?}??
513 static inline int of_property_read_u8(const struct device_node *np,
514                                        const char *propname,
515                                        u8 *out_value)
516 {
517         return of_property_read_u8_array(np, propname, out_value, 1);
518 }
519
520 static inline int of_property_read_u16(const struct device_node *np,
521                                        const char *propname,
522                                        u16 *out_value)
523 {
524         return of_property_read_u16_array(np, propname, out_value, 1);
525 }
526
527 static inline int of_property_read_u32(const struct device_node *np,
528                                        const char *propname,
529                                        u32 *out_value)
530 {
531         return of_property_read_u32_array(np, propname, out_value, 1);
532 }

int of_property_read_string(struct device_node *np, const char

*propname, const char **out_string);

int of_property_read_string_index(struct device_node *np, const char

?? ?*propname, int index, const char **output);

前者讀取字符串屬性,后者讀取字符串數組屬性中的第index個字符串。如drivers/clk/clk.c中的of_clk_get_parent_name()透過of_property_read_string_index()遍歷clkspec結點的所有"clock-output-names"字符串數組屬性。

[cpp]
view plaincopyprint?
  1. 1759?const?char?*of_clk_get_parent_name(struct?device_node?*np,?int?index)??
  2. 1760?{??
  3. 1761?????????struct?of_phandle_args?clkspec;??
  4. 1762?????????const?char?*clk_name;??
  5. 1763?????????int?rc;??
  6. 1764??
  7. 1765?????????if?(index?<?0)??
  8. 1766?????????????????return?NULL;??
  9. 1767??
  10. 1768?????????rc?=?of_parse_phandle_with_args(np,?"clocks",?"#clock-cells",?index,??
  11. 1769?????????????????????????????????????????&clkspec);??
  12. 1770?????????if?(rc)??
  13. 1771?????????????????return?NULL;??
  14. 1772??
  15. 1773?????????if?(of_property_read_string_index(clkspec.np,?"clock-output-names",??
  16. 1774???????????????????????????????????clkspec.args_count???clkspec.args[0]?:?0,??
  17. 1775???????????????????????????????????????????&clk_name)?<?0)??
  18. 1776?????????????????clk_name?=?clkspec.np->name;??
  19. 1777??
  20. 1778?????????of_node_put(clkspec.np);??
  21. 1779?????????return?clk_name;??
  22. 1780?}??
  23. 1781?EXPORT_SYMBOL_GPL(of_clk_get_parent_name);??
1759 const char *of_clk_get_parent_name(struct device_node *np, int index)
1760 {
1761         struct of_phandle_args clkspec;
1762         const char *clk_name;
1763         int rc;
1764
1765         if (index < 0)
1766                 return NULL;
1767
1768         rc = of_parse_phandle_with_args(np, "clocks", "#clock-cells", index,
1769                                         &clkspec);
1770         if (rc)
1771                 return NULL;
1772
1773         if (of_property_read_string_index(clkspec.np, "clock-output-names",
1774                                   clkspec.args_count ? clkspec.args[0] : 0,
1775                                           &clk_name) < 0)
1776                 clk_name = clkspec.np->name;
1777
1778         of_node_put(clkspec.np);
1779         return clk_name;
1780 }
1781 EXPORT_SYMBOL_GPL(of_clk_get_parent_name);

static inline bool of_property_read_bool(const struct device_node *np,

???????????????????????????????????????? const char *propname);

如果設備結點np含有propname屬性,則返回true,否則返回false。一般用于檢查空屬性是否存在。

void __iomem *of_iomap(struct device_node *node, int index);

通過設備結點直接進行設備內存區間的 ioremap(),index是內存段的索引。若設備結點的reg屬性有多段,可通過index標示要ioremap的是哪一段,只有1段的情況,index為0。采用Device Tree后,大量的設備驅動通過of_iomap()進行映射,而不再通過傳統的ioremap。


unsigned int irq_of_parse_and_map(struct device_node *dev, int index);

透過Device Tree或者設備的中斷號,實際上是從.dts中的interrupts屬性解析出中斷號。若設備使用了多個中斷,index指定中斷的索引號。

還有一些OF API,這里不一一列舉,具體可參考include/linux/of.h頭文件。

5.?? ?總結

ARM社區一貫充斥的大量垃圾代碼導致Linus盛怒,因此社區在2011年到2012年進行了大量的工作。ARM Linux開始圍繞Device Tree展開,Device Tree有自己的獨立的語法,它的源文件為.dts,編譯后得到.dtb,Bootloader在引導Linux內核的時候會將.dtb地址告知內核。之后內核會展開Device Tree并創建和注冊相關的設備,因此arch/arm/mach-xxx和arch/arm/plat-xxx中大量的用于注冊platform、I2C、SPI板級信息的代碼被刪除,而驅動也以新的方式和.dts中定義的設備結點進行匹配。

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