Linux設備模型之spi子系統

相比於前面介紹的i2c子系統（見 http://www.linuxidc.com/Linux/2012-01/52782.htm ），spi子系統相對簡單，和i2c的結構也很相似，這裡主要介紹一下平台無關部分的代碼。先概括的說一下，spi總線或者說是spi控制器用結構體struct spi_master來表述，而一般不會明顯的主動實現這個結構而是借助板級的一些信息結構，利用spi的模型填充，存在板級信息的一條鏈表board_list，上面掛接著板級spi設備的描述信息，其掛接的結構是struct boardinfo，這個結構內部嵌入了具體的板級spi設備所需信息結構struct spi_board_info，對於要操控的spi設備，用struct spidev_data來描述，內嵌了具體設備信息結構struct spi_device，並通過struct spi_device->device_entry成員掛接到全局spi設備鏈表device_list，結構struct spi_device就是根據前面board_list上所掛的信息填充的，而driver端比較簡單，用struct spi_driver來描述，一會兒會看到該結構和標准的platform非常相似，總括的說下一般編寫流程：對於soc，spi控制器一般注冊成platform，當driver匹配後，會根據platform_device等信息構造出spi_master，一般發生在driver的probe函數，並且注冊該master，在master的注冊過程中會去遍歷board_list找到bus號相同的spi設備信息，並實例化它，好了先就說這麼多，下面先看一下具體的數據結構。

一、spi相關的數據結構

先看一下spi設備的板級信息填充的結構：

[cpp]

1. struct boardinfo {
2. struct list_head list; //用於掛接到鏈表頭board_list上
3. unsigned n_board_info; //設備信息號，spi_board_info成員的編號
4. struct spi_board_info board_info[0]; //內嵌的spi_board_info結構
5. };
6. //其中內嵌的描述spi設備的具體信息的結構struct spi_board_info為：
7. struct spi_board_info {
8. /* the device name and module name are coupled, like platform_bus;
9. * "modalias" is normally the driver name.
10. *
11. * platform_data goes to spi_device.dev.platform_data,
12. * controller_data goes to spi_device.controller_data,
13. * irq is copied too
14. */
15. char modalias[SPI_NAME_SIZE]; //名字
16. const void *platform_data; //如同注釋寫的指向spi_device.dev.platform_data
17. void *controller_data; //指向spi_device.controller_data
18. int irq; //中斷號
19. /* slower signaling on noisy or low voltage boards */
20. u32 max_speed_hz; //時鐘速率
21. /* bus_num is board specific and matches the bus_num of some
22. * spi_master that will probably be registered later.
23. *
24. * chip_select reflects how this chip is wired to that master;
25. * it's less than num_chipselect.
26. */
27. u16 bus_num; //所在的spi總線編號
28. u16 chip_select;
29. /* mode becomes spi_device.mode, and is essential for chips
30. * where the default of SPI_CS_HIGH = 0 is wrong.
31. */
32. u8 mode; //模式
33. /* ... may need additional spi_device chip config data here.
34. * avoid stuff protocol drivers can set; but include stuff
35. * needed to behave without being bound to a driver:
36. * - quirks like clock rate mattering when not selected
37. */
38. };

利用boardinfo->list成員會將自身掛接到全局的board_list鏈表上。

再來看一下spi控制器的表述結構：

[cpp]

1. struct spi_master {
2. struct device dev; //內嵌的標准dev結構
3. /* other than negative (== assign one dynamically), bus_num is fully
4. * board-specific. usually that simplifies to being SOC-specific.
5. * example: one SOC has three SPI controllers, numbered 0..2,
6. * and one board's schematics might show it using SPI-2. software
7. * would normally use bus_num=2 for that controller.
8. */
9. s16 bus_num; //標識的總線號
10. /* chipselects will be integral to many controllers; some others
11. * might use board-specific GPIOs.
12. */
13. u16 num_chipselect;
14. /* some SPI controllers pose alignment requirements on DMAable
15. * buffers; let protocol drivers know about these requirements.
16. */
17. u16 dma_alignment; //dma對其要求
18. /* spi_device.mode flags understood by this controller driver */
19. u16 mode_bits; //代表操作的spi_device.mode
20. /* other constraints relevant to this driver */
21. u16 flags; //另外的一些標志
22. #define SPI_MASTER_HALF_DUPLEX BIT(0) /* can't do full duplex */
23. #define SPI_MASTER_NO_RX BIT(1) /* can't do buffer read */
24. #define SPI_MASTER_NO_TX BIT(2) /* can't do buffer write */
25. /* lock and mutex for SPI bus locking */
26. spinlock_t bus_lock_spinlock;
27. struct mutex bus_lock_mutex;
28. /* flag indicating that the SPI bus is locked for exclusive use */
29. bool bus_lock_flag;
30. /* Setup mode and clock, etc (spi driver may call many times).
31. *
32. * IMPORTANT: this may be called when transfers to another
33. * device are active. DO NOT UPDATE SHARED REGISTERS in ways
34. * which could break those transfers.
35. */
36. int (*setup)(struct spi_device *spi); //設置模式
37. /* bidirectional bulk transfers
38. *
39. * + The transfer() method may not sleep; its main role is
40. * just to add the message to the queue.
41. * + For now there's no remove-from-queue operation, or
42. * any other request management
43. * + To a given spi_device, message queueing is pure fifo
44. *
45. * + The master's main job is to process its message queue,
46. * selecting a chip then transferring data
47. * + If there are multiple spi_device children, the i/o queue
48. * arbitration algorithm is unspecified (round robin, fifo,
49. * priority, reservations, preemption, etc)
50. *
51. * + Chipselect stays active during the entire message
52. * (unless modified by spi_transfer.cs_change != 0).
53. * + The message transfers use clock and SPI mode parameters
54. * previously established by setup() for this device
55. */
56. int (*transfer)(struct spi_device *spi, //傳輸函數
57. struct spi_message *mesg);
58. /* called on release() to free memory provided by spi_master */
59. void (*cleanup)(struct spi_device *spi);
60. };

而對於要操控的spi總線上的設備，其表述結構為：

[cpp]

1. struct spi_device {
2. struct device dev; //內嵌標准device結構體
3. struct spi_master *master; //spi主控制器
4. u32 max_speed_hz; //時鐘速率
5. u8 chip_select; //設備編號
6. u8 mode; //模式
7. #define SPI_CPHA 0x01 /* clock phase */
8. #define SPI_CPOL 0x02 /* clock polarity */
9. #define SPI_MODE_0 (0|0) /* (original MicroWire) */
10. #define SPI_MODE_1 (0|SPI_CPHA)
11. #define SPI_MODE_2 (SPI_CPOL|0)
12. #define SPI_MODE_3 (SPI_CPOL|SPI_CPHA)
13. #define SPI_CS_HIGH 0x04 /* chipselect active high? */
14. #define SPI_LSB_FIRST 0x08 /* per-word bits-on-wire */
15. #define SPI_3WIRE 0x10 /* SI/SO signals shared */
16. #define SPI_LOOP 0x20 /* loopback mode */
17. #define SPI_NO_CS 0x40 /* 1 dev/bus, no chipselect */
18. #define SPI_READY 0x80 /* slave pulls low to pause */
19. u8 bits_per_word;
20. int irq; //中斷號
21. void *controller_state; //控制狀態
22. void *controller_data; //私有數據
23. char modalias[SPI_NAME_SIZE]; //名字
24. /*
25. * likely need more hooks for more protocol options affecting how
26. * the controller talks to each chip, like:
27. * - memory packing (12 bit samples into low bits, others zeroed)
28. * - priority
29. * - drop chipselect after each word
30. * - chipselect delays
31. * - ...
32. */
33. };