1.I2C子系统架构
- Linux下IIC的架构模型大概可以分为3层:
- 第一层是I2C的从设备驱动,它包含图中的device driver和i2c-dev。device driver需要用户编写,i2c-dev由内核实现,包含了I2C设备的通用方法,但是用户不能直接使用这个驱动,需要编写一个用户层驱动,它们2个合起来才可以实现一个驱动程序。
- 第二层总线驱动,它又叫做总线控制器驱动,比如说芯片内部的I2C控制器的使用需要实现一个驱动程序,比如所需要往I2C总线上传输数据它需要什么方法,需要实现那些函数等等。它包含图中的i2c-adapter和adapter-agio。
- 第三层是i2c-core,I2C总线和I2C设备驱动的中间枢纽,它提供了I2C总线驱动和设备驱动的注册、注销方法等
2.I2C总线驱动
- 图中我们可以知道编写i2c驱动有2种实现方法。
- 第一种是用户自己完全写一个i2c的驱动程序。
- 另一种方法是使用i2c的通用驱动i2c-dev,然后自己设计一个i2c用户模式驱动。
- 对于第一种情况,应用程序读写i2c设备的流程是比较明了的。
- 对于第二种情况的读写流程稍微有点复杂,我们先来介绍另一个比较重要的地方,就是i2c控制器驱动,i2c adapter/algorithm。它是直接操作i2c设备的。里面包含2部分的东西,一个是adapter,适配器的意思,另一个是algorithm,算法的意思。
- 我们先来分析adapter,打开Linux源代码,搜索i2c_adapter:
/*
* i2c_adapter is the structure used to identify a physical i2c bus along
* with the access algorithms necessary to access it.
*/
struct i2c_adapter {
struct module *owner;
unsigned int id;
unsigned int class; /* classes to allow probing for */
const struct i2c_algorithm *algo; /* the algorithm to access the bus */
void *algo_data;
/* data fields that are valid for all devices */
u8 level; /* nesting level for lockdep */
struct mutex bus_lock;
int timeout; /* in jiffies */
int retries;
struct device dev; /* the adapter device */
int nr;
char name[48];
struct completion dev_released;
};
- 在Linux中每一个i2c适配器或者说控制器都会有一个i2c_adapter来描述,里面有一个成员:
const struct i2c_algorithm *algo; /* the algorithm to access the bus */
- 看看它的定义:
/*
* The following structs are for those who like to implement new bus drivers:
* i2c_algorithm is the interface to a class of hardware solutions which can
* be addressed using the same bus algorithms - i.e. bit-banging or the PCF8584
* to name two of the most common.
*/
struct i2c_algorithm {
/* If an adapter algorithm can't do I2C-level access, set master_xfer
to NULL. If an adapter algorithm can do SMBus access, set
smbus_xfer. If set to NULL, the SMBus protocol is simulated
using common I2C messages */
/* master_xfer should return the number of messages successfully
processed, or a negative value on error */
int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs,
int num);
int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr,
unsigned short flags, char read_write,
u8 command, int size, union i2c_smbus_data *data);
/* To determine what the adapter supports */
u32 (*functionality) (struct i2c_adapter *);
};
- 这个结构里面主要是一些函数指针,最重要的要数master_xfer了。这个函数主要来实现需要在i2c总线上传输数据的方法。比如说CPU需要通过i2c总线往一个i2c设备发送一些数据,这时候就可以使用i2c控制器里面实现的传输方法,比如说xxx_transfer来实现对i2c设备的读写,这样就把i2c数据的传输给封装起来了。
- 我们接下来分析一下2440上i2c控制器驱动的实现,它的实现在i2c-s3c2410.c文件中实现。
- 先来分析一下模块初始化和退出函数:
MODULE_DEVICE_TABLE(platform, s3c24xx_driver_ids);
static struct platform_driver s3c24xx_i2c_driver = {
.probe = s3c24xx_i2c_probe,
.remove = s3c24xx_i2c_remove,
.id_table = s3c24xx_driver_ids,
.driver = {
.owner = THIS_MODULE,
.name = "s3c-i2c",
.pm = S3C24XX_DEV_PM_OPS,
},
};
static int __init i2c_adap_s3c_init(void)
{
return platform_driver_register(&s3c24xx_i2c_driver);
}
subsys_initcall(i2c_adap_s3c_init);
static void __exit i2c_adap_s3c_exit(void)
{
platform_driver_unregister(&s3c24xx_i2c_driver);
}
- 这里主要通过平台驱动注册了一个设备。设备名叫做s3c24xx_i2c_driver。上面是s3c24xx_i2c_driver的定义,先来分析一些s3c24xx_i2c_probe函数。里面的一些变量赋值和其他一些简单的函数就不分析
/* s3c24xx_i2c_init
*
* initialise the controller, set the IO lines and frequency
*/
static int s3c24xx_i2c_init(struct s3c24xx_i2c *i2c)
{
unsigned long iicon = S3C2410_IICCON_IRQEN | S3C2410_IICCON_ACKEN;
struct s3c2410_platform_i2c *pdata;
unsigned int freq;
/* get the plafrom data */
pdata = i2c->dev->platform_data;
/* inititalise the gpio */
if (pdata->cfg_gpio)
pdata->cfg_gpio(to_platform_device(i2c->dev));
/* write slave address */
writeb(pdata->slave_addr, i2c->regs + S3C2410_IICADD);
dev_info(i2c->dev, "slave address 0x%02x\n", pdata->slave_addr);
writel(iicon, i2c->regs + S3C2410_IICCON);
/* we need to work out the divisors for the clock... */
if (s3c24xx_i2c_clockrate(i2c, &freq) != 0) {
writel(0, i2c->regs + S3C2410_IICCON);
dev_err(i2c->dev, "cannot meet bus frequency required\n");
return -EINVAL;
}
/* todo - check that the i2c lines aren't being dragged anywhere */
dev_info(i2c->dev, "bus frequency set to %d KHz\n", freq);
dev_dbg(i2c->dev, "S3C2410_IICCON=0x%02lx\n", iicon);
return 0;
}
- 我们可以看到这个函数主要完成了一下的功能:
- 1、unsigned long iicon = S3C2410_IICCON_IRQEN | S3C2410_IICCON_ACKEN;使能中断和应答
- 2、pdata->cfg_gpio(to_platform_device(i2c->dev));初始化GPIO
- 3、writeb(pdata->slave_addr, i2c->regs + S3C2410_IICADD);写从地址
- 4、if (s3c24xx_i2c_clockrate(i2c, &freq) != 0) ;初始化时钟。
- 可以发现,这里的初始化代码和裸机的代码也是非常类似的,初始化之后还调用了i2c_add_numbered_adapter函数来注册一个i2c控制器。
- prob函数中有一个重要的赋值语句i2c->adap.algo = &s3c24xx_i2c_algorithm;之前说过algorithm是用来实现i2c设备读写方法的。这里面包含2个函数,一个是s3c24xx_i2c_xfer,这个函数用来实现i2c设备的读写,他们依次调用s3c24xx_i2c_doxfer->s3c24xx_i2c_message_start,最终实现设备的读写,这个函数如下:
/* s3c24xx_i2c_message_start
*
* put the start of a message onto the bus
*/
static void s3c24xx_i2c_message_start(struct s3c24xx_i2c *i2c,
struct i2c_msg *msg)
{
unsigned int addr = (msg->addr & 0x7f) << 1;
unsigned long stat;
unsigned long iiccon;
stat = 0;
stat |= S3C2410_IICSTAT_TXRXEN;
if (msg->flags & I2C_M_RD) {
stat |= S3C2410_IICSTAT_MASTER_RX;
addr |= 1;
} else
stat |= S3C2410_IICSTAT_MASTER_TX;
if (msg->flags & I2C_M_REV_DIR_ADDR)
addr ^= 1;
/* todo - check for wether ack wanted or not */
s3c24xx_i2c_enable_ack(i2c);
iiccon = readl(i2c->regs + S3C2410_IICCON);
writel(stat, i2c->regs + S3C2410_IICSTAT);
dev_dbg(i2c->dev, "START: %08lx to IICSTAT, %02x to DS\n", stat, addr);
writeb(addr, i2c->regs + S3C2410_IICDS);
/* delay here to ensure the data byte has gotten onto the bus
* before the transaction is started */
ndelay(i2c->tx_setup);
dev_dbg(i2c->dev, "iiccon, %08lx\n", iiccon);
writel(iiccon, i2c->regs + S3C2410_IICCON);
stat |= S3C2410_IICSTAT_START;
writel(stat, i2c->regs + S3C2410_IICSTAT);
}
- 我们对比2440数据手册中的IIC读写时序:
- 可以发现读写的流程和数据手册是一致的(肯定是一致的,不然就读写失败了)。
- 不过后面从ACK period开始就属于中断的内容了,我们需要找到i2c的中断处理函数,肯定是在prob函数里面找的,发现了这么一段话:
ret = request_irq(i2c->irq, s3c24xx_i2c_irq, IRQF_DISABLED,
dev_name(&pdev->dev), i2c);
- 这里面注册了一个中断,s3c24xx_i2c_irq。中断处理程序如下:
/* s3c24xx_i2c_irq
*
* top level IRQ servicing routine
*/
static irqreturn_t s3c24xx_i2c_irq(int irqno, void *dev_id)
{
struct s3c24xx_i2c *i2c = dev_id;
unsigned long status;
unsigned long tmp;
status = readl(i2c->regs + S3C2410_IICSTAT);
if (status & S3C2410_IICSTAT_ARBITR) {
/* deal with arbitration loss */
dev_err(i2c->dev, "deal with arbitration loss\n");
}
if (i2c->state == STATE_IDLE) {
dev_dbg(i2c->dev, "IRQ: error i2c->state == IDLE\n");
tmp = readl(i2c->regs + S3C2410_IICCON);
tmp &= ~S3C2410_IICCON_IRQPEND;
writel(tmp, i2c->regs + S3C2410_IICCON);
goto out;
}
/* pretty much this leaves us with the fact that we've
* transmitted or received whatever byte we last sent */
i2s_s3c_irq_nextbyte(i2c, status);
out:
return IRQ_HANDLED;
}