Linux设备驱动剖析之IIC（四）
558行，又重试2次。
560行，调用s3c24xx_i2c_doxfer函数：
00000482 static int s3c24xx_i2c_doxfer(struct s3c24xx_i2c *i2c, 00000483 struct i2c_msg *msgs, int num) 00000484 { 00000485 unsigned long iicstat, timeout; 00000486 int spins = 20; 00000487 int ret; 00000488 00000489 if (i2c->suspended) 00000490 return -EIO; 00000491 00000492 ret = s3c24xx_i2c_set_master(i2c); 00000493 if (ret != 0) { 00000494 dev_err(i2c->dev, "cannot get bus (error %d)\n", ret); 00000495 ret = -EAGAIN; 00000496 goto out; 00000497 } 00000498 00000499 spin_lock_irq(&i2c->lock); 00000500 00000501 i2c->msg = msgs; 00000502 i2c->msg_num = num; 00000503 i2c->msg_ptr = 0; 00000504 i2c->msg_idx = 0; 00000505 i2c->state = STATE_START; 00000506 00000507 s3c24xx_i2c_enable_irq(i2c); 00000508 s3c24xx_i2c_message_start(i2c, msgs); 00000509 spin_unlock_irq(&i2c->lock); 00000510 00000511 timeout = wait_event_timeout(i2c->wait, i2c->msg_num == 0, HZ * 5); 00000512 00000513 ret = i2c->msg_idx; 00000514 00000515 /* having these next two as dev_err() makes life very 00000516 * noisy when doing an i2cdetect */ 00000517 00000518 if (timeout == 0) 00000519 dev_dbg(i2c->dev, "timeout\n"); 00000520 else if (ret != num) 00000521 dev_dbg(i2c->dev, "incomplete xfer (%d)\n", ret); 00000522 00000523 /* ensure the stop has been through the bus */ 00000524 00000525 dev_dbg(i2c->dev, "waiting for bus idle\n"); 00000526 00000527 /* first, try busy waiting briefly */ 00000528 do { 00000529 iicstat = readl(i2c->regs + S3C2410_IICSTAT); 00000530 } while ((iicstat & S3C2410_IICSTAT_START) && --spins); 00000531 00000532 /* if that timed out sleep */ 00000533 if (!spins) { 00000534 msleep(1); 00000535 iicstat = readl(i2c->regs + S3C2410_IICSTAT); 00000536 } 00000537 00000538 if (iicstat & S3C2410_IICSTAT_START) 00000539 dev_warn(i2c->dev, "timeout waiting for bus idle\n"); 00000540 00000541 out: 00000542 return ret; 00000543 }
489行，如果IIC控制器挂起了的话就不用往下走了，返回出错。
492至497行，调用s3c24xx_i2c_set_master函数，读取IICSTAT寄存器，等待IIC总线空闲。
501至505行，记住这些变量的值，后面的分析会遇到。
507行，使能IIC控制器中断。
508行，调用s3c24xx_i2c_message_start函数开始读写操作，它的定义如下：
00000163 static void s3c24xx_i2c_message_start(struct s3c24xx_i2c *i2c, 00000164 struct i2c_msg *msg) 00000165 { 00000166 unsigned int addr = (msg->addr & 0x7f) << 1; 00000167 unsigned long stat; 00000168 unsigned long iiccon; 00000169 00000170 stat = 0; 00000171 stat |= S3C2410_IICSTAT_TXRXEN; 00000172 00000173 if (msg->flags & I2C_M_RD) { 00000174 stat |= S3C2410_IICSTAT_MASTER_RX; 00000175 addr |= 1; 00000176 } else 00000177 stat |= S3C2410_IICSTAT_MASTER_TX; 00000178 00000179 if (msg->flags & I2C_M_REV_DIR_ADDR) 00000180 addr ^= 1; 00000181 00000182 /* todo - check for wether ack wanted or not */ 00000183 s3c24xx_i2c_enable_ack(i2c); 00000184 00000185 iiccon = readl(i2c->regs + S3C2410_IICCON); 00000186 writel(stat, i2c->regs + S3C2410_IICSTAT); 00000187 00000188 dev_dbg(i2c->dev, "START: %08lx to IICSTAT, %02x to DS\n", stat, addr); 00000189 writeb(addr, i2c->regs + S3C2410_IICDS); 00000190 00000191 /* delay here to ensure the data byte has gotten onto the bus 00000192 * before the transaction is started */ 00000193 00000194 ndelay(i2c->tx_setup); 00000195 00000196 dev_dbg(i2c->dev, "iiccon, %08lx\n", iiccon); 00000197 writel(iiccon, i2c->regs + S3C2410_IICCON); 00000198 00000199 stat |= S3C2410_IICSTAT_START; 00000200 writel(stat, i2c->regs + S3C2410_IICSTAT); 00000201 }
166行，高7位表示从机地址，最低1位表示读或写操作，0表示写，1表示读。
171行，IIC控制器发送和接收使能。
173行，条件不成立，所以执行177行，主机发送使能。
179行，与读操作相关的，因此if条件不成立。
183行，使能IIC控制器ACK应答。
剩下那些语句基本上都是在操作IIC控制器的寄存器，具体含义请看s3c6410的数据手册。
189行，将从机地址写入移位寄存器。
s3c24xx_i2c_message_start函数执行完后硬件就开始进行数据传输，回到s3c24xx_i2c_doxfer函数的第509行，释放锁，与499行是配对使用的。
511行，等待，等待传输操作完成，等待，只因曾经承若。有两种情况会唤醒它，一是超时，二是传输完成。
程序是在等待了，但我们的步伐却不会因此而停留，前面还有很长的路等着我们呢，还等什么，继续前进！
接下来看等待过程中发生的事情，没错，就是在中断里。中断处理函数是s3c24xx_i2c_irq，它的定义：
00000423 static irqreturn_t s3c24xx_i2c_irq(int irqno, void *dev_id) 00000424 { 00000425 struct s3c24xx_i2c *i2c = dev_id; 00000426 unsigned long status; 00000427 unsigned long tmp; 00000428 00000429 status = readl(i2c->regs + S3C2410_IICSTAT); 00000430 00000431 if (status & S3C2410_IICSTAT_ARBITR) { 00000432 /* deal with arbitration loss */ 00000433 dev_err(i2c->dev, "deal with arbitration loss\n"); 00000434 } 00000435 00000436 if (i2c->state == STATE_IDLE) { 00000437 dev_dbg(i2c->dev, "IRQ: error i2c->state == IDLE\n"); 00000438 00000439 tmp = readl(i2c->regs + S3C2410_IICCON); 00000440 tmp &= ~S3C2410_IICCON_IRQPEND; 00000441 writel(tmp, i2c->regs + S3C2410_IICCON); 00000442 goto out; 00000443 } 00000444 00000445 /* pretty much this leaves us with the fact that we've 00000446 * transmitted or received whatever byte we last sent */ 00000447 00000448 i2s_s3c_irq_nextbyte(i2c, status); 00000449 00000450 out: 00000451 return IRQ_HANDLED; 00000452 }
429行，读取状态寄存器。
431至434行，如果总线仲裁失败就打印错误信息。
436至443行，我们知道i2c->state是等于STATE_START的，因此这里的if条件不成立。
448行，i2s_s3c_irq_nextbyte函数执行具体中断处理，i2s_s3c_irq_nextbyte函数的定义：
00000257 static int i2s_s3c_irq_nextbyte(struct s3c24xx_i2c *i2c, unsigned long iicstat) 00000258 { 00000259 unsigned long tmp; 00000260 unsigned char byte; 00000261 int ret = 0; 00000262 00000263 switch (i2c->state) { 00000264 00000265 case STATE_IDLE: 00000266 dev_err(i2c->dev, "%s: called in STATE_IDLE\n", __func__); 00000267 goto out; 00000268 break; 00000269 00000270 case STATE_STOP: 00000271 dev_err(i2c->dev, "%s: called in STATE_STOP\n", __func__); 00000272 s3c24xx_i2c_disable_irq(i2c); 00000273 goto out_ack; 00000274 00000275 case STATE_START: 00000276 /* last thing we did was send a start condition on the 00000277 * bus, or started a new i2c message 00000278 */ 00000279 00000280 if (iicstat & S3C2410_IICSTAT_LASTBIT && 00000281 !(i2c->msg->flags & I2C_M_IGNORE_NAK)) { 00000282 /* ack was not received... */ 00000283 00000284 dev_dbg(i2c->dev, "ack was not received\n"); 00000285 s3c24xx_i2c_stop(i2c, -ENXIO); 00000286 goto out_ack; 00000287 } 00000288 00000289 if (i2c->msg->flags & I2C_M_RD) 00000290 i2c->state = STATE_READ; 00000291 else 00000292 i2c->state = STATE_WRITE; 00000293 00000294 /* terminate the transfer if there is nothing to do 00000295 * as this is used by the i2c probe to find devices. */ 00000296 00000297 if (is_lastmsg(i2c) && i2c->msg->len == 0) { 00000298 s3c24xx_i2c_stop(i2c, 0); 00000299 goto out_ack; 00000300 } 00000301 00000302 if (i2c->state == STATE_READ) 00000303 goto prepare_read; 00000304 00000305 /* fall through to the write state, as we will need to 00000306 * send a byte as well */ 00000307 00000308 case STATE_WRITE: 00000309 /* we are writing data to the device... check for the 00000310 * end of the message, and if so, work out what to do 00000311 */ 00000312 00000313 if (!(i2c->msg->flags & I2C_M_IGNORE_NAK)) { 00000314 if (iicstat & S3C2410_IICSTAT_LASTBIT) { 00000315 dev_dbg(i2c->dev, "WRITE: No Ack\n"); 00000316 00000317 s3c24xx_i2c_stop(i2c, -ECONNREFUSED); 00000318 goto out_ack; 00000319 } 00000320 } 00000321 00000322 retry_write: 00000323 00000324 if (!is_msgend(i2c)) { 00000325 byte = i2c->msg->buf[i2c->msg_ptr++]; 00000326 writeb(byte, i2c->regs + S3C2410_IICDS); 00000327 00000328 /* delay after writing the byte to allow the 00000329 * data setup time on the bus, as writing the 00000330 * data to the register causes the first bit 00000331 * to appear on SDA, and SCL will change as 00000332 * soon as the interrupt is acknowledged */ 00000333 00000334 ndelay(i2c->tx_setup); 00000335 00000336 } else if (!is_lastmsg(i2c)) { 00000337 /* we need to go to the next i2c message */ 00000338 00000339 dev_dbg(i2c->dev, "WRITE: Next Message\n"); 00000340 00000341 i2c->msg_ptr = 0; 00000342 i2c->msg_idx++; 00000343 i2c->msg++; 00000344 00000345 /* check to see if we need to do another message */ 00000346 if (i2c->msg->flags & I2C_M_NOSTART) { 00000347 00000348 if (i2c->msg->flags & I2C_M_RD) { 00000349 /* cannot do this, the controller 00000350 * forces us to send a new START 00000351 * when we change direction */ 00000352 00000353 s3c24xx_i2c_stop(i2c, -EINVAL); 00000354 } 00000355 00000356 goto retry_write; 00000357 } else { 00000358 /* send the new start */ 00000359 s3c24xx_i2c_message_start(i2c, i2c->msg); 00000360 i2c->state = STATE_START; 00000361 } 00000362 00000363 } else { 00000364 /* send stop */ 00000365 00000366 s3c24xx_i2c_stop(i2c, 0); 00000367 } 00000368 break; 00000369 00000370 case STATE_READ: 00000371 /* we have a byte of data in the data register, do 00000372 * something with it, and then work out wether we are 00000373 * going to do any more read/write 00000374 */ 00000375 00000376 byte = readb(i2c->regs + S3C2410_IICDS); 00000377 i2c->msg->buf[i2c->msg_ptr++] = byte; 00000378 00000379 prepare_read: 00000380 if (is_msglast(i2c)) { 00000381 /* last byte of buffer */ 00000382 00000383 if (is_lastmsg(i2c)) 00000384 s3c24xx_i2c_disable_ack(i2c); 00000385 00000386 } else if (is_msgend(i2c)) { 00000387 /* ok, we've read the entire buffer, see if there 00000388 * is anything else we need to do */ 00000389 00000390 if (is_lastmsg(i2c)) { 00000391 /* last message, send stop and complete */ 00000392 dev_dbg(i2c->dev, "READ: Send Stop\n"); 00000393 00000394 s3c24xx_i2c_stop(i2c, 0); 00000395 } else { 00000396 /* go to the next transfer */ 00000397 dev_dbg(i2c->dev, "READ: Next Transfer\n"); 00000398 00000399 i2c->msg_ptr = 0; 00000400 i2c->msg_idx++; 00000401 i2c->msg++; 00000402 } 00000403 } 00000404 00000405 break; 00000406 } 00000407 00000408 /* acknowlegde the IRQ and get back on with the work */ 00000409 00000410 out_ack: 00000411 tmp = readl(i2c->regs + S3C2410_IICCON); 00000412 tmp &= ~S3C2410_IICCON_IRQPEND; 00000413 writel(tmp, i2c->regs + S3C2410_IICCON); 00000414 out: 00000415 return ret; 00000416 }
函数够长的，不过一路走来，早就已经习惯了。
263行，因为i2c->state=STATE_START，因此忽略其他case，直接从275行开始看。
280行，如果没有收到ACK信号并且没有设置忽略ACK则停止这次传输。
289行，if条件不成立，执行292行，i2c->state = STATE_WRITE。
297行，is_lastmsg函数的定义：
00000227 static inline int is_lastmsg(struct s3c24xx_i2c *i2c) 00000228 { 00000229 return i2c->msg_idx >= (i2c->msg_num - 1); 00000230 }
因为i2c->msg_idx=0，i2c->msg_num=1，所以返回1。但是i2c->msg->len=2不为0，所以297行的if条件不成立。
302行，if条件不成立。
注意，这个case里没有并没有break，因此会继续往下执行。
313至320行，也是没收到ACK条件才会成立的。
324行，is_msgend函数的定义：
00000247 static inline int is_msgend(struct s3c24xx_i2c *i2c) 00000248 { 00000249 return i2c->msg_ptr >= i2c->msg->len; 00000250 }
因为i2c->msg_ptr=0，i2c->msg->len=2，因此返回0。324行的if条件成立。
325行，读取第一个要写的字节数据，然后i2c->msg_ptr= i2c->msg_ptr +1。
326行，将数据写入移位寄存器。
334行，延时一下。
368行，跳出switch，到411行。
411至413行，清除pending标志，恢复IIC传输。
下一次进中断的时候会进入308行的case，经过313至320行的判断后来到324行，这次is_msgend函数还是会返回0。325行，读取下一个字节数据，326行，将数据写入移位寄存器，过程和前面的一样。
当第三次进中断的时候，324行的条件就不会成立了，并且336行的if条件也不会成立，因此就会执行366行的s3c24xx_i2c_stop函数，它的定义如下：
00000203 static inline void s3c24xx_i2c_stop(struct s3c24xx_i2c *i2c, int ret) 00000204 { 00000205 unsigned long iicstat = readl(i2c->regs + S3C2410_IICSTAT); 00000206 00000207 dev_dbg(i2c->dev, "STOP\n"); 00000208 00000209 /* stop the transfer */ 00000210 iicstat &= ~S3C2410_IICSTAT_START; 00000211 writel(iicstat, i2c->regs + S3C2410_IICSTAT); 00000212 00000213 i2c->state = STATE_STOP; 00000214 00000215 s3c24xx_i2c_master_complete(i2c, ret); 00000216 s3c24xx_i2c_disable_irq(i2c); 00000217 }
205行，读取状态寄存器。
210、211行，发送停止信号。
213行，i2c->state = STATE_STOP。
215行，调用s3c24xx_i2c_master_complete函数。
216行，禁止IIC控制器中断。下面看s3c24xx_i2c_master_complete函数的定义：
00000109 static inline void s3c24xx_i2c_master_complete(struct s3c24xx_i2c *i2c, int ret) 00000110 { 00000111 dev_dbg(i2c->dev, "master_complete %d\n", ret); 00000112 00000113 i2c->msg_ptr = 0; 00000114 i2c->msg = NULL; 00000115 i2c->msg_idx++; 00000116 i2c->msg_num = 0; 00000117 if (ret) 00000118 i2c->msg_idx = ret; 00000119 00000120 wake_up(&i2c->wait); 00000121 }
113至118行，不用说了。
120行，唤醒那个睡着了的她，谁？就是那个“承若”。忘记了的话就回去看看呗。
至此，可以说ioctl的整个写过程已经说完了，至于读过程就不说了。累，确实有点累。
结束语
i2c-dev.c提供了一套不依赖于具体平台的驱动，让具体的驱动逻辑放在应用程序中，和SPI中的spidev.c的作用是很类似的。
