i2c: at91: manage unexpected RXRDY flag when starting a transfer

In some cases, we could start a new i2c transfer with the RXRDY flag
set. It is not a clean state and it leads to print annoying error
messages even if there no real issue. The cause is only having garbage
data in the Receive Holding Register because of a weird behavior of the
RXRDY flag.

Reported-by: Peter Rosin <peda@lysator.liu.se>
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Tested-by: Peter Rosin <peda@lysator.liu.se>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Fixes: 93563a6a71bb ("i2c: at91: fix a race condition when using the DMA controller")
Cc: stable@vger.kernel.org #4.1

diff --git a/drivers/i2c/busses/i2c-at91.c b/drivers/i2c/busses/i2c-at91.c
index 94c087b0b7220027d5cb8314fb3f18ff6d8fdc0b..10835d1f559ba99f9b0118b19f55b7187a980dea 100644 (file)
--- a/drivers/i2c/busses/i2c-at91.c
+++ b/drivers/i2c/busses/i2c-at91.c
@@ -347,8 +347,14 @@ error:
 
 static void at91_twi_read_next_byte(struct at91_twi_dev *dev)
 {
-       if (!dev->buf_len)
+       /*
+        * If we are in this case, it means there is garbage data in RHR, so
+        * delete them.
+        */
+       if (!dev->buf_len) {
+               at91_twi_read(dev, AT91_TWI_RHR);
                return;
+       }
 
        /* 8bit read works with and without FIFO */
        *dev->buf = readb_relaxed(dev->base + AT91_TWI_RHR);
@@ -465,6 +471,24 @@ static irqreturn_t atmel_twi_interrupt(int irq, void *dev_id)
 
        if (!irqstatus)
                return IRQ_NONE;
+       /*
+        * In reception, the behavior of the twi device (before sama5d2) is
+        * weird. There is some magic about RXRDY flag! When a data has been
+        * almost received, the reception of a new one is anticipated if there
+        * is no stop command to send. That is the reason why ask for sending
+        * the stop command not on the last data but on the second last one.
+        *
+        * Unfortunately, we could still have the RXRDY flag set even if the
+        * transfer is done and we have read the last data. It might happen
+        * when the i2c slave device sends too quickly data after receiving the
+        * ack from the master. The data has been almost received before having
+        * the order to send stop. In this case, sending the stop command could
+        * cause a RXRDY interrupt with a TXCOMP one. It is better to manage
+        * the RXRDY interrupt first in order to not keep garbage data in the
+        * Receive Holding Register for the next transfer.
+        */
+       if (irqstatus & AT91_TWI_RXRDY)
+               at91_twi_read_next_byte(dev);
 
        /*
         * When a NACK condition is detected, the I2C controller sets the NACK,
@@ -507,8 +531,6 @@ static irqreturn_t atmel_twi_interrupt(int irq, void *dev_id)
        if (irqstatus & (AT91_TWI_TXCOMP | AT91_TWI_NACK)) {
                at91_disable_twi_interrupts(dev);
                complete(&dev->cmd_complete);
-       } else if (irqstatus & AT91_TWI_RXRDY) {
-               at91_twi_read_next_byte(dev);
        } else if (irqstatus & AT91_TWI_TXRDY) {
                at91_twi_write_next_byte(dev);
        }
@@ -525,7 +547,6 @@ static int at91_do_twi_transfer(struct at91_twi_dev *dev)
        unsigned long time_left;
        bool has_unre_flag = dev->pdata->has_unre_flag;
        bool has_alt_cmd = dev->pdata->has_alt_cmd;
-       unsigned sr;
 
        /*
         * WARNING: the TXCOMP bit in the Status Register is NOT a clear on
@@ -577,7 +598,7 @@ static int at91_do_twi_transfer(struct at91_twi_dev *dev)
        dev->transfer_status = 0;
 
        /* Clear pending interrupts, such as NACK. */
-       sr = at91_twi_read(dev, AT91_TWI_SR);
+       at91_twi_read(dev, AT91_TWI_SR);
 
        if (dev->fifo_size) {
                unsigned fifo_mr = at91_twi_read(dev, AT91_TWI_FMR);
@@ -600,11 +621,6 @@ static int at91_do_twi_transfer(struct at91_twi_dev *dev)
        } else if (dev->msg->flags & I2C_M_RD) {
                unsigned start_flags = AT91_TWI_START;
 
-               if (sr & AT91_TWI_RXRDY) {
-                       dev_err(dev->dev, "RXRDY still set!");
-                       at91_twi_read(dev, AT91_TWI_RHR);
-               }
-
                /* if only one byte is to be read, immediately stop transfer */
                if (!has_alt_cmd && dev->buf_len <= 1 &&
                    !(dev->msg->flags & I2C_M_RECV_LEN))