[PATCH v3] i2c: at91: manage unexpected RXRDY flag when starting a transfer

Ludovic Desroches
2015-10-26T09:38:19Z

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.

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

Changes from v2:
- fix smatch warning: variable 'sr' set but not used

drivers/i2c/busses/i2c-at91.c | 36 ++++++++++++++++++++++++++----------
1 file changed, 26 insertions(+), 10 deletions(-)

diff --git a/drivers/i2c/busses/i2c-at91.c b/drivers/i2c/busses/i2c-at91.c
index 94c087b..10835d1 100644
--- 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))
--
2.5.0