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Subject[PATCH 3.12 133/142] i2c: at91: manage unexpected RXRDY flag when starting a transfer
Date
From: Ludovic Desroches <ludovic.desroches@atmel.com>

3.12-stable review patch. If anyone has any objections, please let me know.

===============

commit a9bed6b10bd117a300cceb9062003f7a2761ef99 upstream.

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")
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
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 e6f18b241255..70782d560407 100644
--- a/drivers/i2c/busses/i2c-at91.c
+++ b/drivers/i2c/busses/i2c-at91.c
@@ -272,8 +272,14 @@ error:

static void at91_twi_read_next_byte(struct at91_twi_dev *dev)
{
- if (dev->buf_len <= 0)
+ /*
+ * 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;
+ }

*dev->buf = at91_twi_read(dev, AT91_TWI_RHR) & 0xff;
--dev->buf_len;
@@ -370,6 +376,24 @@ 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,
@@ -412,8 +436,6 @@ 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);
}
@@ -428,7 +450,6 @@ static int at91_do_twi_transfer(struct at91_twi_dev *dev)
{
int ret;
bool has_unre_flag = dev->pdata->has_unre_flag;
- unsigned sr;

/*
* WARNING: the TXCOMP bit in the Status Register is NOT a clear on
@@ -465,7 +486,7 @@ 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->buf_len) {
at91_twi_write(dev, AT91_TWI_CR, AT91_TWI_QUICK);
@@ -473,11 +494,6 @@ 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 (dev->buf_len <= 1 && !(dev->msg->flags & I2C_M_RECV_LEN))
start_flags |= AT91_TWI_STOP;
--
2.12.2
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