From: Cyrille Pitchen <cyrille.pitchen@xxxxxxxxx>
In some cases a NACK interrupt may be pending in the Status Register (SR)
as a result of a previous transfer. However at91_do_twi_transfer() did not
read the SR to clear pending interruptions before starting a new transfer.
Hence a NACK interrupt rose as soon as it was enabled again at the I2C
controller level, resulting in a wrong sequence of operations and strange
patterns of behaviour on the I2C bus, such as a clock stretch followed by
a restart of the transfer.
This first issue occurred with both DMA and PIO write transfers.
Also when a NACK error was detected during a PIO write transfer, the
interrupt handler used to wrongly start a new transfer by writing into the
Transmit Holding Register (THR). Then the I2C slave was likely to reply
with a second NACK.
This second issue is fixed in atmel_twi_interrupt() by handling the TXRDY
status bit only if both the TXCOMP and NACK status bits are cleared.
Tested with a at24 eeprom on sama5d36ek board running a linux-4.1-at91
kernel image. Adapted to linux-next.
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@xxxxxxxxx>
Fixes: 93563a6a71bb ("i2c: at91: fix a race condition when using the DMA controller")
Reported-by: Peter Rosin <peda@xxxxxxxxxxxxxx>
Signed-off-by: Ludovic Desroches <ludovic.desroches@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx #4.1
---
drivers/i2c/busses/i2c-at91.c | 58 +++++++++++++++++++++++++++++++++++++------
1 file changed, 50 insertions(+), 8 deletions(-)
diff --git a/drivers/i2c/busses/i2c-at91.c b/drivers/i2c/busses/i2c-at91.c
index 1c758cd..94c087b 100644
--- a/drivers/i2c/busses/i2c-at91.c
+++ b/drivers/i2c/busses/i2c-at91.c
@@ -465,19 +465,57 @@ static irqreturn_t atmel_twi_interrupt(int irq, void *dev_id)
if (!irqstatus)
return IRQ_NONE;
- else if (irqstatus & AT91_TWI_RXRDY)
- at91_twi_read_next_byte(dev);
- else if (irqstatus & AT91_TWI_TXRDY)
- at91_twi_write_next_byte(dev);
-
- /* catch error flags */
- dev->transfer_status |= status;
+ /*
+ * When a NACK condition is detected, the I2C controller sets the NACK,
+ * TXCOMP and TXRDY bits all together in the Status Register (SR).
+ *
+ * 1 - Handling NACK errors with CPU write transfer.
+ *
+ * In such case, we should not write the next byte into the Transmit
+ * Holding Register (THR) otherwise the I2C controller would start a new
+ * transfer and the I2C slave is likely to reply by another NACK.
+ *
+ * 2 - Handling NACK errors with DMA write transfer.
+ *
+ * By setting the TXRDY bit in the SR, the I2C controller also triggers
+ * the DMA controller to write the next data into the THR. Then the
+ * result depends on the hardware version of the I2C controller.
+ *
+ * 2a - Without support of the Alternative Command mode.
+ *
+ * This is the worst case: the DMA controller is triggered to write the
+ * next data into the THR, hence starting a new transfer: the I2C slave
+ * is likely to reply by another NACK.
+ * Concurrently, this interrupt handler is likely to be called to manage
+ * the first NACK before the I2C controller detects the second NACK and
+ * sets once again the NACK bit into the SR.
+ * When handling the first NACK, this interrupt handler disables the I2C
+ * controller interruptions, especially the NACK interrupt.
+ * Hence, the NACK bit is pending into the SR. This is why we should
+ * read the SR to clear all pending interrupts at the beginning of
+ * at91_do_twi_transfer() before actually starting a new transfer.
+ *
+ * 2b - With support of the Alternative Command mode.
+ *
+ * When a NACK condition is detected, the I2C controller also locks the
+ * THR (and sets the LOCK bit in the SR): even though the DMA controller
+ * is triggered by the TXRDY bit to write the next data into the THR,
+ * this data actually won't go on the I2C bus hence a second NACK is not
+ * generated.
+ */
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);
}
+ /* catch error flags */
+ dev->transfer_status |= status;
+
return IRQ_HANDLED;
}
@@ -487,6 +525,7 @@ 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
@@ -537,6 +576,9 @@ static int at91_do_twi_transfer(struct at91_twi_dev *dev)
reinit_completion(&dev->cmd_complete);
dev->transfer_status = 0;
+ /* Clear pending interrupts, such as NACK. */
+ sr = at91_twi_read(dev, AT91_TWI_SR);
+
if (dev->fifo_size) {
unsigned fifo_mr = at91_twi_read(dev, AT91_TWI_FMR);
@@ -558,7 +600,7 @@ 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 (at91_twi_read(dev, AT91_TWI_SR) & AT91_TWI_RXRDY) {
+ if (sr & AT91_TWI_RXRDY) {
dev_err(dev->dev, "RXRDY still set!");
at91_twi_read(dev, AT91_TWI_RHR);
}
--
2.5.0
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