[PATCH v2 1/2] i2c: at91: fix write transfers by clearing pending interrupt first

[Wolfram Sang]

On Wed, Oct 21, 2015 at 03:44:03PM +0200, Ludovic Desroches wrote:
> From: Cyrille Pitchen <cyrille.pitchen at atmel.com>
> 
> 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 at atmel.com>
> Fixes: 93563a6a71bb ("i2c: at91: fix a race condition when using the DMA controller")
> Reported-by: Peter Rosin <peda at lysator.liu.se>
> Signed-off-by: Ludovic Desroches <ludovic.desroches at atmel.com>
> Tested-by: Peter Rosin <peda at lysator.liu.se>
> Cc: stable at vger.kernel.org #4.1

Applied to for-next, thanks!

I considered for-current, but really want this to sit a little in
for-next to make sure there are no regressions. It will go back via
stable.

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