Skip to content

Commit

Permalink
spi: pxa2xx: Fix too early chipselect deassert
Browse files Browse the repository at this point in the history
There is a chance that chipselect is deasserted too early while the last
clock cycle is still running. Protocol analyzers will see this as a failed
last byte. This is more likely to occur with slow bitrates, for instance
at 25 kbps.

Reason for this is when using SPI mode 0 that both SPI host controller and
SPI slave will drive the data lines at the falling edge of clock signal
and sample at the rising edge. Receive FIFO gets the last bit now at the
rising edge and code sees transfer to be finished either by the interrupt
in PIO mode or by the DMA completion in DMA mode.

The SSP Time Out register SSTO should take care of delaying the
completion but it does not seems to have effect at least on Intel
Skylake and Broxton even when using long enough values. Depending on
timing code may get into point where chipselect is deasserted while the
last clock cycle is still running at its second half cycle.

Fix this by adding a wait loop in giveback() that waits until SSP becomes
idle before deasserting the chipselect.

Reported-by: Weifeng Voon <weifeng.voon@intel.com>
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
  • Loading branch information
Jarkko Nikula authored and Mark Brown committed Feb 4, 2016
1 parent 07550df commit 7a8d44b
Showing 1 changed file with 7 additions and 0 deletions.
7 changes: 7 additions & 0 deletions drivers/spi/spi-pxa2xx.c
Original file line number Diff line number Diff line change
Expand Up @@ -496,6 +496,7 @@ static void giveback(struct driver_data *drv_data)
{
struct spi_transfer* last_transfer;
struct spi_message *msg;
unsigned long timeout;

msg = drv_data->cur_msg;
drv_data->cur_msg = NULL;
Expand All @@ -508,6 +509,12 @@ static void giveback(struct driver_data *drv_data)
if (last_transfer->delay_usecs)
udelay(last_transfer->delay_usecs);

/* Wait until SSP becomes idle before deasserting the CS */
timeout = jiffies + msecs_to_jiffies(10);
while (pxa2xx_spi_read(drv_data, SSSR) & SSSR_BSY &&
!time_after(jiffies, timeout))
cpu_relax();

/* Drop chip select UNLESS cs_change is true or we are returning
* a message with an error, or next message is for another chip
*/
Expand Down

0 comments on commit 7a8d44b

Please sign in to comment.