spi/mpc512x-psc: optionally keep PSC SS asserted across xfer segmensts

Some SPI slave devices require asserted chip select signal across
multiple transfer segments of an SPI message. Currently the driver
always de-asserts the internal SS signal for every single transfer
segment of the message and ignores the 'cs_change' flag of the
transfer description. Disable the internal chip select (SS) only
if this is needed and indicated by the 'cs_change' flag.

Without this change, each partial transfer of a surrounding
multi-part SPI transaction might erroneously change the SS
signal, which might prevent slaves from answering the request
that was sent in a previous transfer segment because the
transaction could be considered aborted (SS was de-asserted
before reading the response).

Reported-by: Gerhard Sittig <gerhard.sittig@ifm.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@vger.kernel.org

diff --git a/drivers/spi/spi-mpc512x-psc.c b/drivers/spi/spi-mpc512x-psc.c
index 89480b2..3e490ee 100644 (file)
--- a/drivers/spi/spi-mpc512x-psc.c
+++ b/drivers/spi/spi-mpc512x-psc.c
@@ -164,7 +164,7 @@ static int mpc512x_psc_spi_transfer_rxtx(struct spi_device *spi,
 
                for (i = count; i > 0; i--) {
                        data = tx_buf ? *tx_buf++ : 0;
-                       if (len == EOFBYTE)
+                       if (len == EOFBYTE && t->cs_change)
                                setbits32(&fifo->txcmd, MPC512x_PSC_FIFO_EOF);
                        out_8(&fifo->txdata_8, data);
                        len--;