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

commit 1ad849aee5f53353ed88d9cd3d68a51b03a7d44f upstream.

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/drivers/spi/spi-mpc512x-psc.c b/drivers/spi/spi-mpc512x-psc.c
index 4c63f772780adaf64e1412af167c791123eecb13..2e2b04fc065d1a57d08566848fbf3dccc8916066 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--;