[PATCH] SPI: define null tx_buf to mean "shift out zeroes"
Some issues were recently turned up with the current specification of what
it means for spi_transfer.tx_buf to be null, as part of transfers which are
(from the SPI protocol driver perspective) pure reads.
Specifically, that it seems better to change the TX behaviour there from
"undefined" to "will shift zeroes". This lets protocol drivers (like the
ads7846 driver) depend on that behavior. It's what most controller drivers
in the tree are already doing (with one exception and one case of driver
wanting-to-oops), it's what Microwire hardware will necessarily be doing,
and it removes an issue whereby certain security audits would need to
define such a value anyway as part of removing covert channels.
This patch changes the specification to require shifting zeroes, and
updates all currently merged SPI controller drivers to do so.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
diff --git a/drivers/spi/spi_mpc83xx.c b/drivers/spi/spi_mpc83xx.c
index ff0b048..e9798bf 100644
--- a/drivers/spi/spi_mpc83xx.c
+++ b/drivers/spi/spi_mpc83xx.c
@@ -112,6 +112,8 @@
{ \
u32 data; \
const type * tx = mpc83xx_spi->tx; \
+ if (!tx) \
+ return 0; \
data = *tx++; \
mpc83xx_spi->tx = tx; \
return data; \
diff --git a/drivers/spi/spi_s3c24xx.c b/drivers/spi/spi_s3c24xx.c
index 2ebe1fc..8ca0871 100644
--- a/drivers/spi/spi_s3c24xx.c
+++ b/drivers/spi/spi_s3c24xx.c
@@ -174,7 +174,7 @@
static inline unsigned int hw_txbyte(struct s3c24xx_spi *hw, int count)
{
- return hw->tx ? hw->tx[count] : 0xff;
+ return hw->tx ? hw->tx[count] : 0;
}
static int s3c24xx_spi_txrx(struct spi_device *spi, struct spi_transfer *t)
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index c8bb680..176f6e3 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -289,7 +289,7 @@
* the data being transferred; that may reduce overhead, when the
* underlying driver uses dma.
*
- * If the transmit buffer is null, undefined data will be shifted out
+ * If the transmit buffer is null, zeroes will be shifted out
* while filling rx_buf. If the receive buffer is null, the data
* shifted in will be discarded. Only "len" bytes shift out (or in).
* It's an error to try to shift out a partial word. (For example, by