[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 ff0b04895db06b763b1a225ac983965f4c122372..e9798bf7b8c6a0dff43795edc1ec1ee36ade6c42 100644 (file)
--- a/drivers/spi/spi_mpc83xx.c
+++ b/drivers/spi/spi_mpc83xx.c
@@ -112,6 +112,8 @@ u32 mpc83xx_spi_tx_buf_##type(struct mpc83xx_spi *mpc83xx_spi)      \
 {                                                              \
        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 2ebe1fc4c398e790181ef91da19cf0fbb1fc0cb2..8ca08713528e0724ac2d49527247a8510b0a4b9b 100644 (file)
--- a/drivers/spi/spi_s3c24xx.c
+++ b/drivers/spi/spi_s3c24xx.c
@@ -174,7 +174,7 @@ static int s3c24xx_spi_setup(struct spi_device *spi)
 
 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 c8bb68099eb9e35bf5ae2edba0c43244ca95ff5b..176f6e36dbfacbf4279593500a56c83230947095 100644 (file)
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -289,7 +289,7 @@ extern struct spi_master *spi_busnum_to_master(u16 busnum);
  * 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