spi: Use bitfields for multiple data lines

Trent Piepho observed that since the current realistic maximum number of
data lines is four we can pack the spi_transfer struct more efficiently
if we use a bitfield for the number of bits, allowing the fields to fit
in a single byte along with cs_change.

If space becomes an issue further optimiation is possible by only using
the constants and packing the values chosen for them.

Reported-by: Trent Piepho <tpiepho@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 27a8829..77b529e 100644 (file)
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -576,8 +576,8 @@ struct spi_transfer {
        dma_addr_t      rx_dma;
 
        unsigned        cs_change:1;
-       u8              tx_nbits;
-       u8              rx_nbits;
+       unsigned        tx_nbits:3;
+       unsigned        rx_nbits:3;
 #define        SPI_NBITS_SINGLE        0x01 /* 1bit transfer */
 #define        SPI_NBITS_DUAL          0x02 /* 2bits transfer */
 #define        SPI_NBITS_QUAD          0x04 /* 4bits transfer */