spi: add ability to validate xfer->bits_per_word in SPI core

Allow SPI masters to define the set of bits_per_word values they support.
If they do this, then the SPI core will reject transfers that attempt to
use an unsupported bits_per_word value. This eliminates the need for each
SPI driver to implement this checking in most cases.
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/spi/spi.c

@@ -1377,6 +1377,14 @@ static int __spi_async(struct spi_device *spi, struct spi_message *message)
 			xfer->bits_per_word = spi->bits_per_word;
 		if (!xfer->speed_hz)
 			xfer->speed_hz = spi->max_speed_hz;
+		if (master->bits_per_word_mask) {
+			/* Only 32 bits fit in the mask */
+			if (xfer->bits_per_word > 32)
+				return -EINVAL;
+			if (!(master->bits_per_word_mask &
+					BIT(xfer->bits_per_word - 1)))
+				return -EINVAL;
+		}
 	}
 
 	message->spi = spi;

include/linux/spi/spi.h

@@ -228,6 +228,11 @@ static inline void spi_unregister_driver(struct spi_driver *sdrv)
  *	every chipselect is connected to a slave.
  * @dma_alignment: SPI controller constraint on DMA buffers alignment.
  * @mode_bits: flags understood by this controller driver
+ * @bits_per_word_mask: A mask indicating which values of bits_per_word are
+ *	supported by the driver. Bit n indicates that a bits_per_word n+1 is
+ *	suported. If set, the SPI core will reject any transfer with an
+ *	unsupported bits_per_word. If not set, this value is simply ignored,
+ *	and it's up to the individual driver to perform any validation.
  * @flags: other constraints relevant to this driver
  * @bus_lock_spinlock: spinlock for SPI bus locking
  * @bus_lock_mutex: mutex for SPI bus locking
@@ -301,6 +306,9 @@ struct spi_master {
 	/* spi_device.mode flags understood by this controller driver */
 	u16			mode_bits;
 
+	/* bitmask of supported bits_per_word for transfers */
+	u32			bits_per_word_mask;
+
 	/* other constraints relevant to this driver */
 	u16			flags;
 #define SPI_MASTER_HALF_DUPLEX	BIT(0)		/* can't do full duplex */