spi: introduce max_message_size hook in spi_master

Recently a maximum transfer size was was introduced in struct spi_master.
However there are also spi controllers with a maximum message size, e.g.
fsl-espi has a max message size of 64KB.
Introduce a hook max_message_size to deal with such limitations.

Also make sure that spi_max_transfer_size doesn't return greater values
than spi_max_message_size, even if hook max_transfer_size is not set.
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

include/linux/spi/spi.h

@@ -312,6 +312,8 @@ static inline void spi_unregister_driver(struct spi_driver *sdrv)
  * @flags: other constraints relevant to this driver
  * @max_transfer_size: function that returns the max transfer size for
  *	a &spi_device; may be %NULL, so the default %SIZE_MAX will be used.
+ * @max_message_size: function that returns the max message size for
+ *	a &spi_device; may be %NULL, so the default %SIZE_MAX will be used.
  * @io_mutex: mutex for physical bus access
  * @bus_lock_spinlock: spinlock for SPI bus locking
  * @bus_lock_mutex: mutex for exclusion of multiple callers
@@ -442,10 +444,11 @@ struct spi_master {
 #define SPI_MASTER_MUST_TX      BIT(4)		/* requires tx */
 
 	/*
-	 * on some hardware transfer size may be constrained
+	 * on some hardware transfer / message size may be constrained
 	 * the limit may depend on device transfer settings
 	 */
 	size_t (*max_transfer_size)(struct spi_device *spi);
+	size_t (*max_message_size)(struct spi_device *spi);
 
 	/* I/O mutex */
 	struct mutex		io_mutex;
@@ -905,12 +908,26 @@ extern int spi_async_locked(struct spi_device *spi,
 			    struct spi_message *message);
 
 static inline size_t
-spi_max_transfer_size(struct spi_device *spi)
+spi_max_message_size(struct spi_device *spi)
 {
 	struct spi_master *master = spi->master;
-	if (!master->max_transfer_size)
+	if (!master->max_message_size)
 		return SIZE_MAX;
-	return master->max_transfer_size(spi);
+	return master->max_message_size(spi);
+}
+
+static inline size_t
+spi_max_transfer_size(struct spi_device *spi)
+{
+	struct spi_master *master = spi->master;
+	size_t tr_max = SIZE_MAX;
+	size_t msg_max = spi_max_message_size(spi);
+
+	if (master->max_transfer_size)
+		tr_max = master->max_transfer_size(spi);
+
+	/* transfer size limit must not be greater than messsage size limit */
+	return min(tr_max, msg_max);
 }
 
 /*---------------------------------------------------------------------------*/