regmap: spi: Reserve space for register address/padding

Currently the max_raw_read and max_raw_write limits in regmap_spi struct
do not take into account the additional size of the transmitted register
address and padding.  This may result in exceeding the maximum permitted
SPI message size, which could cause undefined behaviour, e.g. data
corruption.

Fix regmap_get_spi_bus() to properly adjust the above mentioned limits
by reserving space for the register address/padding as set in the regmap
configuration.

Fixes: f231ff38 ("regmap: spi: Set regmap max raw r/w from max_transfer_size")
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Reviewed-by: Lucas Tanure <tanureal@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20220818104851.429479-1-cristian.ciocaltea@collabora.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/base/regmap/regmap-spi.c

@@ -113,6 +113,7 @@ static const struct regmap_bus *regmap_get_spi_bus(struct spi_device *spi,
 						   const struct regmap_config *config)
 {
 	size_t max_size = spi_max_transfer_size(spi);
+	size_t max_msg_size, reg_reserve_size;
 	struct regmap_bus *bus;
 
 	if (max_size != SIZE_MAX) {
@@ -120,9 +121,16 @@ static const struct regmap_bus *regmap_get_spi_bus(struct spi_device *spi,
 		if (!bus)
 			return ERR_PTR(-ENOMEM);
 
+		max_msg_size = spi_max_message_size(spi);
+		reg_reserve_size = config->reg_bits / BITS_PER_BYTE
+				 + config->pad_bits / BITS_PER_BYTE;
+		if (max_size + reg_reserve_size > max_msg_size)
+			max_size -= reg_reserve_size;
+
 		bus->free_on_exit = true;
 		bus->max_raw_read = max_size;
 		bus->max_raw_write = max_size;
+
 		return bus;
 	}