[PATCH v2] regmap: spi: Reserve space for register address/padding

[Cristian Ciocaltea]

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: f231ff38b7b2 ("regmap: spi: Set regmap max raw r/w from max_transfer_size")

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@xxxxxxxxxxxxx>
---
Changes in v2:
 - Make use of BITS_PER_BYTE, as suggested by Lucas
 - Small correction in the commit message

 drivers/base/regmap/regmap-spi.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/base/regmap/regmap-spi.c b/drivers/base/regmap/regmap-spi.c
index 719323bc6c7f..37ab23a9d034 100644
--- a/drivers/base/regmap/regmap-spi.c
+++ b/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;
 	}
 

base-commit: 568035b01cfb107af8d2e4bd2fb9aea22cf5b868
-- 
2.37.2