[PATCH v1 1/1] regmap: spi: Fix potential off-by-one when calculating reserved size
From: Andy Shevchenko
Date: Wed Jun 05 2024 - 16:54:43 EST
If we ever meet a hardware that uses weird register bits and padding,
we may end up in off-by-one error since x/8 + y/8 might not be equal
to (x + y)/8 in some cases.
bits pad x/8+y/8 (x+y)/8
4..7 0..3 0 0 // x + y from 4 up to 7
4..7 4..7 0 1 // x + y from 8 up to 11
4..7 8..11 1 1 // x + y from 12 up to 15
8..15 0..7 1 1 // x + y from 8 up to 15
8..15 8..15 2 2 // x + y from 16 up to 23
Fix this by using (x+y)/8.
Signed-off-by: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
---
drivers/base/regmap/regmap-spi.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/drivers/base/regmap/regmap-spi.c b/drivers/base/regmap/regmap-spi.c
index 094cf2a2ca3c..14b1d88997cb 100644
--- a/drivers/base/regmap/regmap-spi.c
+++ b/drivers/base/regmap/regmap-spi.c
@@ -122,8 +122,7 @@ static const struct regmap_bus *regmap_get_spi_bus(struct spi_device *spi,
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;
+ reg_reserve_size = (config->reg_bits + config->pad_bits) / BITS_PER_BYTE;
if (max_size + reg_reserve_size > max_msg_size)
max_size -= reg_reserve_size;
--
2.45.2