[PATCH v5 05/11] i2c: rtl9300: check if xfer length is valid

[Jonas Jelonek <jelonek.jonas@gmail.com>]

Add an explicit check for the xfer length to 'rtl9300_i2c_config_xfer'
to make sure a length < 1 or > 16 isn't accepted. While there shouldn't
be a length > 16 because this is specified in the i2c_adapter_quirks, a
length of 0 may be passed. This is problematic, because the code adopts
this value with

(len - 1) & 0xf

because the corresponding register documentation states that the value
in the register is always used + 1 ([1]). Obvious reason for this is to
fit allow xfer length of 16 to be specified in this 4-bit wide register
field.

Another consequence of this is also, that an xfer length of 0 cannot be
set. Thus, an explicit check should avoid this. Before, this actually
led to writing 0xf into the register, probably causing unintended
behaviour.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
---
 drivers/i2c/busses/i2c-rtl9300.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/i2c/busses/i2c-rtl9300.c b/drivers/i2c/busses/i2c-rtl9300.c
index fb1ea2961cfb..35b05fb59f88 100644
--- a/drivers/i2c/busses/i2c-rtl9300.c
+++ b/drivers/i2c/busses/i2c-rtl9300.c
@@ -118,6 +118,9 @@ static int rtl9300_i2c_config_xfer(struct rtl9300_i2c *i2c, struct rtl9300_i2c_c
 {
 	int ret;
 
+	if (len < 1 || len > 16)
+		return -EINVAL;
+
 	ret = regmap_field_write(i2c->fields[F_SCL_FREQ], chan->bus_freq);
 	if (ret)
 		return ret;
-- 
2.48.1