[PATCH v7 02/12] i2c: rtl9300: ensure data length is within supported range

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

Add an explicit check for the xfer length to 'rtl9300_i2c_config_xfer'
to ensure the data length isn't within the supported range. In
particular a data length of 0 is not supported by the hardware and
causes unintended or destructive behaviour.

This limitation becomes obvious when looking at the register
documentation [1]. 4 bits are reserved for DATA_WIDTH and the value
of these 4 bits is used as N + 1, allowing a data length range of
1 <= len <= 16.

Affected by this is the SMBus Quick Operation which works with a data
length of 0. Passing 0 as the length causes an underflow of the value
due to:

(len - 1) & 0xf

and effectively specifying a transfer length of 16 via the registers.
This causes a 16-byte write operation instead of a Quick Write. For
example, on SFP modules without write-protected EEPROM this soft-bricks
them by overwriting some initial bytes.

For completeness, also add a quirk for the zero length.

[1] https://svanheule.net/realtek/longan/register/i2c_mst1_ctrl2

Fixes: c366be720235 ("i2c: Add driver for the RTL9300 I2C controller")
Cc: <stable@vger.kernel.org> # v6.13+
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Tested-by: Sven Eckelmann <sven@narfation.org>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Tested-by: Chris Packham <chris.packham@alliedtelesis.co.nz> # On RTL9302C based board
Tested-by: Markus Stockhausen <markus.stockhausen@gmx.de>
---
 drivers/i2c/busses/i2c-rtl9300.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/i2c/busses/i2c-rtl9300.c b/drivers/i2c/busses/i2c-rtl9300.c
index 19c367703eaf..ebd4a85e1bde 100644
--- a/drivers/i2c/busses/i2c-rtl9300.c
+++ b/drivers/i2c/busses/i2c-rtl9300.c
@@ -99,6 +99,9 @@ static int rtl9300_i2c_config_xfer(struct rtl9300_i2c *i2c, struct rtl9300_i2c_c
 {
 	u32 val, mask;
 
+	if (len < 1 || len > 16)
+		return -EINVAL;
+
 	val = chan->bus_freq << RTL9300_I2C_MST_CTRL2_SCL_FREQ_OFS;
 	mask = RTL9300_I2C_MST_CTRL2_SCL_FREQ_MASK;
 
@@ -352,7 +355,7 @@ static const struct i2c_algorithm rtl9300_i2c_algo = {
 };
 
 static struct i2c_adapter_quirks rtl9300_i2c_quirks = {
-	.flags		= I2C_AQ_NO_CLK_STRETCH,
+	.flags		= I2C_AQ_NO_CLK_STRETCH | I2C_AQ_NO_ZERO_LEN,
 	.max_read_len	= 16,
 	.max_write_len	= 16,
 };
-- 
2.48.1