[PATCH 3.16 019/254] drm/i915: Prevent zero length "index" write

[Ben Hutchings]

3.16.55-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Ville SyrjÃlà <ville.syrjala@xxxxxxxxxxxxxxx>

commit 56350fb8978bbf4aafe08f21234e161dd128b417 upstream.

The hardware always writes one or two bytes in the index portion of
an indexed transfer. Make sure the message we send as the index
doesn't have a zero length.

Cc: Daniel Kurtz <djkurtz@xxxxxxxxxxxx>
Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Sean Paul <seanpaul@xxxxxxxxxxxx>
Fixes: 56f9eac05489 ("drm/i915/intel_i2c: use INDEX cycles for i2c read transactions")
Signed-off-by: Ville SyrjÃlà <ville.syrjala@xxxxxxxxxxxxxxx>
Link: https://patchwork.freedesktop.org/patch/msgid/20171123194157.25367-3-ville.syrjala@xxxxxxxxxxxxxxx
Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
(cherry picked from commit bb9e0d4bca50f429152e74a459160b41f3d60fb2)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/i915/intel_i2c.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/gpu/drm/i915/intel_i2c.c
+++ b/drivers/gpu/drm/i915/intel_i2c.c
@@ -449,7 +449,8 @@ gmbus_is_index_read(struct i2c_msg *msgs
 {
 	return (i + 1 < num &&
 		msgs[i].addr == msgs[i + 1].addr &&
-		!(msgs[i].flags & I2C_M_RD) && msgs[i].len <= 2 &&
+		!(msgs[i].flags & I2C_M_RD) &&
+		(msgs[i].len == 1 || msgs[i].len == 2) &&
 		(msgs[i + 1].flags & I2C_M_RD));
 }