drm/i915: Prevent zero length "index" write

BugLink: http://bugs.launchpad.net/bugs/1745043

commit 56350fb8 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@chromium.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <seanpaul@chromium.org>
Fixes: 56f9eac0 ("drm/i915/intel_i2c: use INDEX cycles for i2c read transactions")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171123194157.25367-3-ville.syrjala@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
(cherry picked from commit bb9e0d4b)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

drivers/gpu/drm/i915/intel_i2c.c

@@ -441,7 +441,8 @@ gmbus_is_index_read(struct i2c_msg *msgs, int i, int num)
 {
 	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));
 }