drm/i915/intel_i2c: handle zero-length writes

A common method of probing an i2c bus is trying to do a zero-length write.
Handle this case by checking the length first before decrementing it.

This is actually important, since attempting a zero-length write is one
of the ways that i2cdetect and i2c_new_probed_device detect whether
there is device present on the bus with a given address.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 4 insertions, 3 deletions

diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c
index c12db7265893..99a04f8bcb1f 100644
--- a/drivers/gpu/drm/i915/intel_i2c.c
+++ b/drivers/gpu/drm/i915/intel_i2c.c
@@ -248,9 +248,10 @@ gmbus_xfer_write(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
         u32 val, loop;
 
         val = loop = 0;
-        do {
-                val |= *buf++ << (8 * loop);
-        } while (--len && ++loop < 4);
+        while (len && loop < 4) {
+                val |= *buf++ << (8 * loop++);
+                len -= 1;
+        }
 
         I915_WRITE(GMBUS3 + reg_offset, val);
         I915_WRITE(GMBUS1 + reg_offset,