[PATCH 08/13 v4] drm/i915/intel_i2c: handle zero-length writes

[Daniel Kurtz]

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 at chromium.org>
---
 drivers/gpu/drm/i915/intel_i2c.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c
index c12db72..5a94e9b 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 {
+	while (len && loop < 4) {
 		val |= *buf++ << (8 * loop);
-	} while (--len && ++loop < 4);
+		len -= 1;
+	}
 
 	I915_WRITE(GMBUS3 + reg_offset, val);
 	I915_WRITE(GMBUS1 + reg_offset,
-- 
1.7.7.3