linux-kernel - [PATCH 07/11 v3] drm/i915/intel_i2c: handle zero-length writes

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Date:	Mon, 26 Mar 2012 22:26:46 +0800
From:	Daniel Kurtz <djkurtz@...omium.org>
To:	Keith Packard <keithp@...thp.com>, David Airlie <airlied@...ux.ie>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	Daniel Vetter <daniel@...ll.ch>,
	Chris Wilson <chris@...is-wilson.co.uk>
Cc:	Benson Leung <bleung@...omium.org>,
	Yufeng Shen <miletus@...omium.org>,
	Daniel Kurtz <djkurtz@...omium.org>
Subject: [PATCH 07/11 v3] 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@...omium.org>
---
 drivers/gpu/drm/i915/intel_i2c.c |    7 ++++---
 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 a04f773..c467a2e 100644
--- a/drivers/gpu/drm/i915/intel_i2c.c
+++ b/drivers/gpu/drm/i915/intel_i2c.c
@@ -258,9 +258,10 @@ gmbus_xfer(struct i2c_adapter *adapter,
 			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,
-- 
1.7.7.3

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