linux-kernel - [PATCH 1/2] drm/i915/intel_i2c: handle zero-length reads

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Message-Id: <1334317674-32450-1-git-send-email-djkurtz@chromium.org>
Date:	Fri, 13 Apr 2012 19:47:53 +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:	Daniel Kurtz <djkurtz@...omium.org>
Subject: [PATCH 1/2] drm/i915/intel_i2c: handle zero-length reads

A common method of probing an i2c bus is trying to do a zero-length read.
Handle this case by checking the length first waiting for data to be read.

This is actually important, since attempting a zero-length read 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 |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c
index cab879f..e249160 100644
--- a/drivers/gpu/drm/i915/intel_i2c.c
+++ b/drivers/gpu/drm/i915/intel_i2c.c
@@ -217,7 +217,7 @@ gmbus_xfer_read(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
 		   (len << GMBUS_BYTE_COUNT_SHIFT) |
 		   (msg->addr << GMBUS_SLAVE_ADDR_SHIFT) |
 		   GMBUS_SLAVE_READ | GMBUS_SW_RDY);
-	do {
+	while (len) {
 		int ret;
 		u32 val, loop = 0;
 		u32 gmbus2;
@@ -235,7 +235,7 @@ gmbus_xfer_read(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
 			*buf++ = val & 0xff;
 			val >>= 8;
 		} while (--len && ++loop < 4);
-	} while (len);
+	}
 
 	return 0;
 }
-- 
1.7.7.3

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