drm: Retry i2c transfer of EDID block after failure

Usually EDID retrieval is fine. However, sometimes, especially when the
machine is loaded, it fails, but succeeds after a few retries.

Based on a patch by Michael Buesch.
Reported-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_edid.c

@@ -230,6 +230,15 @@ drm_do_probe_ddc_edid(struct i2c_adapter *adapter, unsigned char *buf,
 		      int block, int len)
 {
 	unsigned char start = block * EDID_LENGTH;
+	int ret, retries = 5;
+
+	/* The core i2c driver will automatically retry the transfer if the
+	 * adapter reports EAGAIN. However, we find that bit-banging transfers
+	 * are susceptible to errors under a heavily loaded machine and
+	 * generate spurious NAKs and timeouts. Retrying the transfer
+	 * of the individual block a few times seems to overcome this.
+	 */
+	do {
 		struct i2c_msg msgs[] = {
 			{
 				.addr	= DDC_ADDR,
@@ -243,11 +252,10 @@ drm_do_probe_ddc_edid(struct i2c_adapter *adapter, unsigned char *buf,
 				.buf	= buf,
 			}
 		};
+		ret = i2c_transfer(adapter, msgs, 2);
+	} while (ret != 2 && --retries);
 
-	if (i2c_transfer(adapter, msgs, 2) == 2)
-		return 0;
-
-	return -1;
+	return ret == 2 ? 0 : -1;
 }
 
 static u8 *