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>

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index af60d9b..9c595e3 100644 (file)
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -230,24 +230,32 @@ drm_do_probe_ddc_edid(struct i2c_adapter *adapter, unsigned char *buf,
                      int block, int len)
 {
        unsigned char start = block * EDID_LENGTH;
-       struct i2c_msg msgs[] = {
-               {
-                       .addr   = DDC_ADDR,
-                       .flags  = 0,
-                       .len    = 1,
-                       .buf    = &start,
-               }, {
-                       .addr   = DDC_ADDR,
-                       .flags  = I2C_M_RD,
-                       .len    = len,
-                       .buf    = buf,
-               }
-       };
+       int ret, retries = 5;
 
-       if (i2c_transfer(adapter, msgs, 2) == 2)
-               return 0;
+       /* 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,
+                               .flags  = 0,
+                               .len    = 1,
+                               .buf    = &start,
+                       }, {
+                               .addr   = DDC_ADDR,
+                               .flags  = I2C_M_RD,
+                               .len    = len,
+                               .buf    = buf,
+                       }
+               };
+               ret = i2c_transfer(adapter, msgs, 2);
+       } while (ret != 2 && --retries);
 
-       return -1;
+       return ret == 2 ? 0 : -1;
 }
 
 static u8 *