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>

1 files changed, 24 insertions, 16 deletions

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index af60d9be9632..9c595e3b9c20 100644
--- 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 *