drm/radeon/kms: retry auxch on 0x20 timeout value.

ATOM appears to return 0x20 which seems to mean some sort of timeout.

retry the transaction up to 10 times before failing, this
makes DP->VGA convertor we bought work at least a bit more predictably.

Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/atombios_dp.c

@@ -332,11 +332,13 @@ bool radeon_process_aux_ch(struct radeon_i2c_chan *chan, u8 *req_bytes,
 	PROCESS_AUX_CHANNEL_TRANSACTION_PS_ALLOCATION args;
 	int index = GetIndexIntoMasterTable(COMMAND, ProcessAuxChannelTransaction);
 	unsigned char *base;
+	int retry_count = 0;
 
 	memset(&args, 0, sizeof(args));
 
 	base = (unsigned char *)rdev->mode_info.atom_context->scratch;
 
+retry:
 	memcpy(base, req_bytes, num_bytes);
 
 	args.lpAuxRequest = 0;
@@ -347,10 +349,12 @@ bool radeon_process_aux_ch(struct radeon_i2c_chan *chan, u8 *req_bytes,
 
 	atom_execute_table(rdev->mode_info.atom_context, index, (uint32_t *)&args);
 
-	if (args.ucReplyStatus) {
+	if (args.ucReplyStatus && !args.ucDataOutLen) {
-		DRM_DEBUG("failed to get auxch %02x%02x %02x %02x 0x%02x %02x\n",
+		if (args.ucReplyStatus == 0x20 && retry_count < 10)
+			goto retry;
+		DRM_DEBUG("failed to get auxch %02x%02x %02x %02x 0x%02x %02x after %d retries\n",
 			  req_bytes[1], req_bytes[0], req_bytes[2], req_bytes[3],
-			  chan->rec.i2c_id, args.ucReplyStatus);
+			  chan->rec.i2c_id, args.ucReplyStatus, retry_count);
 		return false;
 	}