drm/radeon: workaround a hw bug on some radeon chipsets with all-0 EDIDs.

Some RS690 chipsets seem to end up with floating connectors, either
a DVI connector isn't actually populated, or an add-in HDMI card
is available but not installed. In this case we seem to get a NULL byte
response for each byte of the i2c transaction, so we detect this
case and if we see it we don't do anymore DDC transactions on this
connector.

I've tested this on my RS690 without the HDMI card installed and
it seems to work fine.
Signed-off-by: NDave Airlie <airlied@redhat.com>
Reviewed-by: NAlex Deucher <alexdeucher@gmail.com>

drivers/gpu/drm/drm_edid.c

@@ -258,6 +258,17 @@ drm_do_probe_ddc_edid(struct i2c_adapter *adapter, unsigned char *buf,
 	return ret == 2 ? 0 : -1;
 }
 
+static bool drm_edid_is_zero(u8 *in_edid, int length)
+{
+	int i;
+	u32 *raw_edid = (u32 *)in_edid;
+
+	for (i = 0; i < length / 4; i++)
+		if (*(raw_edid + i) != 0)
+			return false;
+	return true;
+}
+
 static u8 *
 drm_do_get_edid(struct drm_connector *connector, struct i2c_adapter *adapter)
 {
@@ -273,6 +284,10 @@ drm_do_get_edid(struct drm_connector *connector, struct i2c_adapter *adapter)
 			goto out;
 		if (drm_edid_block_valid(block))
 			break;
+		if (i == 0 && drm_edid_is_zero(block, EDID_LENGTH)) {
+			connector->null_edid_counter++;
+			goto carp;
+		}
 	}
 	if (i == 4)
 		goto carp;

drivers/gpu/drm/radeon/radeon_connectors.c

@@ -836,6 +836,13 @@ radeon_dvi_detect(struct drm_connector *connector, bool force)
 		if (!radeon_connector->edid) {
 			DRM_ERROR("%s: probed a monitor but no|invalid EDID\n",
 					drm_get_connector_name(connector));
+			/* rs690 seems to have a problem with connectors not existing and always
+			 * return a block of 0's. If we see this just stop polling on this output */
+			if ((rdev->family == CHIP_RS690 || rdev->family == CHIP_RS740) && radeon_connector->base.null_edid_counter) {
+				ret = connector_status_disconnected;
+				DRM_ERROR("%s: detected RS690 floating bus bug, stopping ddc detect\n", drm_get_connector_name(connector));
+				radeon_connector->ddc_bus = NULL;
+			}
 		} else {
 			radeon_connector->use_digital = !!(radeon_connector->edid->input & DRM_EDID_INPUT_DIGITAL);
 

include/drm/drm_crtc.h

@@ -520,6 +520,8 @@ struct drm_connector {
 	uint32_t encoder_ids[DRM_CONNECTOR_MAX_ENCODER];
 	uint32_t force_encoder_id;
 	struct drm_encoder *encoder; /* currently active encoder */
+
+	int null_edid_counter; /* needed to workaround some HW bugs where we get all 0s */
 };
 
 /**