[RFC PATCH xserver] modesetting: re-set the crtc's mode when link-status goes BAD

[Martin Peres]

Despite all the careful planing of the kernel, a link may become
insufficient to handle the currently-set mode. At this point, the
kernel should mark this particular configuration as being broken
and potentially prune the mode before setting the offending connector's
link-status to BAD and send the userspace a hotplug event. This may
happen right after a modeset or later on.

When available, we should use the link-status information to reset
the wanted mode.

Signed-off-by: Martin Peres <martin.peres at linux.intel.com>
---

WARNING: The patches have not been merged in the kernel yet, so this patch is
merely an RFC.

This patch is the result of discussions happening mostly in DRI-devel and
Intel-GFX on how to handle link training failures. I would advise reading the
thread [0] first and then this thread [1] which explain in great length why this
is needed and why the selected approach seems to be the best.

The relevant kernel patches + this patch are enough to pass the relevant
DisplayPort compliance tests, provided that the Desktop Environment or another
program ([2]?) provides the initial modesetting on hotplug.

Would this patch be acceptable to you? Any comments or suggestions?

[0] https://lists.freedesktop.org/archives/dri-devel/2016-November/123366.html
[1] https://lists.freedesktop.org/archives/dri-devel/2016-November/124736.html
[2] https://cgit.freedesktop.org/~mperes/auto-randr/


 hw/xfree86/drivers/modesetting/drmmode_display.c | 51 ++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/hw/xfree86/drivers/modesetting/drmmode_display.c b/hw/xfree86/drivers/modesetting/drmmode_display.c
index 6e755e9482..3144620c67 100644
--- a/hw/xfree86/drivers/modesetting/drmmode_display.c
+++ b/hw/xfree86/drivers/modesetting/drmmode_display.c
@@ -2262,6 +2262,10 @@ drmmode_setup_colormap(ScreenPtr pScreen, ScrnInfoPtr pScrn)
 }
 
 #ifdef CONFIG_UDEV_KMS
+
+#define DRM_MODE_LINK_STATUS_GOOD       0
+#define DRM_MODE_LINK_STATUS_BAD        1
+
 static void
 drmmode_handle_uevents(int fd, void *closure)
 {
@@ -2281,6 +2285,49 @@ drmmode_handle_uevents(int fd, void *closure)
     if (!found)
         return;
 
+    /* Try to re-set the mode on all the connectors with a BAD link-state:
+     * This may happen if a link degrades and a new modeset is necessary, using
+     * different link-training parameters. If the kernel found that the current
+     * mode is not achievable anymore, it should have pruned the mode before
+     * sending the hotplug event. Try to re-set the currently-set mode to keep
+     * the display alive, this will fail if the mode has been pruned.
+     * In any case, we will send randr events for the Desktop Environment to
+     * deal with it, if it wants to.
+     */
+    for (i = 0; i < config->num_output; i++) {
+        xf86OutputPtr output = config->output[i];
+        drmmode_output_private_ptr drmmode_output = output->driver_private;
+        uint32_t con_id = drmmode_output->mode_output->connector_id;
+        drmModeConnectorPtr koutput;
+
+        /* Get an updated view of the properties for the current connector and
+         * look for the link-status property
+         */
+        koutput = drmModeGetConnectorCurrent(drmmode->fd, con_id);
+        for (j = 0; koutput && j < koutput->count_props; j++) {
+            drmModePropertyPtr props;
+            props = drmModeGetProperty(drmmode->fd, koutput->props[j]);
+            if (props && props->flags & DRM_MODE_PROP_ENUM &&
+                !strcmp(props->name, "link-status") &&
+                koutput->prop_values[j] == DRM_MODE_LINK_STATUS_BAD) {
+                xf86CrtcPtr crtc = output->crtc;
+                if (!crtc)
+                    continue;
+
+                /* the connector got a link failure, re-set the current mode */
+                drmmode_set_mode_major(crtc, &crtc->mode, crtc->rotation,
+                                       crtc->x, crtc->y);
+
+                xf86DrvMsg(scrn->scrnIndex, X_WARNING,
+                           "hotplug event: connector %u's link-state is BAD, "
+                           "tried resetting the current mode. You may be left"
+                           "with a black screen if this fails...\n", con_id);
+            }
+            drmModeFreeProperty(props);
+        }
+        drmModeFreeConnector(koutput);
+    }
+
     mode_res = drmModeGetResources(drmmode->fd);
     if (!mode_res)
         goto out;
@@ -2345,6 +2392,10 @@ out_free_res:
 out:
     RRGetInfo(xf86ScrnToScreen(scrn), TRUE);
 }
+
+#undef DRM_MODE_LINK_STATUS_BAD
+#undef DRM_MODE_LINK_STATUS_GOOD
+
 #endif
 
 void
-- 
2.11.0