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

[Daniel Vetter]

On Thu, Jan 26, 2017 at 02:37:28PM +0200, Martin Peres wrote:
> 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.

That's how it's supposed to happen, before we can merge the kernel side,
we need acceptance (=reviewed) by the userspace side. Which is why this
patch here.
-Daniel

> 
> 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
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch