[PATCH 1/2] drm/radeon: Use drm_vblank_off/on to fix vblank counter trouble.

[Vlastimil Babka]

Is there a PATCH 2/2 which I can't find, or is the subject wrong?

On 01/21/2016 09:16 AM, Mario Kleiner wrote:
> The hardware vblank counter of AMD gpu's resets to zero during a
> modeset. The new implementation of drm_update_vblank_count() from
> commit 4dfd6486 "drm: Use vblank timestamps to guesstimate how
> many vblanks were missed", introduced in Linux 4.4, treats that
> as a counter wraparound and causes the software vblank counter
> to jump forward by a large distance of up to 2^24 counts. This
> interacts badly with 32-bit wraparound handling in
> drm_handle_vblank_events(), causing that function to no longer
> deliver pending vblank events to clients.
>
> This leads to client hangs especially if clients perform OpenGL
> or DRI3/Present animations while a modeset happens and triggers
> the hw vblank counter reset. One prominent example is a hang of
> KDE Plasma 5's startup progress splash screen during login, making
> the KDE session unuseable.
>
> Another small potential race exists when executing a modeset while
> vblank interrupts are enabled or just get enabled: The modeset updates
> radeon_crtc->lb_vblank_lead_lines during radeon_display_bandwidth_update,
> so if vblank interrupt handling or enable would try to access that variable
> multiple times at the wrong moment as part of drm_update_vblank_counter,
> while the scanout happens to be within lb_vblank_lead_lines before the
> start of vblank, it could cause inconsistent vblank counting and again
> trigger a jump of the software vblank counter, causing similar client
> hangs. The most easy way to avoid this small race is to not allow
> vblank enable or vblank irq's during modeset.
>
> This patch replaces calls to drm_vblank_pre/post_modeset in the
> drivers dpms code with calls to drm_vblank_off/on, as recommended
> for drivers with hw counters that reset to zero during modeset.
> Those calls disable vblank interrupts during the modeset sequence
> and reinitialize vblank counts and timestamps after the modeset
> properly, taking hw counter reset into account, thereby fixing
> the problem of forward jumping counters.
>
> During a modeset, calls to drm_vblank_get() will no-op/intentionally
> fail, so no vblank events or pageflips can be queued during modesetting.
>
> Radeons static and dynpm power management uses drm_vblank_get to enable
> vblank irqs to synchronize reclocking to start of vblank. If a modeset
> would happen in parallel with such a power management action, drm_vblank_get
> would be suppressed, sync to vblank wouldn't work and a visual glitch could
> happen. However that glitch would hopefully be hidden by the blanking of
> the crtc during modeset. A small fix to power management makes sure to
> check for this and prevent unbalanced vblank reference counts due to
> mismatched drm_vblank_get/put.
>
> Reported-by: Vlastimil Babka <vbabka at suse.cz>
> Signed-off-by: Mario Kleiner <mario.kleiner.de at gmail.com>

FWIW, this seems to work for the kde5 login issue, thanks. Let me know 
if you need also some specific testing/debug output, or testing another 
approach if the "drm_vblank_on/off propaganda" is not acceptable :)