[PATCH 4/9] drm/omap: make modesetting synchronous

[Daniel Vetter]

On Wed, Sep 03, 2014 at 02:55:05PM +0300, Tomi Valkeinen wrote:
> Currently modesetting is not done synchronously, but it queues work that
> is done later. In theory this is fine, but the driver doesn't handle it
> at properly. This means that if an application first does a full
> modeset, then immediately afterwards starts page flipping, the page
> flipping will not work properly as there's modeset work still in the
> queue.
> 
> The result with my test application was that a backbuffer was shown on
> the screen.
> 
> Fixing this properly would be rather big undertaking. Thus this patch
> fixes the issue by making the modesetting synchronous, by waiting for
> the queued work to be done at the end of omap_crtc->commit().
> 
> The ugly part here is that the background work takes crtc->mutex, and
> the modesetting also holds that lock, which means that the background
> work never gets done. To get around this, we unclock, wait, and lock
> again.
> 
> Signed-off-by: Tomi Valkeinen <tomi.valkeinen at ti.com>
> ---
>  drivers/gpu/drm/omapdrm/omap_crtc.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/gpu/drm/omapdrm/omap_crtc.c b/drivers/gpu/drm/omapdrm/omap_crtc.c
> index 193979f97bdb..3261fbf94957 100644
> --- a/drivers/gpu/drm/omapdrm/omap_crtc.c
> +++ b/drivers/gpu/drm/omapdrm/omap_crtc.c
> @@ -277,8 +277,13 @@ static void omap_crtc_prepare(struct drm_crtc *crtc)
>  static void omap_crtc_commit(struct drm_crtc *crtc)
>  {
>  	struct omap_crtc *omap_crtc = to_omap_crtc(crtc);
> +	struct drm_device *dev = crtc->dev;
>  	DBG("%s", omap_crtc->name);
>  	omap_crtc_dpms(crtc, DRM_MODE_DPMS_ON);
> +
> +	drm_modeset_unlock_all(dev);

This will run afoul of the upcoming locking rework in the atomic work. And
I'm fairly sure that the crtc helpers will fall over badly if someone
submits a concurrent setCrtc while you've dropped the locks here.

Can't you instead just drop the locking from the worker? As long as you
synchronize like here at the right places it can't race. I expect that you
might want to synchronize in the crtc_prepare hook, too. But beyond that
it should work.

As-is nacked because future headaches for me ;-)

Cheers, Daniel

> +	omap_crtc_flush(crtc);
> +	drm_modeset_lock_all(dev);
>  }
>  
>  static int omap_crtc_mode_set_base(struct drm_crtc *crtc, int x, int y,
> -- 
> 1.9.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch