[PATCH v2 4/5] drm/tegra: Implement VBLANK support

[Thierry Reding]

On Tue, Jan 22, 2013 at 06:37:39PM +0100, Mario Kleiner wrote:
> On 14.01.13 17:05, Thierry Reding wrote:
> >Implement support for the VBLANK IOCTL. Note that Tegra is somewhat
> >special in this case because it doesn't use the generic IRQ support
> >provided by the DRM core (DRIVER_HAVE_IRQ) but rather registers one
> >interrupt handler for each display controller.
> >
> >While at it, clean up the way that interrupts are enabled to ensure
> >that the VBLANK interrupt only gets enabled when required.
> >
> >Signed-off-by: Thierry Reding <thierry.reding at avionic-design.de>
> 
> ... snip ...
> 
> >  struct drm_driver tegra_drm_driver = {
> >  	.driver_features = DRIVER_BUS_PLATFORM | DRIVER_MODESET | DRIVER_GEM,
> >  	.load = tegra_drm_load,
> >@@ -96,6 +136,10 @@ struct drm_driver tegra_drm_driver = {
> >  	.open = tegra_drm_open,
> >  	.lastclose = tegra_drm_lastclose,
> >
> >+	.get_vblank_counter = drm_vblank_count,
> 
> -> .get_vblank_counter = drm_vblank_count is a no-op.
> 
> .get_vblank_counter() is supposed to return some real hardware
> vblank counter value to reinitialize the software vblank counter at
> vbl irq enable time. That software counter gets queried via
> drm_vblank_count(). If you hook this up to drm_vblank_count() it
> essentially returns a constant, frozen vblank count, it has the same
> effect as returning zero or any other constant value -- You lose all
> vblank counter increments during vblank irq off time. The same
> problem is present in nouveau-kms.
> 
> I think it would be better to either implement a real hw counter
> query, or some function with a /* TODO: Implement me properly */
> comment which returns zero, so it is clear something is missing
> here.

I've finally managed to find some time to look into this some more. The
comment atop the drm_driver.get_vblank_counter() field actually suggests
that drivers should set this to drm_vblank_count if no real hardware
counter is supported.

Now it seems like we may get functionality to obtain the real VBLANK
counter once the syncpoint support is merged, so maybe we can leave this
as-is until then and replace it with a proper implementation at that
point in time?

Alternatively I could use a small wrapper with an explicit comment that
this should be implemented using the upcoming syncpoint support.

Thierry
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