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

[Thierry Reding]

On Tue, Jan 22, 2013 at 10:21:40AM -0800, Jon Mayo wrote:
> On Mon, Jan 14, 2013 at 7:55 AM, Thierry Reding
> <thierry.reding at avionic-design.de> 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>
> > ---
> 
> Sorry Thierry, but is this a useful feature? Are applications really
> making use of it? Because it means that that an IRQ will have to
> trigger every 16.67ms when it is taken, which means the device can't
> sleep. (probably OK because it should go back to idle when the app
> lets go of the vblank). But worse, for one-shot panels there is no
> continuous vblank. I've been doing weird hacks to run a timer when the
> smart panel is idle to trick applications into thinking they have a
> vblank pulse. But really I think the whole feature of a vblank pulse
> is pretty lame and I wish it would die. Were you going to use it for
> something specific, or just adding it for completeness?

This is mainly added for completeness. I know that on Tegra we can do a
lot better by using syncpoints, but applications such as Weston (in
general applications that use the generic DRM API) rely on this to sync
rendering with VBLANK.

I don't think there's another way to achieve the same thing. And as you
already mentioned, this is only enabled when an application actively
uses it, in which case the device won't sleep anyway.

Thierry
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