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

[Jon Mayo]

On Tue, Jan 22, 2013 at 10:39 AM, Lucas Stach <dev at lynxeye.de> wrote:
> Am Dienstag, den 22.01.2013, 18:37 +0100 schrieb Mario Kleiner:
>> 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 looked this up in the TRM a while ago as I know we have the same
> problem in nouveau, but it seems there is no HW vblank counter on Tegra.
>
> Mario, you know a fair bit more about this than I do, what is the
> preferred way of handling this if we are sure we are not able to
> implement anything meaningful here? Just return 0?
>
> Regards,
> Lucas
>
>

In my branch for the old non-DRM version of the tegra driver, I clock
gate and power gate display when using a one-shot smart panel. So not
only are there no more IRQs, but even if Tegra had a hardware vblank
counter it would also be dead too. (it doesn't have one, but I could
make the case to add one in the next chip if we could actually make
use of it, given my previous statement, I don't think it will help)

How badly do people need this feature? Because I really do think smart
panels are going to been the norm in a few years. A bit off-topic to
Thierry's submission, but I'd really like to discourage apps from
relying on this feature, does anyone else agree?

- Jon