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

[Jon Mayo]

On Tue, Jan 22, 2013 at 11:20 AM, Mario Kleiner
<mario.kleiner.de at gmail.com> wrote:
> On 22.01.13 19:39, Lucas Stach 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?
>>
>
> I think 0 would be better, just to make it clear to readers of the source
> code that this function is basically unimplemented. For correctness it
> doesn't matter what you return, drm_vblank_count() is ok as well.
>
> If we wanted, we could probably implement a good enough emulated hw-vblank
> counter, at least on nouveau? If there is some on-chip clock register that
> is driven by the same hardware oscillator as the crtc's so it doesn't drift,
> we could store the clock time in the vblank_disable() hook, and some
> measured duration of a video refresh, wrt. that clock. Then in
> .get_vblank_counter() calculate how many vblanks have elapsed from
> (current_clock_time - vblank_off_clock_time) / frame_duration and increment
> our own private software vblank counter. Something along that line...
>
> -mario
>

Looking through the T30 manuals, I see all the
CLK_RST_CONTROLLER_CLK_SOURCE_xxx entries that support PLLD as a
parent are display related. You won't find any timers or counters that
use the same exact clock. Being out-of-sync is a real possibility, and
something adaptive would have to be implement to hide the desync
between a fake and a real vblank once you make the transition.

That said, I think being inaccurate here doesn't have a very high
cost. Please give me some examples if you disagree though, I'd be
interested in some good use cases to throw into my test plan.

- Jon