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