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@xxxxxxxxxxxxxxxxx>
... 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
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