On 30.01.13 13:36, Daniel Vetter wrote:
On Wed, Jan 30, 2013 at 9:34 AM, Thierry Reding
<thierry.reding@xxxxxxxxxxxxxxxxx> wrote:
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@xxxxxxxxxxxxxxxxx> 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>
---
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.
I think driving animations with the vblank signal is nice, but we
kinda only need that with the pageflip support. Unfortunately the
current drm code is a bit a mess in that area, so pageflips force you
to enable the vblank interrupt for otherwise the timestamp'ed pageflip
completion events won't work.
Extensions like SGI_video_sync, OML_sync_control, INTEL_swap_events
expose vblank counts and related functionality for synchronizing to the
vblank and some applications really use and need them in addition to the
pageflip timestamps, so you need a running reliable vblank counter to
support these. Some scienctific/medical applications need to also
synchronize things like audio playback or capture, or triggering of
special research equipment (fMRI/MEG/EEG brain scanners and recordign
equipment, eye trackers, TMS brain stimulators etc.) to the vblank of a
completed or future bufferswap, with sometimes better than 1 msec
precision, sometimes ahead of the event.
That we can't timestamp non-pageflipped swaps precisely/reliably is a
limitation, not a feature for such uses.
Recent intel hw has pageflip timestamp registers, so we don't really
need that. And I guess other hw is similar. So we probably should
clean up and untangle the infrastructure a bit around the drm vblank
support code.
At least AMD hw as of a while ago didn't, older intel hw didn't (afaik),
and NVidia i don't know. You'd also need hw vblank timestamps and
counters independent of page-flip completion to not regress existing
important functionality if you want to get rid of the irq based method.
-mario
Another issue is that the vblank ioctl itself doesn't deal with
modeset crtc ids, so adding a new interface for that would be good.
Otoh most kms clients don't really use it, but only care about
pageflips.
-Daniel
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