[PATCH 01/11] drm: add drm_send_vblank_event() helper

Laurent Pinchart laurent.pinchart at ideasonboard.com
Tue Oct 16 05:53:01 PDT 2012

Hi Mario,

On Saturday 13 October 2012 02:28:03 Mario Kleiner wrote:
> On 11.10.12 16:19, Laurent Pinchart wrote:
> > On Monday 08 October 2012 14:50:39 Rob Clark wrote:
> >> From: Rob Clark <rob at ti.com>
> ...
> > Do you know why some drivers don't call drm_vblank_count_and_time() ? For
> > instance nouveau sets the sequence to 0 and uses do_gettimeofday(), but it
> > looks like it could just call drm_vblank_count_and_time().
> At least nouveau could use it. Lucas Stach and me wrote patches for
> nouveau-kms, and they went through many iterations and missed many
> kernel merge windows due to slow review until i think both of us got
> tired of resubmitting with tiny changes.

I totally understand the feeling, but please don't give up. You can CC me on 
the next iteration, I'll make sure to review the patches (even though I have 
no experience with the nouveau driver).

> The latest iteration is posted by Lucas on nouveau-devel from 26. April
> 2012. Not sure if they'd still apply after the nouveau-kms rewrite. I'll
> probably give them another try once that has landed when i have some spare
> time.
> In principle it's very simple to use drm_vblank_count_and_time(). A
> driver needs to
> 1. Call drm_handle_vblank() from its vblank irq handler.
> 2. Make sure that in the vblank of pageflip completion
> drm_handle_vblank() is called before drm_vblank_count_and_time(), so the
> latter picks up updated counts and timestamps.
> 3. Big bonus for high precision and robustness: Implement the
> driver->get_vblank_timestamp() hook to provide a precise vblank
> timestamp. One simple way to do that is like radeon-kms or intel-kms do
> it: Call back into drm_calc_vbltimestamp_from_scanoutpos() and provide
> the driver->get_scanout_position() function - a function that returns
> the current hardware scanline counter. This is precise down to ~ 10
> microseconds (at least confirmed by measurements on
> intel,radeon,nouveau) and robust against delayed vblank irq handling.
> -mario

Laurent Pinchart

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