Precise vblank timestamping for VC4 kms.

[Ville Syrjälä]

On Thu, Jun 23, 2016 at 08:17:49AM +0200, Mario Kleiner wrote:
> The following patch implements precise vblank timestamping
> for RaspberryPi's VC4, at least for standard progressive
> scan display modes.
> 
> It has been tested on the HDMI output with half a dozen different
> video modes using special hardware measurement equipment to compare
> generated time stamps against reality. According to the tests it
> works well in its current form.
> 
> Due to hw limitations of the VC4, timestamps can't be scanline
> accurate when taken within vblank, as explained in the patch,
> but at least they will never be off by more than 1 vblank
> duration, and are typically still accurate to ~0.1 msecs
> for the common case when the timestamping is triggered from
> vblank interrupt.
> 
> The patch exposed some problems with how the drm core handles
> calculation of vblank timestamping constants for interlaced
> video modes in drm_calc_timestamping_constants(). Seems it cuts
> the expected frame duration framedur_ns into half for interlaced
> modes two times, so it ends up expecting a field duration half
> of what it should be and then miscalculates vblank counter increments
> as soon as vblank timestamping is supported and the core tries to
> derive vblank counts from it. To work around this bug, for the
> moment the vblank timestamping will disable itself for interlaced
> modes and only work for regular progressive scan.

The code should be correct for i915. The framedur_ns is actually the
duration of a single field (since we get a vblank interrupt for each
field). But it's going to be wrong for anyone that populates the
crtc_ timings with CRTC_INTERLACE_HALVE_V. So to fix it, you'd need
the driver to tell drm_calc_timestamping_constants() whether the
vertical timings were already halved or not.

-- 
Ville Syrjälä
Intel OTC