Precise vblank timestamping for VC4 kms.

[Daniel Vetter]

On Thu, Jun 23, 2016 at 8:17 AM, Mario Kleiner
<mario.kleiner.de at gmail.com> 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.
>
> Eric: In the patch i need to calculate fifo_lines as the capacity
> of what seems to be a multi-line fifo line buffer for composited output
> scanlines between the HVS and the PV, or maybe an input fifo between
> the framebuffer(s) and the HVS? The formula i currently use is ad-hoc,
> found by trial and error. It works reasonably well for a range of
> video modes i could test with my measurement equipment, but it would
> be good to replace it by one that is actually accurately derived from
> your hardware docs.
>
> Other than that, this should be good to go.

Just out of curiosity: Is there no timestamp register that samples a
refclock on each vblank that could be used instead? That seems to be a
somewhat common feature afaict (but I don't know about vc4).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch