Precise vblank timestamping is implemented via the
usual scanout position based method. On VC4 the
pixelvalves PV do not have a scanout position
register. Only the hardware video scaler HVS has a
similar register which describes which scanline for
the output is currently composited and stored in the
HVS fifo for later consumption by the PV.

This causes a problem in that the HVS runs at a much
faster clock (system clock / audio gate) than the PV
which runs at video mode dot clock, so the unless the
fifo between HVS and PV is full, the HVS will progress
faster in its observable read line position than video
scan rate, so the HVS position reading can't be directly
translated into a scanout position for timestamp correction.

Additionally when the PV is in vblank, it doesn't consume
from the fifo, so the fifo gets full very quickly and then
the HVS stops compositing until the PV enters active scanout
and starts consuming scanlines from the fifo again, making
new space for the HVS to composite.

Therefore a simple translation of HVS read position into
elapsed time since (or to) start of active scanout does
not work, but for the most interesting cases we can still
get useful and sufficiently accurate results:

1. The PV enters active scanout of a new frame with the
   fifo of the HVS completely full, and the HVS can refill
   any fifo line which gets consumed and thereby freed up by
   the PV during active scanout very quickly. Therefore the
   PV and HVS work effectively in lock-step during active
   scanout with the fifo never having more than 1 scanline
   freed up by the PV before it gets refilled. The PV's
   real scanout position is therefore trailing the HVS
   compositing position as scanoutpos = hvspos - fifosize
   and we can get the true scanoutpos as HVS readpos minus
   fifo size, so precise timestamping works while in active
   scanout, except for the last few scanlines of the frame,
   when the HVS reaches end of frame, stops compositing and
   the PV catches up and drains the fifo. This special case
   would only introduce minor errors though.

2. If we are in vblank, then we can only guess something
   reasonable. If called from vblank irq, we assume the irq is
   usually dispatched with minimum delay, so we can take a
   timestamp taken at entry into the vblank irq handler as a
   baseline and then add a full vblank duration until the
   guessed start of active scanout. As irq dispatch is usually
   pretty low latency this works with relatively low jitter and
   good results.

   If we aren't called from vblank then we could be anywhere
   within the vblank interval, so we return a neutral result,
   simply the current system timestamp, and hope for the best.

Measurement shows the generated timestamps to be rather precise,
and at least never off more than 1 vblank duration worst-case.

Limitations: Doesn't work well yet for interlaced video modes,
             therefore disabled in interlaced mode for now.

v2: Use the DISPBASE registers to determine the FIFO size (changes
    by anholt)
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-and-tested-by: Mario Kleiner <mario.kleiner.de@gmail.com> (v2)