Re: [PATCH] drm/vc4: Implement precise vblank timestamping.

[Eric Anholt <eric@anholt.net>]

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Mario Kleiner <mario.kleiner.de@gmail.com> writes:

> 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.
>
> Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
> ---
>  drivers/gpu/drm/vc4/vc4_crtc.c | 143 +++++++++++++++++++++++++++++++++++++++++
>  drivers/gpu/drm/vc4/vc4_drv.c  |   2 +
>  drivers/gpu/drm/vc4/vc4_drv.h  |   7 ++
>  drivers/gpu/drm/vc4/vc4_regs.h |   4 ++
>  4 files changed, 156 insertions(+)
>
> diff --git a/drivers/gpu/drm/vc4/vc4_crtc.c b/drivers/gpu/drm/vc4/vc4_crtc.c
> index c82d468..c75166e 100644
> --- a/drivers/gpu/drm/vc4/vc4_crtc.c
> +++ b/drivers/gpu/drm/vc4/vc4_crtc.c

> +int vc4_crtc_get_scanoutpos(struct drm_device *dev, unsigned int crtc_id,
> +			    unsigned int flags, int *vpos, int *hpos,
> +			    ktime_t *stime, ktime_t *etime,
> +			    const struct drm_display_mode *mode)
> +{
...
> +	/* This is the offset we need for translating hvs -> pv scanout pos. */
> +	/* XXX Find proper formula from hw docs instead of guesstimating? */
> +	fifo_lines = 2048 * 7 / mode->crtc_hdisplay;

You got the math really close here!

The COB is laid out as:
4 * 512-pixel, 4 byte per pixel SRAMs
4 * 4672-pixel, 3 byte per pixel SRAMs

The first 4 get allocated for the transposer (fifo 2) for writeback to
memory (which we don't support yet).  Display FIFO 1 (HDMI) gets 1920 *
7 + 16 pixels after that.  Display FIFO 0 gets the rest.  You can see
the current values in the DISPBASE registers (which we should probably
be initializing at boot, if we ever want to support powering on without
the firmware!)  DISPBASE has base address (in units of pixels) in the
low 16 bits, and the last included pixel in the top 16.

Want to respin using reads of the regs?  Reading them once at
initialization of the HVS should be fine.

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