Precise vblank timestamping for VC4 kms.

[Mario Kleiner]

On 06/23/2016 09:28 AM, Daniel Vetter wrote:
> 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
>

Not according to what Eric told me. I also couldn't find anything in the 
documented register set from vc4_regs.h, or poking around the 
neighborhood of those regs. The HVS register i use now seems to be the 
only thing one can use to approximate the typical scanoutpos method well 
enough to be useful and precise enough.

It would be good to know what other SoCs support such timetamp regs. 
Using them probably wouldn't be much more simple than the current 
scanoutpos method, with the need to then synchronize/map the gpu clocks 
time to the hosts CLOCK_MONOTONIC time, but having the precision 
timestamps would be valuable, also for modern compositors.

-mario