[Intel-gfx] [PATCH] drm/vblank: Fixup and document timestamp update/read barriers

[Peter Hurley]

On 04/16/2015 02:39 AM, Mario Kleiner wrote:
> On 04/16/2015 03:29 AM, Peter Hurley wrote:
>> On 04/15/2015 05:26 PM, Mario Kleiner wrote:

>> Because the time scales for these events don't require that level of
>> resolution; consider how much code has to get executed between a
>> hardware vblank irq triggering and the vblank counter being updated.
>>
>> Realistically, the only relevant requirement is that the timestamp
>> match the counter.
>>
> 
> Yes that is the really important part. A msec delay would possibly matter for some timing sensitive apps like mine - some more exotic displays run at 200 Hz, and some apps need to synchronize to the vblank not strictly for graphics. But i assume potential delays here are more on the order of a few microseconds if some pending loads from the cache would get reordered for overall efficiency?

I'd be surprised if the delay were as much as 1 us.

The latency to return to userspace significantly dwarfs any observable
effects having missed the vblank count update by 1 instruction.

Regards,
Peter Hurley