[Intel-gfx] [CI] drm/i915: Skip waking the device to service pwrite

[Tvrtko Ursulin]

On 19/10/2017 11:19, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2017-10-19 11:02:05)
>>
>> On 19/10/2017 07:37, Chris Wilson wrote:
>>> If the device is in runtime suspend, resuming takes time and reduces our
>>> powersaving. If this was for a small write into an object, that resume
>>> will take longer than any savings in using the indirect GGTT access to
>>> avoid the cpu cache.
>>
>> Commit talks about small writes but the patch takes no notice to size of
>> requested writes. Is that intended?
> 
> We are talking gigabytes before the difference in paths outweigh the
> worstcase costs, even before we start measuring power. pwrite is a tricky

No real complaints, just read the commit and expected to see something 
about the size in the patch itself.

> one, for anything big and tiled or for anything where you may repeat the
> read/write/mmap, you don't want to use pwrite but the direct mmap. pwrite
> should only wins for small one-off uses, so that's what I have in mind
> as the typical user -- and I encourage them to re-evaluate their use.
> 
> So why bother? Because it made a big difference to igt pwrite runtime
> when we didn't have a display connector. Is the addition in complexity
> worth it for the few real users? Should we be more aware of the wider
> implications (both power and performance) of certain operations within
> the driver? Do we need more mostly idle and/or headless testing?

Regards,

Tvrtko