linux-kernel - Re: [PATCH] drm/radeon: Remove deprecated create_singlethread

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Message-ID: <7f4299b1-a118-50ce-1f40-fbcfdfb19942@daenzer.net>
Date:	Fri, 8 Jul 2016 14:52:30 +0900
From:	Michel Dänzer <michel@...nzer.net>
To:	Christian König <christian.koenig@....com>,
	Tejun Heo <tj@...nel.org>
Cc:	Bhaktipriya Shridhar <bhaktipriya96@...il.com>,
	Alex Deucher <alexander.deucher@....com>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/radeon: Remove deprecated
 create_singlethread_workqueue

On 07.07.2016 16:43, Christian König wrote:
> Am 07.07.2016 um 05:32 schrieb Michel Dänzer:
>> On 06.07.2016 22:45, Tejun Heo wrote:
>>> On Wed, Jul 06, 2016 at 12:12:52PM +0900, Michel Dänzer wrote:
>>>
>>>> Not being very familiar with the workqueue APIs, I'll describe how it's
>>>> supposed to work from a driver POV, which will hopefully help you guys
>>>> decide on the most appropriate alloc_workqueue parameters.
>>>>
>>>> There is one flip work queue for each hardware CRTC. At most one
>>>> radeon_flip_work_func item can be queued for any of them at any time.
>>>> When a radeon_flip_work_func item is queued, it should be executed ASAP
>>>> (so WQ_HIGHPRI might be appropriate?).
>>> Hmmm... the only time WQ_HIGHPRI should be used is when it'd otherwise
>>> require a kthread w/ nice value at -20.  Would that be the case here?
>>> What are the consequences of the work item getting delayed?
>> A page flip may be delayed to a later display refresh cycle.
>>
>>
>>> Also, what kind of delays matter here?  Is it millisec range or micro?
>> It can be the latter in theory, but normally rather the former.
> 
> Well to be precise with a typical 1920x1080@60 resolution you have about
> 2.16ms time under ideal conditions for the flip.
> 
> So using the high priority queue still sounds like a good idea to me.

How did you arrive at 2.16ms?

Userspace can call the ioctl up to one full refresh cycle ahead of time,
which is ~16ms at 60 Hz. On the other hand userspace can also call the
ioctl arbitrarily close to the vertical blank period, in which case even
a delay of just 1ms (or even significantly less) may cause the flip to
be delayed by one refresh cycle.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer



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