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

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Date:	Tue, 12 Jul 2016 13:52:52 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Michel Dänzer <michel@...nzer.net>
Cc:	Christian König <christian.koenig@....com>,
	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

Hello,

On Fri, Jul 08, 2016 at 02:52:30PM +0900, Michel Dänzer wrote:
> On 07.07.2016 16:43, Christian König wrote:
> >>> 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.

If there's too long a delay, the outcome is missing the refresh cycle,
right?  Hmmm... yeah, WQ_HIGHPRI probably is the right answer here.
Bhaktipriya, can you please update the patch to use a dedicated
workqueue with WQ_HIGHPRI?

Thanks.

-- 
tejun