Re: [PATCH] drm/radeon: Remove deprecated create_singlethread_workqueue

From: "Michel Dänzer" <michel@daenzer.net>
To: Tejun Heo <tj@kernel.org>
Cc: "Alex Deucher" <alexander.deucher@amd.com>,
	"Bhaktipriya Shridhar" <bhaktipriya96@gmail.com>,
	"Christian König" <christian.koenig@amd.com>,
	dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] drm/radeon: Remove deprecated create_singlethread_workqueue
Date: Wed, 6 Jul 2016 12:12:52 +0900	[thread overview]
Message-ID: <936c5bae-b8e6-bf64-8be2-d27608814fac@daenzer.net> (raw)
In-Reply-To: <20160705210644.GB25394@htj.duckdns.org>

On 06.07.2016 06:06, Tejun Heo wrote:
> On Mon, Jul 04, 2016 at 12:58:32PM +0900, Michel Dänzer wrote:
>> On 02.07.2016 22:46, Tejun Heo wrote:
>>> On Sat, Jul 02, 2016 at 04:33:50PM +0530, Bhaktipriya Shridhar wrote:
>>>> alloc_workqueue replaces deprecated create_singlethread_workqueue().
>>>>
>>>> A dedicated workqueue has been used since work items need to be flushed
>>>> as a group rather than individually.
> 
> There seem to be two work items involved, the flip one and unpin one.
> Provided that there's no ordering requirement between the two, can't
> we just flush them individually?

There is an ordering requirement between the two queues, but it's
enforced by the driver (by only queuing the unpin work once a flip has
completed, which only happens after the corresponding flip work has run).

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?).

In contrast, the radeon_unpin_work_func items aren't particularly
urgent, and it's okay for several of them to be queued up. So I guess it
would actually make sense to use a different workqueue for them, maybe
even the default one?

>>>> Since there are only a fixed number of work items, explicit concurrency
>>>> limit is unnecessary here.
>>>
>>> What are the involved work items?
>>
>> drivers/gpu/drm/radeon/radeon_display.c:radeon_flip_work_func()
>>
>>> Is it safe to run them concurrently?
>>
>> Concurrently with what exactly?
> 
> The flip and unpin work items.

Yes, it's safe to run those concurrently.

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