linux-kernel - Re: [PATCH drm-misc-next v3] drm/sched: implement dynamic job-flow control

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Date:   Fri, 27 Oct 2023 16:34:26 +0200
From:   Danilo Krummrich <dakr@...hat.com>
To:     Boris Brezillon <boris.brezillon@...labora.com>
Cc:     airlied@...il.com, daniel@...ll.ch, matthew.brost@...el.com,
        christian.koenig@....com, faith@...strand.net,
        luben.tuikov@....com, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH drm-misc-next v3] drm/sched: implement dynamic job-flow
 control

On 10/27/23 09:17, Boris Brezillon wrote:
> Hi Danilo,
> 
> On Thu, 26 Oct 2023 18:13:00 +0200
> Danilo Krummrich <dakr@...hat.com> wrote:
> 
>> +
>> +	/**
>> +	 * @update_job_credits: Called once the scheduler is considering this
>> +	 * job for execution.
>> +	 *
>> +	 * Drivers may use this to update the job's submission credits, which is
>> +	 * useful to e.g. deduct the number of native fences which have been
>> +	 * signaled meanwhile.
>> +	 *
>> +	 * The callback must either return the new number of submission credits
>> +	 * for the given job, or zero if no update is required.
>> +	 *
>> +	 * This callback is optional.
>> +	 */
>> +	u32 (*update_job_credits)(struct drm_sched_job *sched_job);
> 
> I'm copying my late reply to v2 here so it doesn't get lost:
> 
> I keep thinking it'd be simpler to make this a void function that
> updates s_job->submission_credits directly. I also don't see the
> problem with doing a sanity check on job->submission_credits. I mean,
> if the driver is doing something silly, you can't do much to prevent it
> anyway, except warn the user that something wrong has happened. If you
> want to
> 
> 	WARN_ON(job->submission_credits == 0 ||
> 		job->submission_credits > job_old_submission_credits);
> 
> that's fine. But none of this sanity checking has to do with the
> function prototype/semantics, and I'm still not comfortable with this 0
> => no-change. If there's no change, we should just leave
> job->submission_credits unchanged (or return job->submission_credits)
> instead of inventing a new special case.

If we can avoid letting drivers change fields of generic structures directly
without any drawbacks I think we should avoid it. Currently, drivers shouldn't
have the need to mess with job->credits directly. The initial value is set
through drm_sched_job_init() and is updated through the return value of
update_job_credits().

I'm fine getting rid of the 0 => no-change semantics though. Instead we can just
WARN() on 0. However, if we do that I'd also want to change it for
drm_sched_job_init() (where 0 currently defaults to 1) such that we accept 0, but
WARN() accordingly.

I think it's consequent to either consistently give 0 a different meaning or just
accept it but WARN() on it.

> 
> Regards,
> 
> Boris
>