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

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Message-ID: <20231027093238.2ff8172e@collabora.com>
Date:   Fri, 27 Oct 2023 09:32:38 +0200
From:   Boris Brezillon <boris.brezillon@...labora.com>
To:     Christian König <christian.koenig@....com>
Cc:     Danilo Krummrich <dakr@...hat.com>, airlied@...il.com,
        daniel@...ll.ch, matthew.brost@...el.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 Fri, 27 Oct 2023 09:22:12 +0200
Christian König <christian.koenig@....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);  
> 
> Why do we need an extra callback for this?
> 
> Just document that prepare_job() is allowed to reduce the number of 
> credits the job might need.

->prepare_job() is called only once if the returned fence is NULL, but
we need this credit-update to happen every time a job is considered for
execution by the scheduler. If you're saying this control-flow should
be implemented with a dma_fence that's signaled when enough space is
available, I fear Danilo's work won't be that useful to the PowerVR
driver, unfortunately.