Re: [PATCH v2 3/8] drm/panfrost: Hold runtime PM reference until jobs complete

From: Steven Price <steven.price@arm.com>
To: Rob Herring <robh@kernel.org>, dri-devel@lists.freedesktop.org
Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>,
	Maxime Ripard <maxime.ripard@bootlin.com>,
	Robin Murphy <robin.murphy@arm.com>,
	David Airlie <airlied@linux.ie>,
	Boris Brezillon <boris.brezillon@collabora.com>,
	Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>,
	Sean Paul <sean@poorly.run>
Subject: Re: [PATCH v2 3/8] drm/panfrost: Hold runtime PM reference until jobs complete
Date: Fri, 23 Aug 2019 15:50:55 +0100	[thread overview]
Message-ID: <2d4febdb-4db8-7e69-7798-9fea67c1cc8e@arm.com> (raw)
In-Reply-To: <20190823021216.5862-4-robh@kernel.org>

On 23/08/2019 03:12, Rob Herring wrote:
> Doing a pm_runtime_put as soon as a job is submitted is wrong as it should
> not happen until the job completes. It works because we are relying on the
> autosuspend timeout to keep the h/w enabled.
> 
> Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
> Cc: Steven Price <steven.price@arm.com>
> Cc: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Signed-off-by: Rob Herring <robh@kernel.org>

Nice find, but I'm a bit worried about one thing.

> ---
> v2: new patch
> 
>   drivers/gpu/drm/panfrost/panfrost_job.c | 16 +++++++++-------
>   1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_job.c b/drivers/gpu/drm/panfrost/panfrost_job.c
> index 05c85f45a0de..80c9cab9a01b 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_job.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_job.c
> @@ -151,7 +151,7 @@ static void panfrost_job_hw_submit(struct panfrost_job *job, int js)
>   		return;
>   
>   	if (WARN_ON(job_read(pfdev, JS_COMMAND_NEXT(js))))
> -		goto end;
> +		return;
>   
>   	cfg = panfrost_mmu_as_get(pfdev, &job->file_priv->mmu);
>   
> @@ -187,10 +187,6 @@ static void panfrost_job_hw_submit(struct panfrost_job *job, int js)
>   	job_write(pfdev, JS_COMMAND_NEXT(js), JS_COMMAND_START);
>   
>   	spin_unlock_irqrestore(&pfdev->hwaccess_lock, flags);
> -
> -end:
> -	pm_runtime_mark_last_busy(pfdev->dev);
> -	pm_runtime_put_autosuspend(pfdev->dev);
>   }
>   
>   static void panfrost_acquire_object_fences(struct drm_gem_object **bos,
> @@ -388,9 +384,13 @@ static void panfrost_job_timedout(struct drm_sched_job *sched_job)
>   
>   	mutex_lock(&pfdev->reset_lock);
>   
> -	for (i = 0; i < NUM_JOB_SLOTS; i++)
> +	for (i = 0; i < NUM_JOB_SLOTS; i++) {
>   		drm_sched_stop(&pfdev->js->queue[i].sched, sched_job);
> -
> +		if (pfdev->jobs[i]) {
> +			pm_runtime_put_noidle(pfdev->dev);
> +			pfdev->jobs[i] = NULL;

I can't see what prevents this racing with panfrost_job_irq_handler() - 
the job could be completing at the same time as we assign NULL. Then 
panfrost_job_irq_handler() will happily dereference the NULL pointer...

Admittedly this patch is an improvement over the situation before :)

Steve

> +		}
> +	}
>   	if (sched_job)
>   		drm_sched_increase_karma(sched_job);
>   
> @@ -455,7 +455,9 @@ static irqreturn_t panfrost_job_irq_handler(int irq, void *data)
>   			pfdev->jobs[j] = NULL;
>   			panfrost_mmu_as_put(pfdev, &job->file_priv->mmu);
>   			panfrost_devfreq_record_transition(pfdev, j);
> +
>   			dma_fence_signal(job->done_fence);
> +			pm_runtime_put_autosuspend(pfdev->dev);
>   		}
>   
>   		status &= ~mask;
> 

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