linux-kernel - Re: [PATCH RFC 2/4] media: v4l2-mem2mem: Add an optional job

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Date:   Tue, 15 Jan 2019 06:42:37 -0300
From:   Ezequiel Garcia <ezequiel@...labora.com>
To:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        devel@...verdev.osuosl.org, linux-arm-kernel@...ts.infradead.org,
        linux-sunxi@...glegroups.com
Cc:     Pawel Osciak <pawel@...iak.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Randy Li <ayaka@...lik.info>,
        Hans Verkuil <hverkuil@...all.nl>,
        Tomasz Figa <tfiga@...omium.org>,
        Alexandre Courbot <acourbot@...omium.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH RFC 2/4] media: v4l2-mem2mem: Add an optional job_done
 operation

On Mon, 2019-01-14 at 14:38 +0100, Paul Kocialkowski wrote:
> Introduce a new optional job_done operation, which allows calling back
> to the driver when a job is done. Since the job might be completed
> from interrupt context where some operations are not available, having
> a callback from non-atomic context allows performing these operations
> upon completion of a job. This is particularly useful for releasing
> access to a reference buffer, which cannot be done in atomic context.
> 

I'm not exactly sure it makes a lot of sense to review this patch,
since the approach could change.

However, let me point out a few fundamental issues here.
 
> Use the already existing v4l2_m2m_device_run_work work queue for that
> and clear the M2M device current context after calling job_done in the
> worker thread, so that the private data can be passed to the operation.
> 
> Delaying the current context clearing should not be a problem since the
> next call to v4l2_m2m_try_run happens right after that.
> 

Careful here. It's misleading to think an event will happen
"right after". I'd say it's either synchronously, or asynchronously.

It's quite the opposite I'd say, the clearing will happen
"who-knows-when the scheduler picks the thread to run" :-)

Before this patch the curr_ctx was cleared in v4l2_m2m_job_finish,
atomically with the ctx job flags clearing and before waking up
threads waiting in v4l2_m2m_cancel_job.

You are now changing this, by clearing curr_ctx in a worker.
It's perfectly possible that v4l2_m2m_try_schedule will run
before the worker, trying to run with the old context, which
apparently would be safely refused.

> Signed-off-by: Paul Kocialkowski <paul.kocialkowski@...tlin.com>
> ---
>  drivers/media/v4l2-core/v4l2-mem2mem.c | 8 ++++++--
>  include/media/v4l2-mem2mem.h           | 4 ++++
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-mem2mem.c b/drivers/media/v4l2-core/v4l2-mem2mem.c
> index 631f4e2aa942..d5bccb0192f9 100644
> --- a/drivers/media/v4l2-core/v4l2-mem2mem.c
> +++ b/drivers/media/v4l2-core/v4l2-mem2mem.c
> @@ -376,6 +376,11 @@ static void v4l2_m2m_device_run_work(struct work_struct *work)
>  	struct v4l2_m2m_dev *m2m_dev =
>  		container_of(work, struct v4l2_m2m_dev, job_work);
>  
> +	if (m2m_dev->m2m_ops->job_done && m2m_dev->curr_ctx)
> +		m2m_dev->m2m_ops->job_done(m2m_dev->curr_ctx->priv);
> +
> +	m2m_dev->curr_ctx = NULL;
> +

I don't think you can access this without taking the job spinlock.

>  	v4l2_m2m_try_run(m2m_dev);
>  }
> 

Aside from this, it seems we might need this hook sooner or later.

Thanks,
Eze