linux-kernel - Re: [PATCH v2 02/10] media-request: Add a request complete operation to allow m2m scheduling

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Date:   Tue, 24 Apr 2018 11:28:32 +0300
From:   Sakari Ailus <sakari.ailus@...ux.intel.com>
To:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>
Cc:     linux-media@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-sunxi@...glegroups.com,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Chen-Yu Tsai <wens@...e.org>, Pawel Osciak <pawel@...iak.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Arnd Bergmann <arnd@...db.de>,
        Alexandre Courbot <acourbot@...omium.org>,
        Tomasz Figa <tfiga@...omium.org>
Subject: Re: [PATCH v2 02/10] media-request: Add a request complete operation
 to allow m2m scheduling

Hi Paul,

On Thu, Apr 19, 2018 at 05:41:16PM +0200, Paul Kocialkowski wrote:
> When using the request API in the context of a m2m driver, the
> operations that come with a m2m run scheduling call in their
> (m2m-specific) ioctl handler are delayed until the request is queued
> (for instance, this includes queuing buffers and streamon).
> 
> Thus, the m2m run scheduling calls are not called in due time since the
> request AP's internal plumbing will (rightfully) use the relevant core
> functions directly instead of the ioctl handler.
> 
> This ends up in a situation where nothing happens if there is no
> run-scheduling ioctl called after queuing the request.
> 
> In order to circumvent the issue, a new media operation is introduced,
> called at the time of handling the media request queue ioctl. It gives
> m2m drivers a chance to schedule a m2m device run at that time.
> 
> The existing req_queue operation cannot be used for this purpose, since
> it is called with the request queue mutex held, that is eventually needed
> in the device_run call to apply relevant controls.

Based on the description I'm not entirely sure what's the intent here.

What would be the purpose for acquiring the request queue mutex during
the execution of the request? It is only intended for serialising access to
the request objects while the request is being validated and queued.

If a driver does a significant amount of processing that is not related to
managing the request as such, then one option could be to add a work queue
for it.

> 
> Signed-off-by: Paul Kocialkowski <paul.kocialkowski@...tlin.com>
> ---
>  drivers/media/media-request.c | 3 +++
>  include/media/media-device.h  | 2 ++
>  2 files changed, 5 insertions(+)
> 
> diff --git a/drivers/media/media-request.c b/drivers/media/media-request.c
> index 415f7e31019d..28ac5ccfe6a2 100644
> --- a/drivers/media/media-request.c
> +++ b/drivers/media/media-request.c
> @@ -157,6 +157,9 @@ static long media_request_ioctl_queue(struct media_request *req)
>  		media_request_get(req);
>  	}
>  
> +	if (mdev->ops->req_complete)
> +		mdev->ops->req_complete(req);
> +
>  	return ret;
>  }
>  
> diff --git a/include/media/media-device.h b/include/media/media-device.h
> index 07e323c57202..c7dcf2079cc9 100644
> --- a/include/media/media-device.h
> +++ b/include/media/media-device.h
> @@ -55,6 +55,7 @@ struct media_entity_notify {
>   * @req_alloc: Allocate a request
>   * @req_free: Free a request
>   * @req_queue: Queue a request
> + * @req_complete: Complete a request
>   */
>  struct media_device_ops {
>  	int (*link_notify)(struct media_link *link, u32 flags,
> @@ -62,6 +63,7 @@ struct media_device_ops {
>  	struct media_request *(*req_alloc)(struct media_device *mdev);
>  	void (*req_free)(struct media_request *req);
>  	int (*req_queue)(struct media_request *req);
> +	void (*req_complete)(struct media_request *req);
>  };
>  
>  /**
> -- 
> 2.16.3
> 

-- 
Sakari Ailus
sakari.ailus@...ux.intel.com