From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from perceval.ideasonboard.com ([95.142.166.194]:45556 "EHLO perceval.ideasonboard.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751195Ab3IIKYJ (ORCPT ); Mon, 9 Sep 2013 06:24:09 -0400 From: Laurent Pinchart To: Hans Verkuil Cc: Sylwester Nawrocki , Sylwester Nawrocki , linux-media@vger.kernel.org, Kyungmin Park Subject: Re: [PATCH] V4L: Drop meaningless video_is_registered() call in v4l2_open() Date: Mon, 09 Sep 2013 12:24:13 +0200 Message-ID: <7183549.W0I9Cqdz4K@avalon> In-Reply-To: <522DA03E.8010808@xs4all.nl> References: <1375446449-27066-1-git-send-email-s.nawrocki@samsung.com> <26516577.dQgL4XrfDY@avalon> <522DA03E.8010808@xs4all.nl> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-media-owner@vger.kernel.org List-ID: Hi Hans, On Monday 09 September 2013 12:17:34 Hans Verkuil wrote: > On 09/09/2013 12:10 PM, Laurent Pinchart wrote: > > On Monday 09 September 2013 12:07:18 Hans Verkuil wrote: > >> On 09/09/2013 12:00 PM, Laurent Pinchart wrote: > >>> On Monday 09 September 2013 11:07:43 Hans Verkuil wrote: > >>>> On 09/06/2013 12:33 AM, Sylwester Nawrocki wrote: > >>> [snip] > >>> > >>>>> The main issue as I see it is that we need to track both driver > >>>>> remove() and struct device .release() calls and free resources only > >>>>> when last of them executes. Data structures which are referenced in > >>>>> fops must not be freed in remove() and we cannot use dev_get_drvdata() > >>>>> in fops, e.g. not protected with device_lock(). > >>>> > >>>> You can do all that by returning 0 if probe() was partially successful > >>>> (i.e. one or more, but not all, nodes were created successfully) by > >>>> doing what I described above. I don't see another way that doesn't > >>>> introduce a race condition. > >>> > >>> But isn't this just plain wrong ? If probing fails, I don't see how > >>> returning success could be a good idea. > >> > >> Well, the nodes that are created are working fine. So it's partially OK > >> :-) > >> > >> That said, yes it would be better if it could safely clean up and return > >> an error. But it is better than returning an error and introducing a race > >> condition. > >> > >>>> That doesn't mean that there isn't one, it's just that I don't know of > >>>> a better way of doing this. > >>> > >>> We might need support from the device core. > >> > >> I do come back to my main question: has anyone actually experienced this > >> error in a realistic scenario? Other than in very low-memory situations I > >> cannot imagine this happening. > > > > What about running out of minors, which could very well happen with subdev > > nodes in complex SoCs ? > > Is that really realistic? What's the worst-case SoC we have in terms of > device nodes? Frankly, if this might happen then we should allow for more > minors or make the minor allocation completely dynamic. For the 4 VSP1 instances on the R-Car H2, I need 33 video nodes and 65 (if I'm not mistaken) subdev nodes. That doesn't include the camera interface. On a side note, this seems to indicate that the subdev API should probably move to the /dev/media device node. That's something else to discuss. > If you run into this situation then you have bigger problems than a > potential race condition. -- Regards, Laurent Pinchart