From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.4 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 24C9AC388F9 for ; Wed, 11 Nov 2020 12:48:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B3F1C20709 for ; Wed, 11 Nov 2020 12:48:00 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=xs4all.nl header.i=@xs4all.nl header.b="UcTDTQC9" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726591AbgKKMrx (ORCPT ); Wed, 11 Nov 2020 07:47:53 -0500 Received: from lb3-smtp-cloud7.xs4all.net ([194.109.24.31]:45183 "EHLO lb3-smtp-cloud7.xs4all.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726207AbgKKMrx (ORCPT ); Wed, 11 Nov 2020 07:47:53 -0500 Received: from cust-b5b5937f ([IPv6:fc0c:c16d:66b8:757f:c639:739b:9d66:799d]) by smtp-cloud7.xs4all.net with ESMTPA id cpWykn8MgRiwVcpX6kaLlF; Wed, 11 Nov 2020 13:47:47 +0100 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=xs4all.nl; s=s1; t=1605098870; bh=764s9qizFJslTNse1o+RIcZfci3G2jnzqqGavb8U8fU=; h=Subject:To:From:Message-ID:Date:MIME-Version:Content-Type:From: Subject; b=UcTDTQC9KinU1GIue9igoUmlL7rTofJMNgbu9pP0jUvpUhloyNCp9bQ48kCstGiDr 1T5YvA8tx5l5BwZMZBddmqfXvMUHNDCJ0ca6y/mqjnlC2jcfhxUL277+sTNUGjI96q /MCyvlX4CSKBkrvCTjfLqwtOV/lb+5sCniFbKPFDoqV3j8jA1M6Oeiq8j9MLsptQ9L wyBTALqyRV66Z9QsYKEqlgGhkMkC2/NpWKE/yJ2lOlbjhk7baxJP1DR+GN4hyGD7vn qgP0G7xBDM5eblas7l6Z5JNWpaePF7GCDHgrP+cUQklr1LmXNrMLk9KgdffN1YSD+3 SfVQ1bMMqpc3A== Subject: Re: [PATCH] media: v4l2-mem2mem: always call poll_wait() on queues To: Alexandre Courbot Cc: Mauro Carvalho Chehab , linux-media , Linux Kernel Mailing List References: <20201022122421.133976-1-gnurou@gmail.com> <695e6163-7bdc-d120-cd02-0cff6efb53ef@xs4all.nl> <92db8b0e-c348-70ef-a607-eb5c42f86fac@xs4all.nl> From: Hans Verkuil Message-ID: <817895e0-6207-9fbb-5581-0bf784c63915@xs4all.nl> Date: Wed, 11 Nov 2020 13:47:20 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-CMAE-Envelope: MS4xfHnp4hHBd9MTp8LNzGjgTCMUOOl35R6VtC4U+s6qbwRKa7BfYvBFoum4DXwDQNSP25wQxA6lbzd/JXJT9uWMKi7TK1RGTPBCST2Eq8Q9mJBOrYhhEprT aoiKij7QKULw6Cy08moFtWPuKYpb8RAeOn/LHAIIlSc4vhzsfQLAeiBISqU4gvaqOiTmhv8CUKaM7FKQ6ij+Jlq7IOmwQdgpXa6BA6b4j/JvvVJPZ/BiRTXA 50Fw7LVgMlgW1bnej0LP5xVKVlIflIPgs1KFfbnplEbpC/4AipQaIiMqjd05F2oHZNyw96a+sY0UiNPeQfCZ8A== Precedence: bulk List-ID: X-Mailing-List: linux-media@vger.kernel.org On 11/11/2020 13:41, Alexandre Courbot wrote: > On Thu, Nov 5, 2020 at 11:05 PM Alexandre Courbot wrote: >> >> On Thu, Nov 5, 2020 at 10:12 PM Hans Verkuil wrote: >>> >>> On 05/11/2020 13:52, Alexandre Courbot wrote: >>>> On Thu, Nov 5, 2020 at 9:36 PM Hans Verkuil wrote: >>>>> >>>>> On 05/11/2020 13:21, Alexandre Courbot wrote: >>>>>> On Tue, Nov 3, 2020 at 6:48 PM Hans Verkuil wrote: >>>>>>> >>>>>>> On 03/11/2020 09:51, Alexandre Courbot wrote: >>>>>>>> Hi Hans, >>>>>>>> >>>>>>>> On Sat, Oct 31, 2020 at 12:09 AM Hans Verkuil wrote: >>>>>>>>> >>>>>>>>> On 22/10/2020 14:24, Alexandre Courbot wrote: >>>>>>>>>> do_poll()/do_select() seem to set the _qproc member of poll_table to >>>>>>>>>> NULL the first time they are called on a given table, making subsequent >>>>>>>>>> calls of poll_wait() on that table no-ops. This is a problem for mem2mem >>>>>>>>>> which calls poll_wait() on the V4L2 queues' waitqueues only when a >>>>>>>>>> queue-related event is requested, which may not necessarily be the case >>>>>>>>>> during the first poll. >>>>>>>>>> >>>>>>>>>> For instance, a stateful decoder is typically only interested in >>>>>>>>>> EPOLLPRI events when it starts, and will switch to listening to both >>>>>>>>>> EPOLLPRI and EPOLLIN after receiving the initial resolution change event >>>>>>>>>> and configuring the CAPTURE queue. However by the time that switch >>>>>>>>>> happens and v4l2_m2m_poll_for_data() is called for the first time, >>>>>>>>>> poll_wait() has become a no-op and the V4L2 queues waitqueues thus >>>>>>>>>> cannot be registered. >>>>>>>>>> >>>>>>>>>> Fix this by moving the registration to v4l2_m2m_poll() and do it whether >>>>>>>>>> or not one of the queue-related events are requested. >>>>>>>>> >>>>>>>>> This looks good, but would it be possible to add a test for this to >>>>>>>>> v4l2-compliance? (Look for POLL_MODE_EPOLL in v4l2-test-buffers.cpp) >>>>>>>>> >>>>>>>>> If I understand this right, calling EPOLL_CTL_ADD for EPOLLPRI, then >>>>>>>>> calling EPOLL_CTL_ADD for EPOLLIN/OUT would trigger this? Or does there >>>>>>>>> have to be an epoll_wait call in between? >>>>>>>> >>>>>>>> Even without an epoll_wait() in between the behavior is visible. >>>>>>>> v4l2_m2m_poll() will be called once during the initial EPOLL_CTL_ADD >>>>>>>> and this will trigger the bug. >>>>>>>> >>>>>>>>> Another reason for adding this test is that I wonder if regular capture >>>>>>>>> or output V4L2 devices don't have the same issue. >>>>>>>>> >>>>>>>>> It's a very subtle bug and so adding a test for this to v4l2-compliance >>>>>>>>> would be very useful. >>>>>>>> >>>>>>>> I fully agree, this is very counter-intuitive since what basically >>>>>>>> happens is that the kernel's poll_wait() function becomes a no-op >>>>>>>> after the poll() hook of a driver is called for the first time. There >>>>>>>> is no way one can expect this behavior just from browsing the code so >>>>>>>> this is likely to affect other drivers. >>>>>>>> >>>>>>>> As for the test itself, we can easily reproduce the conditions for >>>>>>>> failure in v4l2-test-buffers.cpp's captureBufs() function, but doing >>>>>>>> so will make the streaming tests fail without being specific about the >>>>>>>> cause. Or maybe we should add another pollmode to specifically test >>>>>>>> epoll in this setup? Can I get your thoughts? >>>>>>> >>>>>>> No, just keep it as part of the poll test. Just add comments at the place >>>>>>> where it fails describing this error. >>>>>>> >>>>>>> After all, it *is* a poll() bug, so it is only fair that it is tested as >>>>>>> part of the epoll test. >>>>>>> >>>>>>> Can you call EPOLL_CTL_ADD with ev.events set to 0? And then call it again >>>>>>> with the actual value that you need? If that triggers this issue as well, >>>>>>> then that is a nice test (but perhaps EPOLL_CTL_ADD won't call poll() if >>>>>>> ev.events is 0, but perhaps EPOLLERR would work instead of 0). >>>>>> >>>>>> Yup, actually the following is enough to make v4l2-compliance -s fail >>>>>> with vicodec: >>>>> >>>>> Does it also fail with vivid? I am curious to know whether this issue is >>>>> m2m specific or a more general problem. >>>> >>>> It does fail actually! And that made me notice that vb2_poll() uses >>>> the same pattern as v4l2_m2m_poll() (probably because the latter is >>>> inspired by the former?) and needs to be fixed similarly. I will send >>>> another patch to fix vb2_poll() as well, thanks for pointing it out! >>> >>> I was afraid of that. >>> >>> Testing epoll for control events would be interesting as well. The >>> vivid radio device is an example of a device that has controls, but >>> does not do streaming (so is not using vb2). >>> >>> But from what I can see v4l2_ctrl_poll() does the right thing, so this >>> should be fine. >> >> Indeed, it unconditionally calls poll_wait() with all the wait queues >> that may wake us up (that is, only one), so there is no problem there. > > Sorry, I noticed that this patch was marked with "Changes Requested" > in patchwork, but isn't it valid as-is? We need a similar change to > VB2, but that should go as a separate patch IMHO. I'm fine with doing > both in one go if you prefer that though. > In at least one reply you mentioned that you wanted to add a comment (reply from 23 Oct). That's why I changed it to 'Changes Requested'. Also, I prefer to fix both m2m and vb2 at the same time (separate patches, but part of the same patch series). And together with a separate patch improving v4l2-compliance. Regards, Hans