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=-3.6 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 6DA23C388F9 for ; Wed, 11 Nov 2020 12:42:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 07BEB207BB for ; Wed, 11 Nov 2020 12:42:32 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="jsacUJVf" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726980AbgKKMlw (ORCPT ); Wed, 11 Nov 2020 07:41:52 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45866 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726955AbgKKMlt (ORCPT ); Wed, 11 Nov 2020 07:41:49 -0500 Received: from mail-lf1-x144.google.com (mail-lf1-x144.google.com [IPv6:2a00:1450:4864:20::144]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 73C25C0613D1; Wed, 11 Nov 2020 04:41:49 -0800 (PST) Received: by mail-lf1-x144.google.com with SMTP id w142so2935985lff.8; Wed, 11 Nov 2020 04:41:49 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=p09sB7YHU/bDDzgMxuLQXPTX1TBYFFjC8vLK3IStYYs=; b=jsacUJVfZifsMa0Od0ifyXIYeQBOyfqKY1dbaeYP1+U9OUQdn+hdRyObztAbzcnx/p uk++tbnY2njal5E0vTh/ByNS5edhg1cnmwO0W1QO7jZCHQkCwbvXo6TqElo7HqOw05Z9 L4KpZ+Yzd9ZHXSsbtNej6L1ebjaqigRDmPe+xkHWN+XfTsCn/g96sMCBN5hp4K7k9NKB ksueb4mmGgqR3zq8U9FLjOH2EjrKem904+I8ckgWVMeEyar2a5QwfowcoKqK6o3pNAEp dcM6cx7zf9DZ8j4RAC555BCLYLxOe6gOHlXRzWVT35VWg8r9A4LNpmSp6uwKtUtDNFfx VkzQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=p09sB7YHU/bDDzgMxuLQXPTX1TBYFFjC8vLK3IStYYs=; b=BfvRNJU2/2Ysu3vfZ6bpYe+Yqz6Tdt+MmdJXV0MtDOstKRDZa5AbcqcCVI+eIhG6UQ 8ofewDISqqYS98AT+Z14nQZaWHmN72E+hoIeRVr2Ym480VsnUx6zoW0CTIVmmAKttT5z VdMCQUPNktYFfMjYN+vvHfSgokOMQUGb2oTY+JZHDIBnzkdvznWCKDDPaLgPS9es43pv crGNomEeriSnFe/SKc1VzDD+kOCsupy6oCzuZX7T1oKIjIIlIOk94Ar0Kz0SEeFrBkMg LcCqW+Luf3TlqQSyNsudCl9mgjPGJThvX89HAc2s4CdEbG+NhGLXFsvoq0DyF2h3OZxx bENA== X-Gm-Message-State: AOAM530IOoVSuM51G4IfcMj79mK614GuQ1bfU81w7KcXoG+7AzRcwvQp fO8UHkjbbTVvL9ReHRt7W0PDcvFQBqNReSF/xveyFdsekpB6qA== X-Google-Smtp-Source: ABdhPJyeoMTvxm99hJFLil3+3qpGIB8eKHBJ0gbr+qzyieVuMOkPPf61uLVEpXxwtJRNLz5hZvU35qVMLpDGRDBw/xo= X-Received: by 2002:a19:2292:: with SMTP id i140mr607457lfi.77.1605098507820; Wed, 11 Nov 2020 04:41:47 -0800 (PST) MIME-Version: 1.0 References: <20201022122421.133976-1-gnurou@gmail.com> <695e6163-7bdc-d120-cd02-0cff6efb53ef@xs4all.nl> <92db8b0e-c348-70ef-a607-eb5c42f86fac@xs4all.nl> In-Reply-To: From: Alexandre Courbot Date: Wed, 11 Nov 2020 21:41:35 +0900 Message-ID: Subject: Re: [PATCH] media: v4l2-mem2mem: always call poll_wait() on queues To: Hans Verkuil Cc: Mauro Carvalho Chehab , linux-media , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-media@vger.kernel.org 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.