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=-5.3 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 9E5C6C388F9 for ; Thu, 5 Nov 2020 13:12:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3C2662087D for ; Thu, 5 Nov 2020 13:12:23 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=xs4all.nl header.i=@xs4all.nl header.b="Bh/oIf5e" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730511AbgKENMU (ORCPT ); Thu, 5 Nov 2020 08:12:20 -0500 Received: from lb3-smtp-cloud8.xs4all.net ([194.109.24.29]:46877 "EHLO lb3-smtp-cloud8.xs4all.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730461AbgKENMT (ORCPT ); Thu, 5 Nov 2020 08:12:19 -0500 Received: from cust-b5b5937f ([IPv6:fc0c:c16d:66b8:757f:c639:739b:9d66:799d]) by smtp-cloud8.xs4all.net with ESMTPA id af3lkD87yNanzaf3pkoRCh; Thu, 05 Nov 2020 14:12:17 +0100 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=xs4all.nl; s=s1; t=1604581937; bh=zWfaLPUPmGSKADLsEvQGgUKfHwanDJoHAaPVUnqE/zo=; h=Subject:To:From:Message-ID:Date:MIME-Version:Content-Type:From: Subject; b=Bh/oIf5e7DVflq9xLhsk2ORQK6hXohFGJjkwZT8VY3MH+ioh/fqngD96sIlfj668y pKb+zM7SxCQyCpQhyNyjkhGCB0sfclp9rN08AIbqVll+V1rhKPADFHhbJCdt6BpNmy FJiAgVdUwdLxfQEvo1pZjukYZ7POSNx1EAuZX+1MUwqHAq4/4XJAeUAS2mH9FdWmZX NlqoRNJiFqiuAfwaUpJNvYPBEzJIngwjG68GVHYBzkor/onNVLNnBqDSAnCLGcEfku k4f9GBg2YL4K1KofW8Y7gEKSCEa40wHdVpUzAwmNndbSBtIA0ZKPxFEdFcx0UaDSMf c6bPCpzUkYeNw== 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> From: Hans Verkuil Message-ID: <92db8b0e-c348-70ef-a607-eb5c42f86fac@xs4all.nl> Date: Thu, 5 Nov 2020 14:12:13 +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: MS4xfDCqL8w6yJAdjDRvyDcRN+qoABJ8s9oCYEwCLoqnisY4vxo34Ztk45vuSk0a7A1pjFIAOG58w8CP4c3XQid1+/IvixhWOaFDUsCDoKHbMHG2q5TbxmeQ 4J0AHywSt1KxjcLLbvxrm0V7hI5u+inqSMO4acEDg/G4jVau0spz7rDlzKvpFlKCvqn8meXjDbuuLsKPyW8F0w5+tcTisA3lHLXXiOWRuvLVCbXiBtq4k4Ub hsL6S362JxQm8WJlPcBS2wtlZfktqOBzalg+EF74roik/DHOAs228ifTNsiBOk7WSUrCG1aRoH6iklSK9jXAVg== Precedence: bulk List-ID: X-Mailing-List: linux-media@vger.kernel.org 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. Regards, Hans