From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from mail-qt1-f195.google.com ([209.85.160.195]:41594 "EHLO mail-qt1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2407885AbeKWKfb (ORCPT ); Fri, 23 Nov 2018 05:35:31 -0500 Received: by mail-qt1-f195.google.com with SMTP id d18so8940357qto.8 for ; Thu, 22 Nov 2018 15:53:41 -0800 (PST) Message-ID: Subject: Re: [PATCH] media: venus: fix reported size of 0-length buffers From: Nicolas Dufresne To: Alexandre Courbot Cc: Stanimir Varbanov , Mauro Carvalho Chehab , Linux Media Mailing List , linux-arm-msm@vger.kernel.org, LKML Date: Thu, 22 Nov 2018 18:53:39 -0500 In-Reply-To: References: <20181113093048.236201-1-acourbot@chromium.org> <463ac42b795933a54daa8d2bbba3ff1ac2b733db.camel@ndufresne.ca> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-media-owner@vger.kernel.org List-ID: Le jeudi 22 novembre 2018 à 17:31 +0900, Alexandre Courbot a écrit : > On Fri, Nov 16, 2018 at 1:49 AM Nicolas Dufresne wrote: > > Le mercredi 14 novembre 2018 à 13:12 +0900, Alexandre Courbot a écrit : > > > On Wed, Nov 14, 2018 at 3:54 AM Nicolas Dufresne wrote: > > > > > > > > Le mar. 13 nov. 2018 04 h 30, Alexandre Courbot a écrit : > > > > > The last buffer is often signaled by an empty buffer with the > > > > > V4L2_BUF_FLAG_LAST flag set. Such buffers were returned with the > > > > > bytesused field set to the full size of the OPB, which leads > > > > > user-space to believe that the buffer actually contains useful data. Fix > > > > > this by passing the number of bytes reported used by the firmware. > > > > > > > > That means the driver does not know on time which one is last. Why not just returned EPIPE to userspace on DQBUF and ovoid this useless roundtrip ? > > > > > > Sorry, I don't understand what you mean. EPIPE is supposed to be > > > returned after a buffer with V4L2_BUF_FLAG_LAST is made available for > > > dequeue. This patch amends the code that prepares this LAST-flagged > > > buffer. How could we avoid a roundtrip in this case? > > > > Maybe it has changed, but when this was introduced, we found that some > > firmware (Exynos MFC) could not know which one is last. Instead, it > > gets an event saying there will be no more buffers. > > > > Sending buffers with payload size to 0 just for the sake of setting the > > V4L2_BUF_FLAG_LAST was considered a waste. Specially that after that, > > every polls should return EPIPE. So in the end, we decided the it > > should just unblock the userspace and return EPIPE. > > > > If you look at the related GStreamer code, it completely ignores the > > LAST flag. With fake buffer of size 0, userspace will endup dequeuing > > and throwing away. This is not useful to the process of terminating the > > decoding. To me, this LAST flag is not useful in this context. > > Note that this patch does not interfere with DQBUF returning -EPIPE > after the last buffer has been dequeued. It just fixes an invalid size > that was returned for the last buffer. > > Note also that if I understand the doc properly, the kernel driver > *must* set the V4L2_BUF_FLAG_LAST on the last buffer. With Venus the > last buffer is signaled by the firmware with an empty buffer. That's > not something we can change or predict earlier, so in order to respect > the specification we need to return that empty buffer. After that > DQBUF will behave as expected (returning -EPIPE), so GStreamer should > be happy as well. > > Without the proposed fix however, GStreamer would receive the last > buffer with an incorrect size, and thus interpret random data as a > frame. > > So to me this fix seems to be both correct, and needed. Isn't it? Totally, thanks for the extra clarification. > > Cheers, > Alex.