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Wed, 28 Jul 2021 17:21:00 +0000 Subject: Re: [RFC 0/4] dma-fence: Deadline awareness To: Rob Clark , Pekka Paalanen Cc: Rob Clark , Matthew Brost , Jack Zhang , =?UTF-8?Q?Christian_K=c3=b6nig?= , =?UTF-8?Q?Michel_D=c3=a4nzer?= , open list , dri-devel , "moderated list:DMA BUFFER SHARING FRAMEWORK" , Luben Tuikov , Roy Sun , Gustavo Padovan , Alex Deucher , Tian Tao , Lee Jones , "open list:DMA BUFFER SHARING FRAMEWORK" References: <20210726233854.2453899-1-robdclark@gmail.com> <28ca4167-4a65-0ccc-36be-5fb017f6f49d@daenzer.net> <99984703-c3ca-6aae-5888-5997d7046112@daenzer.net> <04d44873-d8e6-6ae7-f0f9-17bcb484d697@amd.com> <9d5f4415-d470-3bc1-7d52-61ba739706ae@daenzer.net> <20210728165700.38c39cf8@eldfell> From: =?UTF-8?Q?Christian_K=c3=b6nig?= Message-ID: Date: Wed, 28 Jul 2021 19:20:53 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-ClientProxiedBy: FR3P281CA0052.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:4a::23) To MN2PR12MB3775.namprd12.prod.outlook.com (2603:10b6:208:159::19) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [IPv6:2a02:908:1252:fb60:f1e3:17b4:fe37:19a9] (2a02:908:1252:fb60:f1e3:17b4:fe37:19a9) by FR3P281CA0052.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:4a::23) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4373.7 via Frontend Transport; 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Therefore, a Wayland compositor has to set a deadline for the next refresh cycle, and when the deadline passes, it has to select the best buffer available for the fullscreen surface. To make sure the flip will not miss the next refresh cycle, the compositor has to pick an idle buffer. If it picks a non-idle buffer, and the pending rendering does not finish in time for vertical blank, the flip will be delayed by at least one refresh cycle, which results in visible stuttering. >>>>>> >>>>>> (Until the deadline passes, the Wayland compositor can't even know if a previously fullscreen surface will still be fullscreen for the next refresh cycle) >>>>> Well then let's extend the KMS API instead of hacking together workarounds in userspace. >>>> That's indeed a possible solution for the fullscreen / direct scanout case. >>>> >>>> Not for the general compositing case though, since a compositor does not want to composite multiple output frames per display refresh cycle, so it has to make sure the one frame hits the target. >>> Yeah, that's true as well. >>> >>> At least as long as nobody invents a mechanism to do this decision on >>> the GPU instead. >> That would mean putting the whole window manager into the GPU. >> > Hmm, seems like we could come up with a way for a shader to figure out > if a fence has signaled or not on the GPU, and then either sample from > the current or previous window surface? Yeah, exactly that's my thinking as well. Christian. > > BR, > -R