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([240b:10:c9a0:ca00:1102:c007:eee:4478]) by smtp.gmail.com with ESMTPSA id d13sm1552726pfn.136.2021.06.23.01.21.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 23 Jun 2021 01:21:26 -0700 (PDT) Subject: Re: [PATH 0/4] [RFC] Support virtual DRM To: =?UTF-8?Q?Michel_D=c3=a4nzer?= , Pekka Paalanen Cc: devicetree@vger.kernel.org, Takanari Hayama , linux-doc@vger.kernel.org, David Airlie , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-renesas-soc@vger.kernel.org, Kieran Bingham , Laurent Pinchart , Thomas Zimmermann , Damian Hobson-Garcia References: <20210621062742.26073-1-etom@igel.co.jp> <9853d0a9-6053-db64-9c79-40b7e0689eec@suse.de> <85593f2f-5aa9-6023-ecba-c5275a468b71@igel.co.jp> <20210622105757.2b9dec32@eldfell> <01a20860-8fe2-2762-5678-d9a75f70e20a@daenzer.net> From: Esaki Tomohito Message-ID: <0fd18324-f396-ba76-1c8b-d048e5b72dca@igel.co.jp> Date: Wed, 23 Jun 2021 17:21:24 +0900 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <01a20860-8fe2-2762-5678-d9a75f70e20a@daenzer.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/06/23 17:04, Michel Dänzer wrote: > On 2021-06-22 9:57 a.m., Pekka Paalanen wrote: >> On Tue, 22 Jun 2021 13:02:59 +0900 >> Esaki Tomohito wrote: >> >>> Hi, Thomas >>> Thank you for reply. >>> >>> On 2021/06/21 16:10, Thomas Zimmermann wrote: >>>> Hi >>>> >>>> Am 21.06.21 um 08:27 schrieb Tomohito Esaki: >>>>> Virtual DRM splits the overlay planes of a display controller into >>>>> multiple >>>>> virtual devices to allow each plane to be accessed by each process. >>>>> >>>>> This makes it possible to overlay images output from multiple >>>>> processes on a >>>>> display. For example, one process displays the camera image without >>>>> compositor >>>>> while another process overlays the UI. >>>> >>>> I briefly looked over your patches. I didn't understand how this is >>>> different to the functionality of a compositor? Shouldn't this be solved >>>> in userspace? >>> >>> I think when latency is important (e.g., AR, VR, for displaying camera >>> images in IVI systems), there may be use cases where the compositor >>> cannot be used. >> >> Hi, >> >>> Normally, when the image is passed through the compositor, it is >>> displayed after 2 VSYNC at most, because the compositor combines the >>> image with VSYNC synchronization. >> >> This is not a universal fact. You can write a Wayland compositor that >> consistently reaches app-to-screen latency of less than one monitor >> refresh cycle, while also using KMS planes. >> >> I believe Weston succeeds in this already if you write the Wayland >> application accordingly. > > For a specific example, https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1620 allows app-to-screen latency as low as ~6 ms (including a fixed 2 ms buffer to avoid skipped frames). mutter doesn't use KMS planes yet, but if anything I'd expect that to help rather than hurt for latency (if the compositor doesn't need to draw anything). Thank you for providing specific examples. Best regards Esaki