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Tue, 27 Oct 2026 15:30:44 -0700 (PDT) Received: from localhost ([178.214.243.78]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-5bfca5c9ebdsm205370e87.84.2026.10.27.15.30.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 27 Oct 2026 15:30:43 -0700 (PDT) From: Mikhail Gavrilov To: Calvin Owens Cc: linux-kernel@vger.kernel.org, Harry Wentland , Leo Li , Rodrigo Siqueira , Alex Deucher , =?UTF-8?q?Christian=20K=C3=B6nig?= , David Airlie , Simona Vetter , Alex Hung , amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, Aurabindo Pillai , Fangzhi Zuo , George Zhang , regressions@lists.linux.dev Subject: Re: [PATCH] Revert "drm/amd/display: Allocate DCN32/1 dmub memory to GTT" Date: Wed, 28 Oct 2026 03:30:37 +0500 Message-ID: <20261027223038.299768-1-mikhail.v.gavrilov@gmail.com> X-Mailer: git-send-email 2.56.0 In-Reply-To: <25995707c39c0c191e3ac46c4813f55a7f928403.1793131583.git.calvin@wbinvd.org> References: <25995707c39c0c191e3ac46c4813f55a7f928403.1793131583.git.calvin@wbinvd.org> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit I see the same kind of single-frame band across the whole screen on an RX 7900 XTX driving a 3840x2160 TV over HDMI, and measured a637a140ab86 against the same parent: 35 times more events, over about eleven hours each [1]. What I found since may help decide how to fix it: here the band needs both the commit and DC switching the memory clock through sub-viewport (SubVP). With either one missing, the rate stays at the same low level. Events per 1000 eligible frames (frames whose neighbours show unchanged content), counted with the CRTC CRC as described in my report [4]: kernel refresh SubVP events / eligible per 1000 1d8327353de6 (parent) 4K@120 yes 7 / 3,019,571 0.0023 a637a140ab86 4K@120 yes 293 / 3,632,444 0.0807 7.3-rc6 4K@120 no 5 / 1,582,072 0.0032 7.3-rc6 4K@100 yes 139 / 712,333 0.195 7.3-rc6 contains the commit. The 4K@120 runs with and without SubVP differ in the TV's VRR setting, which decides whether DC may use SubVP at that rate; without SubVP, DC keeps the memory clock at its maximum [2]. In the 4K@100 run, 138 of the 139 bad frames have CRCs that occur nowhere else in the capture, so the content of the band is new nearly every time, which fits your "non-framebuffer data". A second user, on an RX 7900 XT, sees the band over HDMI and over DisplayPort whenever amdgpu_dm_capabilities reports SubVP enabled, and not when it is disabled [3]. The DC debugging documentation describes this kind of symptom: "a horizontal fillet of flickering happens in some specific part of the screen. This could be an indication of Sub-Viewport issues" (Documentation/gpu/amdgpu/display/dc-debug.rst). It suggests force_disable_subvp to check; on a stock kernel that is amdgpu.dcdebugmask=0x20000. Since it reproduces within 30 minutes for you, could you check three things on an unreverted kernel? 1. The refresh rate, HDMI or DisplayPort, whether VRR/FreeSync is on, and what amdgpu_dm_capabilities says: sudo sh -c 'head -v /sys/kernel/debug/dri/0000:*/amdgpu_dm_capabilities' 2. Whether the bars always appear in the same part of the screen, as in the documentation's description, and if so, where. 3. Whether they come back when booted with amdgpu.dcdebugmask=0x20000. amdgpu_dm_capabilities should then say "sub-viewport supported: yes, enabled: no", and the memory clock may stay at its maximum. I have not run the third test myself. The commit moved DMUB memory to GTT because data from DMCUB sometimes reached the driver late, after DMCUB had signalled it. SubVP is a path where DMCUB itself works to a deadline: DC budgets 15 us for its processing when it sizes the SubVP region (subvp_fw_processing_delay_us in dcn32_resource.c). Since the commit, the windows the driver sets up for the firmware's data, mailbox and state are in system memory. Whether that can make the firmware miss its deadline is what I asked AMD earlier [2]; it is still open. If this goes in, Fixes: a637a140ab86 with Cc: stable would take it to 7.3.y, and a Closes: tag pointing at my report [4] would let the regression tracker close it. [1] https://lore.kernel.org/all/20260919200820.799243-1-mikhail.v.gavrilov@gmail.com/ [2] https://lore.kernel.org/all/20261011201833.853907-1-mikhail.v.gavrilov@gmail.com/ [3] https://gitlab.freedesktop.org/drm/amd/-/work_items/5714 [4] https://lore.kernel.org/all/20260829010525.29935-1-mikhail.v.gavrilov@gmail.com/ -- Thanks, Mikhail