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From: Val Packett Subject: Re: [PATCH v2 1/2] drm/rockchip: vop: clear DMA stop bit upon vblank on RK3066 To: Heiko =?iso-8859-1?q?St=FCbner?= Cc: stable@vger.kernel.org, Sandy Huang , Andy Yan , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Daniel Vetter , dri-devel@lists.freedesktop.org, linux-arm-kernel@lists.infradead.org, linux-rockchip@lists.infradead.org, linux-kernel@vger.kernel.org Message-Id: In-Reply-To: <1817371.3VsfAaAtOV@diego> References: <2024051930-canteen-produce-1ba7@gregkh> <20240527071736.21784-1-val@packett.cool> <1817371.3VsfAaAtOV@diego> MIME-Version: 1.0 X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240527_151430_985144_618CAE71 X-CRM114-Status: GOOD ( 17.27 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="iso-8859-1"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, May 27 2024 at 22:43:18 +02:00:00, Heiko St=FCbner = wrote: > Hi Val, > = > Am Montag, 27. Mai 2024, 09:16:33 CEST schrieb Val Packett: >> On the RK3066, there is a bit that must be cleared, otherwise >> the picture does not show up >> on the display (at least for RGB). >> = >> Fixes: f4a6de8 ("drm: rockchip: vop: add rk3066 vop definitions") >> Cc: stable@vger.kernel.org >> Signed-off-by: Val Packett >> --- >> v2: doing this on vblank makes more sense; added fixes tag > = > can you give a rationale for this please? > = > I.e. does this dma-stop bit need to be set on each vblank that happens > to push this frame to the display somehow? The only things I'm 100% sure about: - that bit is called dma_stop in the Android kernel's header; - without ever setting that bit to 1, it was getting set to 1 by the = chip itself, as logging the register on flush was showing a 1 in that = position (it was the only set bit - I guess others aren't readable = after cfg_done?); - without clearing it "between" frames, the whole screen is always = filled with noise, the picture is not visible. The rest is at least a bit (ha) speculative: As I understand from what the name implies, the hardware sets it to = indicate that it has scanned out the frame and is waiting for = acknowledgment (clearing) to be able to scan out the next frame. I = guess it's a redundant synchronization mechanism that was removed in = later iterations of the VOP hardware block. I've been trying to see if moving where I clear the bit affects the = sort-of-tearing-but-vertical glitches that sometimes happen, especially = early on after the system has just booted, but that seems to be = completely unrelated pixel clock craziness (the Android kernel runs the = screen at 66 fps, interestingly). I'm fairly confident that both places are "correct". The reason I'm = more on the side of vblank now is that it made logical sense to me when = I thought about it more: acknowledging that the frame has been scanned = out is a reaction to the frame having been scanned out. It's a = consequence of *that* that the acknowledgment is required for the next = frame to be drawn. Unless we can get the opinion of someone closely familiar with this = decade-old hardware, we only have this reasoning to go off of :) ~val > = _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel