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Mon, 06 Apr 2026 01:35:47 -0700 (PDT) Message-ID: <35eed359-8088-4ec0-9e16-e5cb31e0952e@gmail.com> Date: Mon, 6 Apr 2026 14:05:39 +0530 Precedence: bulk X-Mailing-List: devicetree@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: tessolveupstream@gmail.com Subject: Re: [PATCH v2 2/2] drm: bridge: ti-sn65dsi83: Add support for dual-link LVDS video mode To: Luca Ceresoli , andrzej.hajda@intel.com, neil.armstrong@linaro.org, rfoss@kernel.org Cc: Laurent.pinchart@ideasonboard.com, jonas@kwiboo.se, jernej.skrabec@gmail.com, maarten.lankhorst@linux.intel.com, mripard@kernel.org, tzimmermann@suse.de, airlied@gmail.com, simona@ffwll.ch, robh@kernel.org, krzk+dt@kernel.org, conor+dt@kernel.org, marex@denx.de, valentin@compulab.co.il, philippe.schenker@toradex.com, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, devicetree@vger.kernel.org References: <20260312043743.261475-1-tessolveupstream@gmail.com> <20260312043743.261475-3-tessolveupstream@gmail.com> <9a9e13a5-411a-40bc-b52f-4345e7f6b92e@gmail.com> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 18-03-2026 14:22, Luca Ceresoli wrote: > Hello Sudarshan, > > On Wed Mar 18, 2026 at 6:53 AM CET, tessolveupstream wrote: >>>> + if (ctx->dual_link_video_mode) { >>>> + regmap_write(ctx->regmap, REG_RC_LVDS_PLL, 0x05); >>>> + regmap_write(ctx->regmap, REG_RC_PLL_EN, 0x00); >>>> + regmap_write(ctx->regmap, REG_DSI_CLK, 0x53); >>>> + regmap_write(ctx->regmap, REG_LVDS_FMT, 0x6f); >>>> + regmap_write(ctx->regmap, REG_LVDS_VCOM, 0x00); >>>> + regmap_write(ctx->regmap, >>>> + REG_VID_CHA_VERTICAL_DISPLAY_SIZE_LOW, 0x00); >>>> + regmap_write(ctx->regmap, >>>> + REG_VID_CHA_VERTICAL_DISPLAY_SIZE_HIGH, 0x00); >>>> + regmap_write(ctx->regmap, >>>> + REG_VID_CHA_HSYNC_PULSE_WIDTH_LOW, 0x10); >>>> + regmap_write(ctx->regmap, >>>> + REG_VID_CHA_HORIZONTAL_BACK_PORCH, 0x28); >>>> + regmap_write(ctx->regmap, >>>> + REG_VID_CHA_VERTICAL_BACK_PORCH, 0x00); >>>> + regmap_write(ctx->regmap, >>>> + REG_VID_CHA_HORIZONTAL_FRONT_PORCH, 0x00); >>>> + regmap_write(ctx->regmap, >>>> + REG_VID_CHA_VERTICAL_FRONT_PORCH, 0x00); >>>> + } >>> >>> I guess these hard-coded values are sepcific to your panel. They must >>> instead be computed based on the timings in order to work for every panel. >>> >> >> The hard-coded values were initially derived from the TI DSI Tuner output >> during our bring-up testing. TI had also mentioned that when PATGEN is >> enabled with dual-LVDS output on the SN65DSI84, the horizontal timings >> must be divided by 2. They also noted that the current driver does not >> appear to divide the horizontal timings when PATGEN is enabled in >> dual-LVDS mode. >> >> Based on that suggestion, we had tried adjusting the horizontal timing >> registers accordingly to match the tuner output. >> Could you please advise how these register values are expected to be >> derived from the mode timings so that they work correctly for different >> panels? > > Well, the principle is quite simple: > > 1. the panel docs tell you which timings the panel needs, e.g. HBP must be > 10 clock cycles > > 2. your panel description in dts or implementation in a panel driver will > then be written accordingly > > 3. the ti-sn65dsi83 driver will receive a struct drm_display_mode* with > these values > > 4. based on those values it sets the registers so the SN65DSI84 uses the > timings required by the panel (with a bit of math if needed): > > regmap_write(ctx->regmap, REG_VID_CHA_HORIZONTAL_BACK_PORCH, > mode->htotal - mode->hsync_end); > > Same for all other timings. > > Ti is more complicated if more cases need to be handled, such as dual-LVDS, > and the chip documentation is vague about what must be done in those cases. > > I suggested next steps to move forward in reply to the cover letter. > Thank you so much for your suggestion. >>>> @@ -965,9 +1001,15 @@ static int sn65dsi83_host_attach(struct sn65dsi83 *ctx) >>>> >>>> dsi->lanes = dsi_lanes; >>>> dsi->format = MIPI_DSI_FMT_RGB888; >>>> - dsi->mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_VIDEO_BURST | >>>> - MIPI_DSI_MODE_VIDEO_NO_HFP | MIPI_DSI_MODE_VIDEO_NO_HBP | >>>> - MIPI_DSI_MODE_VIDEO_NO_HSA | MIPI_DSI_MODE_NO_EOT_PACKET; >>>> + if (ctx->dual_link_video_mode) >>>> + dsi->mode_flags = MIPI_DSI_MODE_VIDEO; >>>> + else >>>> + dsi->mode_flags = MIPI_DSI_MODE_VIDEO | >>>> + MIPI_DSI_MODE_VIDEO_BURST | >>>> + MIPI_DSI_MODE_VIDEO_NO_HFP | >>>> + MIPI_DSI_MODE_VIDEO_NO_HBP | >>>> + MIPI_DSI_MODE_VIDEO_NO_HSA | >>>> + MIPI_DSI_MODE_NO_EOT_PACKET; >>> >>> There is no explanation about this, can you elaborate on why? >>> >>> I'm working on bringing up a dual-LVDS panel on a board with the SN65DSI84, >>> and the removing MIPI_DSI_MODE_VIDEO_BURST seems to help, but I still have >>> no idea why. Should you have any info, maybe from TI, it would be very >>> interesting. >>> >> >> During our earlier bring-up, TI mentioned that one possible reason for the DSI >> REFCLK not behaving as expected could be that the DSI output is configured in >> burst mode instead of non-burst mode. In burst mode the DSI clock may not be >> continuous, whereas non-burst mode provides a more predictable DSI clock. > > Uhm, this is a bit vague. They basically said "burst can be more > problematic than continuous", which is obvious, and "try disabling burst > and see whether it helps" with no explanation on why one works and not the > other. Shoudl you have more info from them you'd be welcome to share it. In > particular, is disabling burst mode specifically related to dual-LVDS, or > just a way to (try to) get rid of some problems without a clear > understanding? > > On my side I also have a dual-LVDS panel connected to a SN65DSI84, which > works only by disabling burst mode. I haven't tried upstreaming it because > I don't have an explanation of why it fixes the panel and so I have no idea > how to teach the driver when it should disable burst mode. > > Additionally inyour patch you remove many other flags. Any explanation from > those? > Thanks for your inputs. I wanted to share a quick observation from our side. With your suggested 3 patches (links below), the panel started working after simplifying the dsi-> mode_flags: https://lore.kernel.org/all/20260226-ti-sn65dsi83-dual-lvds-fixes-and-test-pattern-v1-1-2e15f5a9a6a0@bootlin.com/ https://lore.kernel.org/all/20260226-ti-sn65dsi83-dual-lvds-fixes-and-test-pattern-v1-2-2e15f5a9a6a0@bootlin.com/ https://lore.kernel.org/lkml/20260309-ti-sn65dsi83-dual-lvds-fixes-and-test-pattern-v2-1-e6aaa7e1d181@bootlin.com/ Earlier configuration: MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_VIDEO_BURST | MIPI_DSI_MODE_VIDEO_NO_HFP | MIPI_DSI_MODE_VIDEO_NO_HBP | MIPI_DSI_MODE_VIDEO_NO_HSA | MIPI_DSI_MODE_NO_EOT_PACKET; Working configuration: MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_VIDEO_NO_HSA | MIPI_DSI_MODE_NO_EOT_PACKET; >From our testing, removing MIPI_DSI_MODE_VIDEO_BURST along with the NO_HFP/NO_HBP flags results in stable LVDS output in dual-link mode. Could you please suggest how you would prefer to handle this change for upstreaming? > Best regards, > Luca > > -- > Luca Ceresoli, Bootlin > Embedded Linux and Kernel engineering > https://bootlin.com