linux-kernel - [PATCH v3 3/3] drm/msm/dsi: Support dual panel use case with single CRTC

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Message-Id: <20250924-dsi-dual-panel-upstream-v3-3-6927284f1098@linaro.org>
Date: Wed, 24 Sep 2025 23:08:12 +0800
From: Jun Nie <jun.nie@...aro.org>
To: Rob Clark <robin.clark@....qualcomm.com>, 
 Abhinav Kumar <abhinav.kumar@...ux.dev>, 
 Jessica Zhang <jessica.zhang@....qualcomm.com>, 
 Dmitry Baryshkov <lumag@...nel.org>, Sean Paul <sean@...rly.run>, 
 Marijn Suijten <marijn.suijten@...ainline.org>, 
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, 
 Conor Dooley <conor+dt@...nel.org>, 
 Krishna Manikandan <quic_mkrishn@...cinc.com>
Cc: linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
 freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
 devicetree@...r.kernel.org, Jun Nie <jun.nie@...aro.org>
Subject: [PATCH v3 3/3] drm/msm/dsi: Support dual panel use case with
 single CRTC

Support a hardware configuration where two independent DSI panels are
driven by a single, synchronous CRTC. This configuration uses a bonded
DSI link to provide a unified vblank for both displays.

This allows application software to treat the two displays as a single,
wide framebuffer with a synchronized refresh cycle, simplifying rendering
logic for side-by-side panel arrangements.

At the DSI host level, the frame width for each link must be that of an
individual panel. The driver therefore halves the CRTC's horizontal
resolution before configuring the DSI host and any DSC encoders, ensuring
each panel receives the correct half of the framebuffer.

While the DSI panel driver should manage two panels togehter.
1. During probe, the driver finds the sibling dsi host via device tree
phandle and register the 2nd panel to get another mipi_dsi_device.
2. Set dual_panel flag on both mipi_dsi_device.
3. Prepare DSC data per requirement from single panel.
4. All DSI commands should be send on every DSI link.
5. Handle power supply for 2 panels in one shot, the same is true to
   brightness.
6. From the CRTC's perspective, the two panels appear as one wide display.
   The driver exposes a DRM mode where the horizontal timings (hdisplay,
   hsync_start, etc.) are doubled, while the vertical timings remain those
   of a single panel. Because 2 panels are expected to be mounted in
   left/right position.

To maintain synchronization, both DSI links are configured to share a
single clock source, with the DSI1 controller using the clock provided
to DSI0 as below.

&mdss_dsi1 {
   assigned-clocks = <&dispcc DISP_CC_MDSS_BYTE1_CLK_SRC>,
		     <&dispcc DISP_CC_MDSS_PCLK1_CLK_SRC>;
   assigned-clock-parents = <&mdss_dsi0_phy 0>, <&mdss_dsi0_phy 1>;
}

Signed-off-by: Jun Nie <jun.nie@...aro.org>
---
 drivers/gpu/drm/msm/dsi/dsi_host.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
index de51cb02f267205320c5a94fc4188413e05907e6..1fddcea7f86547258be18a51a0a3e3f96f6c3838 100644
--- a/drivers/gpu/drm/msm/dsi/dsi_host.c
+++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
@@ -177,6 +177,7 @@ struct msm_dsi_host {
 	bool registered;
 	bool power_on;
 	bool enabled;
+	bool is_dual_panel;
 	int irq;
 };

@@ -935,7 +936,10 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
 			return;
 		}

-		dsc->pic_width = mode->hdisplay;
+		if (msm_host->is_dual_panel)
+			dsc->pic_width = hdisplay;
+		else
+			dsc->pic_width = mode->hdisplay;
 		dsc->pic_height = mode->vdisplay;
 		DBG("Mode %dx%d\n", dsc->pic_width, dsc->pic_height);

@@ -1609,6 +1613,7 @@ static int dsi_host_attach(struct mipi_dsi_host *host,
 	if (dsi->lanes > msm_host->num_data_lanes)
 		return -EINVAL;

+	msm_host->is_dual_panel = dsi->dual_panel;
 	msm_host->channel = dsi->channel;
 	msm_host->lanes = dsi->lanes;
 	msm_host->format = dsi->format;
@@ -2492,6 +2497,9 @@ enum drm_mode_status msm_dsi_host_check_dsc(struct mipi_dsi_host *host,
 	if (!msm_host->dsc)
 		return MODE_OK;

+	if (msm_host->is_dual_panel)
+		pic_width = mode->hdisplay / 2;
+
 	if (pic_width % dsc->slice_width) {
 		pr_err("DSI: pic_width %d has to be multiple of slice %d\n",
 		       pic_width, dsc->slice_width);

-- 
2.34.1