Message-ID: <gmtcfmanngfrap5fl5622a46nstx662e7kfgvcf3jx7eqrm6qs@tpxarssvehkd>
Date: Fri, 26 Sep 2025 00:49:54 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Jun Nie <jun.nie@...aro.org>
Cc: 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>,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH v3 3/3] drm/msm/dsi: Support dual panel use case with
 single CRTC

On Thu, Sep 25, 2025 at 12:43:43PM +0800, Jun Nie wrote:
> Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com> 于2025年9月25日周四 03:21写道：
> >
> > On Wed, Sep 24, 2025 at 11:08:12PM +0800, Jun Nie wrote:
> > > 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.
> >
> > I'd like to understand how is it framed on the overall system design
> > level. If it's a panel controlled over the DSI interface, it's fine
> > since we can broadcast commands over two DSI links. What if the panel
> > (or bridge) is controlled via I2C / SPI?
> 
> You mean there is only DSI data and all configuration is done via I2C/SPI,
> right? I do not have a real use case so far, but it can be supported in the
> same way. Panel driver finds the sibling panel via device tree to get the
> peer I2C/SPI first. All commands are send to every bus in panel driver.

Not quite, because why and how should a normal DSI bridge driver know
and care about the 'peer' device? How would you describe that in DT and
in the DRM bridge chains framework?

> 
> >
> > >
> > > 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
> > >
> >
> > --
> > With best wishes
> > Dmitry

-- 
With best wishes
Dmitry

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