linux-kernel - Re: [PATCH v1] drm/bridge: ti-sn65dsi86: Transition to LP mode on every video line

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Message-ID: <CAD=FV=XMT=2UJjtcEk4DDAtFn9KhJ=UX3Vyzabk_shBChbvNuA@mail.gmail.com>
Date: Wed, 17 Sep 2025 07:38:38 -0700
From: Doug Anderson <dianders@...omium.org>
To: Emanuele Ghidoli <ghidoliemanuele@...il.com>
Cc: Emanuele Ghidoli <emanuele.ghidoli@...adex.com>, Andrzej Hajda <andrzej.hajda@...el.com>, 
	Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, 
	Laurent Pinchart <Laurent.pinchart@...asonboard.com>, Jonas Karlman <jonas@...boo.se>, 
	Jernej Skrabec <jernej.skrabec@...il.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Jayesh Choudhary <j-choudhary@...com>, dri-devel@...ts.freedesktop.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] drm/bridge: ti-sn65dsi86: Transition to LP mode on
 every video line

Hi,

On Tue, Sep 16, 2025 at 8:28 AM Emanuele Ghidoli
<ghidoliemanuele@...il.com> wrote:
>
> On 16/09/2025 16:45, Doug Anderson wrote:
> > Hi,
> >
> > On Tue, Sep 16, 2025 at 7:22 AM Emanuele Ghidoli
> > <ghidoliemanuele@...il.com> wrote:
> >>
> >> From: Emanuele Ghidoli <emanuele.ghidoli@...adex.com>
> >>
> >> The component datasheet recommends, to reduce power consumption,
> >> transitioning to LP mode on every video line.
> >>
> >> Enable the MIPI_DSI_MODE_VIDEO_NO_HFP and MIPI_DSI_MODE_VIDEO_NO_HBP
> >> flags so that the bridge can enter LP mode during the horizontal front
> >> porch and back porch periods.
> >>
> >> Signed-off-by: Emanuele Ghidoli <emanuele.ghidoli@...adex.com>
> >> ---
> >> Cc: Douglas Anderson <dianders@...omium.org>
> >> Cc: Andrzej Hajda <andrzej.hajda@...el.com>
> >> Cc: Neil Armstrong <neil.armstrong@...aro.org>
> >> Cc: Robert Foss <rfoss@...nel.org>
> >> Cc: Laurent Pinchart <Laurent.pinchart@...asonboard.com>
> >> Cc: Jonas Karlman <jonas@...boo.se>
> >> Cc: Jernej Skrabec <jernej.skrabec@...il.com>
> >> Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
> >> Cc: Maxime Ripard <mripard@...nel.org>
> >> Cc: Thomas Zimmermann <tzimmermann@...e.de>
> >> Cc: David Airlie <airlied@...il.com>
> >> Cc: Simona Vetter <simona@...ll.ch>
> >> Cc: Jayesh Choudhary <j-choudhary@...com>
> >> Cc: <dri-devel@...ts.freedesktop.org>
> >> Cc: <linux-kernel@...r.kernel.org>
> >> ---
> >>  drivers/gpu/drm/bridge/ti-sn65dsi86.c | 3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > I put this on a sc7180-trogdor based Chromebook and the display no
> > longer comes up. I don't personally know the MIPI side of the spec too
> > well so I have no idea why that would be.
> >
> > -Doug
>
> Hi Doug,
> thanks for the test.
> According to the datasheet, LP is recommended for the front porch and optional
> for the back porch.
> Could you please run another test by keeping only MIPI_DSI_MODE_VIDEO_NO_HFP
> and removing MIPI_DSI_MODE_VIDEO_NO_HBP?
>
> dsi->mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_VIDEO_NO_HFP;

With just MIPI_DSI_MODE_VIDEO_NO_HFP:
-> Display doesn't work

With just MIPI_DSI_MODE_VIDEO_NO_HBP:
-> Display doesn't work

So if I set _either_ flag (or both) then the display doesn't work (it
just comes up black). If I set neither flag then things are still OK.

...and, if it helps, when the screen isn't working I can still force
the color bars to show up with:

i2cset -f -y 2 0x2d 0x3c 0x10

...so I know that the device has probed OK and the eDP side of things is OK.

-Doug