linux-kernel - Re: [RFC PATCH 03/10] drm/mipi-dsi: add API for manual control over the DSI link power state

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Message-ID: <CAA8EJpphwXoKnzDkY3cBqzsDZwdw+nze-Ev2toPBJm-2VJvY_g@mail.gmail.com>
Date:   Tue, 28 Nov 2023 18:49:36 +0200
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Michael Walle <mwalle@...nel.org>
Cc:     Laurent.pinchart@...asonboard.com, andrzej.hajda@...el.com,
        dave.stevenson@...pberrypi.com, dianders@...omium.org,
        dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
        jernej.skrabec@...il.com, jonas@...boo.se,
        konrad.dybcio@...aro.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org, marex@...x.de,
        marijn.suijten@...ainline.org, mripard@...nel.org,
        neil.armstrong@...aro.org, quic_abhinavk@...cinc.com,
        quic_jesszhan@...cinc.com, rfoss@...nel.org, sean@...rly.run,
        tzimmermann@...e.de, tony@...mide.com,
        alexander.stein@...tq-group.com
Subject: Re: [RFC PATCH 03/10] drm/mipi-dsi: add API for manual control over
 the DSI link power state

On Mon, 27 Nov 2023 at 18:07, Michael Walle <mwalle@...nel.org> wrote:
>
> Hi,
>
> > DSI device lifetime has three different stages:
> > 1. before the DSI link being powered up and clocking,
> > 2. when the DSI link is in LP state (for the purpose of this question,
> > this is the time between the DSI link being powered up and the video
> > stream start)
> > 3. when the DSI link is in HS state (while streaming the video).
>
> It's not clear to me what (2) is. What is the state of the clock and
> data lanes?

Clk an Data0 should be in the LP mode, ready for LP Data Transfer.

I don't think we support ULPS currently.


>
> I'm facing similar issues with the tc358775 bridge. This bridge needs
> to release its reset while both clock and data lanes are in LP-11 mode.
> But then it needs to be configured (via I2C) while the clock lane is
> in enabled (HS mode), but the data lanes are still in LP-11 mode.
>
> To me it looks like there is a fouth case then:
> 1. unpowered
> 2. DSI clock and data are in LP-11
> 3. DSI clock is in HS and data are in LP-11
> 4. DSI clock is in HS and data is in HS
>
> (And of course the bridge needs continuous clock mode).
>
> > Different DSI bridges have different requirements with respect to the
> > code being executed at stages 1 and 2. For example several DSI-to-eDP
> > bridges (ps8640, tc358767 require for the link to be quiet during
> > reset time.
> > The DSI-controlled bridges and DSI panels need to send some commands
> > in stage 2, before starting up video
> >
> > In the DRM subsystem stage 3 naturally maps to the
> > drm_bridge_funcs::enable, stage 1 also naturally maps to the
> > drm_bridge_funcs::pre_enable. Stage 2 doesn't have its own place in
> > the DRM call chain.
> > Earlier we attempted to solve that using the pre_enable_prev_first,
> > which remapped pre-enable callback execution order. However it has led
> > us to the two issues. First, at the DSI host driver we do not know
> > whether the panel / bridge were updated to use pre_enable_prev_first
> > or not. Second, if the bridge has to perform steps during both stages
> > 1 and 2, it can not do that.
> >
> > I'm trying to find a way to express the difference between stages 1
> > and 2 in the generic code, so that we do not to worry about particular
> > DSI host and DSI bridge / panel peculiarities when implementing the
> > DSI host and/or DSI panel driver.
>
> For now, I have a rather hacky ".dsi_lp11_notify" callback in
> drm_bridge_funcs which is supposed to be called by the DSI host while the
> clock and data lanes are in LP-11 mode. But that is rather an RFC and me
> needing something to get the driver for this bridge working. Because it's
> badly broken. FWIW, you can find my work-in-progress patches at
> https://github.com/mwalle/linux/tree/feature-tc358775-fixes
>
> -michael
>


--
With best wishes
Dmitry