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: <CAA8EJpoYkH-0onMmNRk1iO5YeLN+5hpZMsfvtNo-7p6y2mjZwg@mail.gmail.com>
Date:   Tue, 28 Nov 2023 22:23:15 +0200
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Michael Walle <michael@...le.cc>
Cc:     Michael Walle <mwalle@...nel.org>,
        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 Tue, 28 Nov 2023 at 21:50, Michael Walle <michael@...le.cc> wrote:
>
> >> >> > 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.
> >>
> >> Then this is somehow missing
> >> https://docs.kernel.org/gpu/drm-kms-helpers.html#mipi-dsi-bridge-operation
> >>
> >>    A DSI host should keep the PHY powered down until the pre_enable
> >> operation
> >>    is called. All lanes are in an undefined idle state up to this point,
> >> and
> >>    it must not be assumed that it is LP-11. pre_enable should initialise
> >> the
> >>    PHY, set the data lanes to LP-11, and the clock lane to either LP-11
> >> or HS
> >>    depending on the mode_flag MIPI_DSI_CLOCK_NON_CONTINUOUS.
> >>
> >> So I don't think these three states are sufficient, see below, that
> >> there
> >> should be at least four.
> >
> >Which one is #4?
>
> enabled clock lane (HS mode), data lanes in LP-11

What is the purpose of such a mode?

>
> -michael
>
> >>
> >> >
> >> > 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
> >
> >
> >
>


-- 
With best wishes
Dmitry