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

Dmitry Baryshkov
2023-11-28T17:12:44Z

On Tue, 28 Nov 2023 at 18:56, Michael Walle <mwalle@kernel.org> 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?

>
> -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