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

[Michael Walle]

>> > 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.

-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