[PATCH v7 10/12] drm/bridge: Implement enable_next_first to alter bridge init order

[Jagan Teki]

On Wed, Mar 29, 2023 at 10:16 PM Maxime Ripard <maxime at cerno.tech> wrote:
>
> On Wed, Mar 29, 2023 at 05:28:28PM +0100, Dave Stevenson wrote:
> > On Wed, 29 Mar 2023 at 14:19, Jagan Teki <jagan at amarulasolutions.com> wrote:
> > >
> > > DSI sink devices typically send the MIPI-DCS commands to the DSI host
> > > via general MIPI_DSI_DCS read and write API.
> > >
> > > The classical DSI sequence mentioned that the DSI host receives MIPI-DCS
> > > commands from the DSI sink first in order to switch HS mode properly.
> > > Once the DSI host switches to HS mode any MIPI-DCS commands from the
> > > DSI sink are unfunctional.
> >
> > That statement contradicts the spec.
> > The DSI spec section 8.11.1 Transmission Packet Sequences says that
> > during any BLLP (Blanking or Low Power) period the host can do any of:
> > - remain in LP-11
> > - transmit one or more non-video packets from host to peripheral in escape mode
> > - transmit one or more non-video packets from host to peripheral in
> > using HS mode
> > - receive one or more packets from peripheral to host using escape mode
> > - transmit data on a different virtual channel.
> >
> > Indeed if the sink doesn't set MIPI_DSI_MODE_LPM /
> > MIPI_DSI_MSG_USE_LPM, then the expectation is that any data transfer
> > will be in HS mode.
> >
> > That makes me confused as to the need for this patch.
>
> Yeah, and it looks like that would break the expectation that, in
> enable, a bridge can expect its controller to be in HS mode.
>
> I think that was Jagan is trying to do is to work around an issue with
> the Allwinner DSI driver:
> https://elixir.bootlin.com/linux/v6.3-rc4/source/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c#L775

Correct and I can see it seems to be a classic DSI sequence observed
in dw-mipi-dsi as well - based on Neil's comments.
https://lore.kernel.org/all/9aa3d19d-4378-aaf3-6857-c40be5d252c7@baylibre.com/

In fact, I did follow and initialize the command-mode mode_set which
set low-speed DCS and switch back to video-mode @enable and switch to
HS but could see the same issue as the host cannot accept DCS as
before (I might implement improper sequence, but not sure due to lack
of documentation). But this sequence has issues with calling
post_disable twice even on dw-mipi-dsi.

May be Neill, can comment here?

Thanks,
Jagan.