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

[Dmitry Baryshkov]

On Mon, 23 Oct 2023 at 11:14, Alexander Stein
<alexander.stein@xxxxxxxxxxxxxxx> wrote:
>
> Am Montag, 23. Oktober 2023, 09:34:42 CEST schrieb Dmitry Baryshkov:
> > On Mon, 23 Oct 2023 at 09:52, Alexander Stein
> >
> > <alexander.stein@xxxxxxxxxxxxxxx> wrote:
> > > Hi Dmitry,
> > >
> > > Am Sonntag, 22. Oktober 2023, 12:49:41 CEST schrieb Dmitry Baryshkov:
> > > > On Thu, 19 Oct 2023 at 14:42, Alexander Stein
> > > >
> > > > <alexander.stein@xxxxxxxxxxxxxxx> wrote:
> > > > > Hi,
> > > > >
> > > > > Am Donnerstag, 19. Oktober 2023, 13:19:51 CEST schrieb Dmitry
> Baryshkov:
> > > > > > On Thu, 19 Oct 2023 at 12:26, Maxime Ripard <mripard@xxxxxxxxxx>
> wrote:
> > > > > > > On Mon, Oct 16, 2023 at 07:53:48PM +0300, Dmitry Baryshkov wrote:
> > > > > > > > The MIPI DSI links do not fully fall into the DRM callbacks
> > > > > > > > model.
> > > > > > >
> > > > > > > Explaining why would help
> > > > > >
> > > > > > A kind of explanation comes afterwards, but probably I should change
> > > > > > the order of the phrases and expand it:
> > > > > >
> > > > > > The atomic_pre_enable / atomic_enable and correspondingly
> > > > > > atomic_disable / atomic_post_disable expect that the bridge links
> > > > > > follow a simple paradigm: either it is off, or it is on and
> > > > > > streaming
> > > > > > video. Thus, it is fine to just enable the link at the enable time,
> > > > > > doing some preparations during the pre_enable.
> > > > > >
> > > > > > But then it causes several issues with DSI. First, some of the DSI
> > > > > > bridges and most of the DSI panels would like to send commands over
> > > > > > the DSI link to setup the device. Next, some of the DSI hosts have
> > > > > > limitations on sending the commands. The proverbial sunxi DSI host
> > > > > > can
> > > > > > not send DSI commands after the video stream has started. Thus most
> > > > > > of
> > > > > > the panels have opted to send all DSI commands from pre_enable (or
> > > > > > prepare) callback (before the video stream has started).
> > > > > >
> > > > > > However this leaves no good place for the DSI host to power up the
> > > > > > DSI
> > > > > > link. By default the host's pre_enable callback is called after the
> > > > > > DSI bridge's pre_enable. For quite some time we were powering up the
> > > > > > DSI link from mode_set. This doesn't look fully correct. And also we
> > > > > > got into the issue with ps8640 bridge, which requires for the DSI
> > > > > > link
> > > > > > to be quiet / unpowered at the bridge's reset time.
> > > > >
> > > > > There are also bridges (e.g. tc358767) which require DSI-LP11 upon
> > > > > bridge
> > > > > reset. And additionally this DSI-(e)DP bridge requires LP11 while
> > > > > accessing
> > > > > DP-AUX channel, e.g. reading EDID. So bridges need at least some
> > > > > control
> > > > > over DSI line state.
> > > >
> > > > For sending commands in LP11 it is typical to toggle the
> > > > MIPI_DSI_MODE_LPM flag, for example see panel-=jdi-lt070me05000.c or
> > > > some other drives. It might be a good idea to make that more explicit.
> > > > All suggestions here would be appreciated.
> > >
> > > The biggest difference between that display and the tc358767 bridge is
> > > that
> > > the display uses DSI commands, while the bridge is using i2c transfer to
> > > issue DP-AUX commands. There is no host_transfer [1] which would enable
> > > LP-11. It seems this DSI-DP bridge requires LP-11/HS on DSI lanes all
> > > times. This contradicts current Linux behaviour.
> >
> > I see. I took a quick glance at the driver. Does the device mark AUX
> > as busy when there is a HS transfer?
> > Because otherwise it might be pretty hard to synchronise DP-AUX
> > transfers with the DSI link state. We might need to add an API for
> > this, if the DSI hosts actually can signal the blanking / DSI LP.
>
> I don't see that a synchronization would be required. AUX should be
> independent from DSI transfers. ASFAICS the bridge internals just requires DSI
> lines to be LP-00 or HS for AUX channel to be functioning.

Ah, LP or HS. Then it should be fine. I probably misread your original
email. I thought that AUX transfers work only in the LP mode.

>
> >
> > Side note: the driver needs some care. It doesn't support the aux-bus
> > bindings for eDP panels, it doesn't support other bridges on top of DP
> > connectors (but there can be e..g. dp-connector device).
>
> I don't think that this is necessary as you add an optional endpoint to port2
> which will then add an eDP display panel bridge. This should then handle aux-
> bus bindings.

Not quite, see Documentation/devicetree/bindings/display/dp-aux-bus.yaml
and devm_of_dp_aux_populate_bus().

It is expected that eDP panels are to be placed under the edp_bridge /
aux-bus device node. But this is a separate topic, I just wanted to
point out other missing pieces.

>
> Best regards,
> Alexander
>
> > > Best regards,
> > > Alexander
> > >
> > > [1]
> > > https://www.kernel.org/doc/html/latest/gpu/drm-kms-helpers.html#mipi-dsi-> > bridge-operation
>
>
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>


-- 
With best wishes
Dmitry