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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Date:   Tue, 7 Nov 2023 13:18:14 +0100
From:   Maxime Ripard <mripard@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Douglas Anderson <dianders@...omium.org>,
        Rob Clark <robdclark@...il.com>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Jessica Zhang <quic_jesszhan@...cinc.com>,
        Marek Vasut <marex@...x.de>, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        freedreno@...ts.freedesktop.org
Subject: Re: [RFC PATCH 03/10] drm/mipi-dsi: add API for manual control over
 the DSI link power state

On Tue, Nov 07, 2023 at 12:22:21PM +0100, Greg Kroah-Hartman wrote:
> On Tue, Nov 07, 2023 at 11:57:49AM +0100, Maxime Ripard wrote:
> > +GKH
> 
> Why?  I don't see a question for me here, sorry.

I guess the question is: we have a bus with various power states
(powered off, low power, high speed)

low power is typically used to send commands to a device, high speed to
transmit pixels, but still allows to send commands.

Depending on the devices, there's different requirements about the state
devices expect the bus to be in to send commands. Some will need to send
all the commands in the low power state, some don't care, etc. See
the mail I was replying too for more details.

We've tried so far to model that in KMS itself, so the framework the
drivers would register too, but we're kind of reaching the limits of
what we can do there. It also feels to me that "the driver can't access
its device" is more of a problem for the bus to solve rather than the
framework.

Do you agree? Are you aware of any other bus in Linux with similar
requirements we could look at? Or any suggestion on how to solve it?

Thanks
Maxime

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