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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Message-ID: <kuzbg4a52onojyfh2cc7h7c4iruzfbbbo2lm2jve72rnrpfk6f@2hars5pwujg3>
Date:   Wed, 8 Nov 2023 16:34:39 +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

Hi,

Thanks for your answer

On Tue, Nov 07, 2023 at 04:26:34PM +0100, Greg Kroah-Hartman wrote:
> On Tue, Nov 07, 2023 at 01:18:14PM +0100, Maxime Ripard wrote:
> > 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)
> 
> Great, have fun!  And is this per-device or per-bus-instance?

Per bus instance

> > 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.
> 
> This is up to the specific bus to resolve, there's nothing special
> needed in the driver core for it, right?

Yeah, we weren't really looking to handle this into the driver core, but
rather if there was a set of guidelines or feedback on implementing
those kind of features for a bus.

> > 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?
> 
> There might be others, yes, look at how the dynamic power management
> works for different devices on most busses, that might help you out
> here.

Thanks for the pointers, we'll have a look
Maxime

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