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

[Neil Armstrong]

On 08/11/2023 16:58, Laurent Pinchart wrote:
> On Wed, Nov 08, 2023 at 04:34:39PM +0100, Maxime Ripard wrote:
> > 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
>
> To be precise, those power states are link states. They don't
> necessarily translate directly to device power states, and they're not
> so much about power management than speed (and bus turn-around for
> reads) management.

So the DSI core should support handling and tracking the current DSI
link state, and DSI devices should be able to request for a particular
link state.

> Also, while DSI allows for multiple peripherals on a bus, the link is
> point-to-point, with the peripherals being all behind a single DSI RX. >
> > > > low power is typically used to send commands to a device, high speed to
> > > > transmit pixels, but still allows to send commands.
>
> Low power (LP) is a link state where commands can be transmitted at a
> low speed, as opposed to the high speed (HS) link state that is used to
> transmit both video data and commands at high speed. Any device-to-host
> data transfer (in response to read commands) occurs exclusively in LP
> mode (at least with DSI v1.3, I don't have acces to newer
> specifications).
>
> > > > 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