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

[Neil Armstrong <neil.armstrong@linaro.org>]

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
>