Re: [RFC v1 1/2] scmi: Introduce pinctrl SCMI protocol driver

[Cristian Marussi]

On Fri, Apr 21, 2023 at 08:40:47AM +0000, Oleksii Moisieiev wrote:
> Hi Peng Fan,
> 
> On 17.04.23 05:55, Peng Fan wrote:
> >
> >
> > On 4/13/2023 6:04 AM, Cristian Marussi wrote:
> >> On Fri, Apr 07, 2023 at 10:18:27AM +0000, Oleksii Moisieiev wrote:
> >>> Implementation of the SCMI client driver, which implements
> >>> PINCTRL_PROTOCOL. This protocol has ID 19 and is described
> >>> in the latest DEN0056 document.
> >>
> >> Hi,
> >>
> >>> This protocol is part of the feature that was designed to
> >>> separate the pinctrl subsystem from the SCP firmware.
> >>> The idea is to separate communication of the pin control
> >>> subsystem with the hardware to SCP firmware
> >>> (or a similar system, such as ATF), which provides an interface
> >>> to give the OS ability to control the hardware through SCMI protocol.
> >>> This is a generic driver that implements SCMI protocol,
> >>> independent of the platform type.
> >>>
> >>> DEN0056 document:
> >>> https://urldefense.com/v3/__https://developer.arm.com/documentation/den0056/latest__;!!GF_29dbcQIUBPA!y2hR3PEGGxiPjVeXBcgGyV03DPDhzgUKR0uHvsTpiafKgBar8Egc6oOOs-IkFIquhSf-qBzltqEMyzRZHq8eC4g$ 
> >>> [developer[.]arm[.]com]
> >>>
> >>
> >> No need to specify all of this in the commit message, just a note that
> >> you are adding a new SCMIv3.2 Pincontrol protocol, highlighting anything
> >> that has been left out in this patch (if any) will be enough.
> >
> > Is it possible to extend the spec to support multilple uint32_t for PIN
> > CONFIG SET?
> >
> > With only one uint32_t could not satisfy i.MX requirement.
> >
> > Thanks,
> > Peng.
> >
> IIUC you are expecting to have an ability to set some kind of array of 
> uint32_t config values to some specific ConfigType?
> 
> I'm not sure if it's supported by pintctrl subsystem right now. I was 
> unable to find an example in the existing device-tree pinctrl bindings. 
> This makes me think that this kind of binding is OEM specific.
> 
> Maybe it can be implemented by adding new IDs to OEM specific range 
> (192-255) which is reserved for OEM specific units (See Table 23 of 
> DEN0056E).
> 

If I understood correctly the aim of Peng multi-valued request, I think
that even if Linux does not support using this kind of multiple valued
requests (as of now), if it is useful or required by some of the possibly
supported hardware, it should be described and allowed by the specification
and supported by the core SCMI protocol support at least, while the pinctrl
SCMI driver can ignore this and keep using a one-sized array protocol_ops
call internally (since it cannot do any different anyway as of now)

IOW I dont think we should model too strictly the SCMI spec against only
what the Linux pinctrl subsystem support today, since Linux it is just
really only one of the possible SCMI agents and Linux implementation itself
can possibly change: it is better to model the spec on the HW requirements
or the possible usage patterns across all the possibly participating agents.

As an example, for similar reasons, when the SCMI Voltage protocol was added
to the spec, at the very last minute, a change was made to the spec to allow
for negative voltages, even though the Linux regulator subsystem was not
and still is not supporting at all negative voltages as of now; so basically
the SCMI voltage protocol API now exposes a per-domain flag (negative_volts_allowed),
that allows any kind of voltage domain to be enumerated and handled at the SCMI
spec and core layer but that also allows any SCMI driver user, like the SCMI
Regulator driver, to decide on his own if negative voltages domains can be
supported: indeed the scmi-regulator driver just skips the initialization of
any voltage domain that is found to be describing negative voltages.

Here is a bit different, it is more of an optimization in the call path
than an HW difference, but I would follow the same approach: with the
SCMI spec and the core SCMI stack (the protocol) that supports a multi-uint32
call as a general case, if useful for some scenarios, and instead the SCMI
pinctrl driver that just ignores this possibility and keep using a single-value
array anyway....then, it will be up to the guys leveraging this multi-valued
call to come up with a way to use it on their systems, possibly maybe contributing
back to upstream any needed modification if general enough
(not sure about the details of how this multi-vals operation should be...we'll have
to discuss that about the spec all together I think.)

In any case, I would definitely NOT relegate such possibility to vendor space,
since it is something generic and, especially being just (as it seems to me) an
optimization on the call path at the end, it will just lead to uneeded duplication
of functionalities in the vendor implementation of stuff that it is already
very slightly differently supported by the standard.

...just my opinion anyway, I'll happily let other guys in this thread discuss and
decide about this :P

Thanks,
Cristian