Re: [PATCH v4 11/12] [RFC] firmware: arm_scmi: Add sync_cmds_atomic_replies transport flag

[Cristian Marussi]

On Wed, Aug 25, 2021 at 01:17:47PM -0400, Jim Quinlan wrote:
> On Wed, Aug 25, 2021 at 12:38 PM Florian Fainelli <f.fainelli@xxxxxxxxx> wrote:
> >
> >
> >

Hi Florian and Jim,

> > On 8/24/2021 3:59 PM, Cristian Marussi wrote:
> > > A flag is added to let the transport signal the core that its handling of
> > > synchronous command messages implies that, after .send_message has returned
> > > successfully, the requested command can be assumed to be fully and
> > > completely executed on SCMI platform side so that any possible response
> > > value is already immediately available to be retrieved by a .fetch_reponse:
> > > in other words the polling phase can be skipped in such a case and the
> > > response values accessed straight away.
> > >
> > > Note that all of the above applies only when polling mode of operation was
> > > selected by the core: if instead a completion IRQ was found to be available
> > > the normal response processing path based on completions will still be
> > > followed.
> >
> > This might actually have to be settable on a per-message basis ideally
> > since we may be transporting short lived SCMI messages for which the
> > completion can be done at SMC time, and long lived SCMI messages (e.g.:
> > involving a voltage change) for which we would prefer a completion
> > interrupt. Jim, what do you think?
> Even if the SCMI main driver could be configured this way in an
> elegant manner, I'm not sure that there is a clean way of specifying
> this  attribute on a per-message basis.  Certainly we could do this
> with our own protocols, but  many of our "long lived" messages are the
> Perf protocol's set_level command.  At any rate, let me give it some
> thought.
> 

The new flag .sync_cmds_atomic_replies applies only when polling mode
has been selected for a specific cmd transaction, which means when no
completion IRQ was found available OR if xfer.poll_completion was
excplicitly set for a specific command.

At the moment in this series (unknown bugs apart :D), if you have a
channel configured with a completion IRQ and the .sync_cmds_atomic_replies
set for the transport, this latter flag would be generally ignored and a
wait_for_completion() will be normally used upon reception of the
completionIRQ, UNLESS you specify that one specific command has to be
polled using the per message xfer.poll_completion flag: so you should be
already able to selectively use a polling which immediately returns after
the smc by setting xfer.poll_completion for that specific short lived
message (since sync_cmds_atomic_replies is set and applies to pollmode).
On the other side any other LONG lived message will be naturally handled
via completionIRQ + wait_for_completion. (at least that was the aim..)

!!! NOTE that you'll have also to drop

 [PATCH v4 10/12] [RFC] firmware: arm_scmi: Make smc transport atomic

from this series for the wait_completion to happen as you wish.

As said I'm not sure that this whole mixing of polling and IRQs on the
same channel on a regular won't cause any issues: any feedback on this
from your setup is much appreciated.
(maybe it's fine for SMC transport, but it led to a bit of hell in the
past with mboxes AFAIK...)

Thanks a lot again for your feedback, I'll have to chat with Sudeep
about the various issues/configs possibility that we discussed and I'll
keep you in the loop.

Thanks,
Cristian

P.S.: I'll be off for a few weeks, so even though I'll keep an eye on
the mail, I cannot guarantee any responsiveness :D