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

From: Jim Quinlan
Date: Thu Aug 26 2021 - 14:29:39 EST

On Wed, Aug 25, 2021 at 2:49 PM Cristian Marussi
<cristian.marussi@xxxxxxx> wrote:
> 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.

Hi Cristian,

I've tested all commits on our SMC-based system. I also tested all commits
minus "10/12 [RFC] firmware: arm_scmi: Make smc transport atomic".
This was a basic stress test, not a comprehensive one. So

Tested-by: Jim Quinlan <james.quinlan@xxxxxxxxxxxx>

Of course I have a strong preference for omitting "10/12 [RFC]" :-).
FWIW, if you are not planning on dropping this commit, perhaps there
could be a transport
node in the DT, and that could contain the a bool property

Jim Quinlan
Broadcom STB

> 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

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