linux-kernel - Re: [PATCH v2 2/2] firmware: arm_scmi: Augment SMC/HVC to allow optional interrupt

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Date:   Fri, 20 Nov 2020 08:27:38 -0500
From:   Jim Quinlan <james.quinlan@...adcom.com>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     "maintainer:BROADCOM BCM7XXX ARM ARCHITECTURE" 
        <bcm-kernel-feedback-list@...adcom.com>,
        Cristian Marussi <cristian.marussi@....com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] firmware: arm_scmi: Augment SMC/HVC to allow
 optional interrupt

On Fri, Nov 20, 2020 at 6:14 AM Sudeep Holla <sudeep.holla@....com> wrote:
>
> On Thu, Nov 19, 2020 at 01:34:18PM -0500, Jim Quinlan wrote:
> > On Fri, Nov 13, 2020 at 10:12 AM Jim Quinlan <james.quinlan@...adcom.com> wrote:
> > >
> > > On Fri, Nov 13, 2020 at 9:36 AM Sudeep Holla <sudeep.holla@....com> wrote:
> > > >
> > > > On Fri, Nov 13, 2020 at 09:26:43AM -0500, Jim Quinlan wrote:
> > > > > Hi, these are fast calls.  Regards, Jim
> > > > >
> > > > >
> > > > > On Fri, Nov 13, 2020 at 4:47 AM Sudeep Holla <sudeep.holla@....com> wrote:
> > > > > >
> > > > > > On Thu, Nov 12, 2020 at 12:56:27PM -0500, Jim Quinlan wrote:
> > > > > > > The SMC/HVC SCMI transport is modified to allow the completion of an SCMI
> > > > > > > message to be indicated by an interrupt rather than the return of the smc
> > > > > > > call.  This accommodates the existing behavior of the BrcmSTB SCMI
> > > > > > > "platform" whose SW is already out in the field and cannot be changed.
> > > > > > >
> > > > > >
> > > > > > Sorry for missing to check with you earlier. Are these not fast smc calls ?
> > > > > > Can we check the SMC Function IDs for the same and expect IRQ to be present
> > > > > > if they are not fast calls ?
> > > > > Hi, if I understand you correctly you want to do something like this:
> > > > >
> > > > >  if (! ARM_SMCCC_IS_FAST_CALL(func_id)) {
> > > > >         /* look for irq and request it */
> > > > > }
> > > > >
> > > >
> > > > Yes.
> > > >
> > > > > But we  do use fast calls.
> > > >
> > > > What was the rationale for retaining fast SMC calls but use IRQ for Tx
> > > > completion ?
> > > >
> > > > Is it because you offload it to some other microprocessor and don't
> > > > continue execution on secure side in whcih case you can afford fast call ?
> > Hi Sudeep,
> >
>
> Thanks for the details. Unfortunately more questions:
>
> > Here is my understanding:  Some SMC calls may take a few longer to
> > complete than others. The longer ones tie up the CPU core that is
> > handling the SMC call, and so nothing can be scheduled on that
> > specific core.
>
> So far good.
>
> > Unfortunately, we have a real-time OS that runs
> > sporadically on one specific core and if that happens to be the same
> > core that is handling the SMC, the RTOS will miss its deadline.  So we
> > need to have the SMC return immediately and use an SGI for task
> > completion.
> >
>
> So it sounds more like it can't be fast call then.
Hi Sudeep,

To be honest,  I'm not sure what the big difference between fast and
slow SMC calls are other than the latter is "yielding" and
interruptible.  We cannot tolerate them being interruptible.

>
> Does that me, it will always return early and send SGI when the request
> is complete ?
Most calls send the SGI and return immediately.  The ones that take
longer return from the SMC and send the SGI when the operation is
completed.
>
> 1. If yes, what happens if there are multiple requests in parallel and
>    second one completes before the first. Can we handle that with this
>    patch set. Of will the second request fails until the first one is
>    complete ? It extends to number of cpus in the system worst case.
With SCMI we only have one message pending  at  a time;  perhaps I do
not understand your question.  Having the SMC return is mostly a no-op
as far as the SCMI  driver is concerned.

Our SCMI messages cannot fail.  When we do have timeouts it indicates
that something is wrong and needs to be fixed.

Regards,
Jim Quinlan
Broadcom STB
>
> 2. If no, will this not cause issues if we unconditional wait for interrupt
>    every single time ?
>
> --
> Regards,
> Sudeep

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