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

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Message-ID: <CA+-6iNw2+uGX3W3boiNokbVwaK2MseJORPq8mDpc+SihoLSOQw@mail.gmail.com>
Date:   Thu, 19 Nov 2020 13:34:18 -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 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,
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.  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.

Regards,
Jim Quinlan
Broadcom STB


>
> Hi Sudeep,
> I have an answer for this but allow me time to contact the platform FW
> engineer to make sure I have the full picture -- this may take a day
> or two.  Regardless, our implementation has already "shipped" to
> customers for some time so we may not be able to change it.
> Regards, Jim
> >
> >
> > --
> > Regards,
> > Sudeep

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