linux-kernel - Re: [PATCH V2 2/2] mailbox: introduce ARM SMC based mailbox

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Message-ID: <CABb+yY1aVbKfuqX=GvTzyjkgRXB3DXLvgjZARGn8k8m2R2vSqA@mail.gmail.com>
Date:   Thu, 27 Jun 2019 10:32:27 -0500
From:   Jassi Brar <jassisinghbrar@...il.com>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Peng Fan <peng.fan@....com>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        ", Sascha Hauer" <kernel@...gutronix.de>,
        dl-linux-imx <linux-imx@....com>,
        Shawn Guo <shawnguo@...nel.org>,
        "festevam@...il.com" <festevam@...il.com>,
        Devicetree List <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Andre Przywara <andre.przywara@....com>,
        "van.freenix@...il.com" <van.freenix@...il.com>
Subject: Re: [PATCH V2 2/2] mailbox: introduce ARM SMC based mailbox

On Thu, Jun 27, 2019 at 4:09 AM Sudeep Holla <sudeep.holla@....com> wrote:
>
> On Wed, Jun 26, 2019 at 01:27:41PM -0500, Jassi Brar wrote:
> > On Wed, Jun 26, 2019 at 11:44 AM Florian Fainelli <f.fainelli@...il.com> wrote:
> > >
> > > On 6/26/19 6:31 AM, Peng Fan wrote:
> > > >>> The firmware driver might not have func-id, such as SCMI/SCPI.
> > > >>> So add an optional func-id to let smc mailbox driver could
> > > >>> use smc SiP func id.
> > > >>>
> > > >> There is no end to conforming to protocols. Controller drivers should
> > > >> be written having no particular client in mind.
> > > >
> > > > If the func-id needs be passed from user, then the chan_id suggested
> > > > by Sudeep should also be passed from user, not in mailbox driver.
> > > >
> > > > Jassi, so from your point, arm_smc_send_data just send a0 - a6
> > > > to firmware, right?
> > > >
> > > > Sudeep, Andre, Florian,
> > > >
> > > > What's your suggestion? SCMI not support, do you have
> > > > plan to add smc transport in SCMI?
> > >
> > > On the platforms that I work with, we have taken the liberty of
> > > implementing SCMI in our monitor firmware because the other MCU we use
> > > for dynamic voltage and frequency scaling did not have enough memory to
> > > support that and we still had the ability to make that firmware be
> > > trusted enough we could give it power management responsibilities. I
> > > would certainly feel more comfortable if the SCMI specification was
> > > amended to indicate that the Agent could be such a software entity,
> > > still residing on the same host CPU as the Platform(s), but if not,
> > > that's fine.
> > >
> > > This has lead us to implement a mailbox driver that uses a proprietary
> > > SMC call for the P2A path ("tx" channel) and the return being done via
> > > either that same SMC or through SGI. You can take a look at it in our
> > > downstream tree here actually:
> > >
> > > https://github.com/Broadcom/stblinux-4.9/blob/master/linux/drivers/mailbox/brcmstb-mailbox.c
> > >
> > > If we can get rid of our own driver and uses a standard SMC based
> > > mailbox driver that supports our use case that involves interrupts (we
> > > can always change their kind without our firmware/boot loader since FDT
> > > is generated from that component), that would be great.
> > >
> > static irqreturn_t brcm_isr(void)
> > {
> >          mbox_chan_received_data(&chans[0], NULL);
> >          return IRQ_HANDLED;
> > }
> >
> > Sorry, I fail to understand why the irq can't be moved inside the
> > client driver itself? There can't be more cost to it and there
> > definitely is no functionality lost.
>
> What if there are multiple clients ?
>
There is a flag IRQF_SHARED for such situations.
(good to see you considering multiple clients per channel as a legit scenario)

> And I assume you are referring to case like this where IRQ is not tied
> to the mailbox IP.
>
Yes, and that is the reason the irq should not be managed by the mailbox driver.