linux-kernel - Re: [PATCH 0/6] mailbox: arm_mhu: add support to use in doorbell mode

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Message-ID: <20190613150804.GA11854@e107155-lin>
Date:   Thu, 13 Jun 2019 16:08:04 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Jassi Brar <jassisinghbrar@...il.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Arnd Bergmann <arnd@...db.de>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Cristian Marussi <cristian.marussi@....com>
Subject: Re: [PATCH 0/6] mailbox: arm_mhu: add support to use in doorbell mode

On Thu, Jun 06, 2019 at 04:40:45PM +0100, Sudeep Holla wrote:
> On Thu, Jun 06, 2019 at 10:20:40AM -0500, Jassi Brar wrote:
> > On Thu, Jun 6, 2019 at 7:51 AM Sudeep Holla <sudeep.holla@....com> wrote:
> >
> > >
> > > > BTW, this is not going to be the end of SCMI troubles (I believe
> > > > that's what his client is). SCMI will eventually have to be broken up
> > > > in layers (protocol and transport) for many legit platforms to use it.
> > > > That is mbox_send_message() will have to be replaced by, say,
> > > > platform_mbox_send()  in drivers/firmware/arm_scmi/driver.c  OR  the
> > > > platforms have to have shmem and each mailbox controller driver (that
> > > > could ever be used under scmi) will have to implement "doorbell
> > > > emulation" mode. That is the reason I am not letting the way paved for
> > > > such emulations.
> > > >
> > >
> > > While I don't dislike or disagree with separate transport in SCMI which
> > > I have invested time and realised that I will duplicate mailbox framework
> > > at the end.
> > >
> > Can you please share the code? Or is it no more available?
> >
> > > So I am against it only because of duplication and extra
> > > layer of indirection which has performance impact(we have this seen in
> > > sched governor for DVFS).
> > >
> > I don't see why the overhead should increase noticeably.
> >
>
> Simple, if 2 protocols share the same channel, then the requests are
> serialised. E.g. if bits 0 and 1 are allocated for protocol#1
> and bits 2 and 3 for protocol#2 and protocol#1 has higher latency
> requirements like sched-governor DVFS and there are 3-4 pending requests
> on protocol#2, then the incoming request for protocol#1 is blocked.
>

Any idea on addressing the above with abstraction layer above the driver ?
And the bit allotment without the virtual channel representation in DT.
These 2 are main issues that needs to be resolved.

--
Regards,
Sudeep