linux-kernel - Re: [RFC] dt-bindings: mailbox: add doorbell support to ARM MHU

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Message-ID: <CABb+yY1wJMzakpz0h6ZxAh4Z3OB718f+Wq3RP0R4NZ_U=vRMkw@mail.gmail.com>
Date:   Fri, 15 May 2020 11:46:29 -0500
From:   Jassi Brar <jassisinghbrar@...il.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Arnd Bergmann <arnd@...db.de>, Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Devicetree List <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] dt-bindings: mailbox: add doorbell support to ARM MHU

On Fri, May 15, 2020 at 12:17 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> - The hardware gives us the capability to write the register in
>   parallel, i.e. we can write 0x800 and 0x400 together without any
>   software locks, and so these 32 bits should be considered as separate
>   channel even if only one interrupt is issued by the hardware finally.
>   This shouldn't be called as virtualization of the channels, as the
>   hardware supports this (as clearly mentioned in the TRM) and it takes
>   care of handling the signal properly.
>
I'll leave this one open to bikeshed arguments.

> - With serialization, if we use only one channel as today at every
>   priority, if there are 5 requests to send signal to the receiver and
>   the dvfs request is the last one in queue (which may be called from
>   scheduler's hot path with fast switching), it unnecessarily needs to
>   wait for the first four transfers to finish due to the software
>   locking imposed by the mailbox framework. This adds additional delay,
>   maybe of few ms only, which isn't required by the hardware but just by
>   the software and few ms can be important in scheduler's hotpath.
>
As I asked you yesterday over the call, it may help if you could share
some numbers to back up the doomsday scenario.
I don't believe mailbox will be a bottleneck, unless you send commands
in a while(1) ... but even then you have to compare against the
virtual-channel implementation. (Not to forget one usually doesn't
need/want the dvfs, power, clock, hotplug all happening at the _same_
time)

Please note, SCMI... lets not pretend it is not about making scmi work
with mhu :) ...  itself uses shared-memory transfers and
wait_for_completion_timeout  in scmi_do_xfer().   If some platform
_really-really_ faced speed bottlenecks, it would come to want to
exchange 32-bit encoded command/response over the mhu register,
asynchronously and totally bypassing shmem... which is possible only
now.


> - With the current approach it isn't possible to assign different bits
>   (or doorbell numbers) to clients from DT and the only way of doing
>   that without adding new bindings is by extending #mbox-cells to accept
>   a value of 2 as done in this patch.
>
I am afraid you are confused. You can use bit/doorbell-6 by passing
0x40 to mhu as the data to send.

Cheers!