From: Jassi Brar <jassisinghbrar@gmail.com>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>, Rob Herring <robh+dt@kernel.org>,
Frank Rowand <frowand.list@gmail.com>,
Bjorn Andersson <bjorn.andersson@linaro.org>,
Vincent Guittot <vincent.guittot@linaro.org>,
linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
Sudeep Holla <sudeep.holla@arm.com>,
Devicetree List <devicetree@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC] dt-bindings: mailbox: add doorbell support to ARM MHU
Date: Fri, 15 May 2020 11:46:29 -0500 [thread overview]
Message-ID: <CABb+yY1wJMzakpz0h6ZxAh4Z3OB718f+Wq3RP0R4NZ_U=vRMkw@mail.gmail.com> (raw)
In-Reply-To: <0a50f0cf5593baeb628dc8606c523665e5e2ae6c.1589519600.git.viresh.kumar@linaro.org>
On Fri, May 15, 2020 at 12:17 AM Viresh Kumar <viresh.kumar@linaro.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!
next prev parent reply other threads:[~2020-05-15 16:46 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-15 5:17 [RFC] dt-bindings: mailbox: add doorbell support to ARM MHU Viresh Kumar
2020-05-15 16:46 ` Jassi Brar [this message]
2020-05-18 7:35 ` Viresh Kumar
2020-05-19 0:53 ` Jassi Brar
2020-05-19 4:39 ` Viresh Kumar
2020-05-19 1:29 ` Bjorn Andersson
2020-05-19 3:40 ` Viresh Kumar
2020-05-19 4:05 ` Jassi Brar
2020-06-03 18:31 ` Sudeep Holla
2020-06-03 18:42 ` Jassi Brar
2020-06-03 18:28 ` Sudeep Holla
2020-05-28 19:20 ` Rob Herring
2020-05-29 4:07 ` Viresh Kumar
2020-06-03 18:04 ` Sudeep Holla
2020-06-03 18:17 ` Sudeep Holla
2020-06-04 5:59 ` Viresh Kumar
2020-06-04 8:28 ` Sudeep Holla
2020-06-03 18:32 ` Jassi Brar
2020-06-04 9:20 ` Sudeep Holla
2020-06-04 15:15 ` Jassi Brar
2020-06-05 4:56 ` Sudeep Holla
2020-06-05 6:30 ` Jassi Brar
2020-06-05 8:58 ` Sudeep Holla
2020-06-05 15:42 ` Jassi Brar
2020-06-10 9:33 ` Viresh Kumar
2020-06-11 10:00 ` Sudeep Holla
2020-06-12 0:34 ` Jassi Brar
2020-06-12 5:28 ` Viresh Kumar
2020-09-08 9:14 ` Arnd Bergmann
2020-09-08 9:27 ` Viresh Kumar
2020-09-08 13:26 ` Sudeep Holla
2020-09-09 3:23 ` Jassi Brar
2020-09-09 4:46 ` Viresh Kumar
2020-09-09 9:31 ` Sudeep Holla
2020-05-29 5:20 ` Jassi Brar
2020-05-29 6:27 ` Viresh Kumar
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