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

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Message-ID: <20200908092729.sv2pzkvqzpbloe2o@vireshk-i7>
Date:   Tue, 8 Sep 2020 14:57:29 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Jassi Brar <jassisinghbrar@...il.com>,
        Sudeep Holla <sudeep.holla@....com>,
        Rob Herring <robh@...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>,
        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 08-09-20, 11:14, Arnd Bergmann wrote:
> Picking up the old thread again after and getting pinged by multiple
> colleagues about it (thanks!) reading through the history.

Thanks for your inputs Arnd.

> Earlier, Jassi also commented "Linux does not provide real-time
> guarantees", which to me is what actually causes the issue here:
> 
> Linux having timeouts when communicating to the firmware means
> that it relies on the hardware and firmware having real-time behavior
> even when not providing real-time guarantees to its processes.
> 
> When comparing the two usage models, it's clear that the minimum
> latency for a message delivery is always at least the time time
> to process an interrupt, plus at least one expensive MMIO read
> and one less expensive posted MMIO write for an Ack. If we
> have a doorbell plus out-of-band message, we need an extra
> DMA barrier and a read from coherent memory, both of which can
> be noticeable. As soon as messages are queued in the current
> model, the maximum latency increases by a potentially unbounded
> number of round-trips, while in the doorbell model that problem
> does not exist, so I agree that we need to handle both modes
> in the kernel deal with all existing hardware as well as firmware
> that requires low-latency communication.

Right.

> It also sounds like that debate is already settled because there
> are platforms using both modes, and in the kernel we usually
> end up supporting the platforms that our users have, whether
> we think it's a good idea or not.
> 
> The only questions that I see in need of being answered are:
> 
> 1. Should the binding use just different "#mbox-cells" values or
>    also different "compatible" strings to tell that difference?
> 2. Should one driver try to handle both modes or should there
>    be two drivers?
> 
> It sounds like Jassi strongly prefers separate drivers, which
> would make separate compatible strings the more practical
> approach. While the argument can be made that a single
> piece of hardware should only have one DT description,
> the counter-argument would be that the behavior described
> by the DT here is made up by both the hardware and the
> firmware behind it, and they are in fact different.

I would be fine with both the ideas and that isn't a blocker for me.
Though I still feel this is exactly why we have #mbox-cells here and
that should be enough in this case, even if we opt for multiple
drivers.

But whatever everyone agrees to will be fine.

-- 
viresh