linux-kernel - Re: [PATCH 2/3] syscon: add support for "syscon-smc" compatible

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Message-ID: <20210723164158.GJ5221@sirena.org.uk>
Date:   Fri, 23 Jul 2021 17:41:58 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Clément Léger <clement.leger@...tlin.com>,
        Lee Jones <lee.jones@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        DTML <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peng Fan <peng.fan@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>
Subject: Re: [PATCH 2/3] syscon: add support for "syscon-smc" compatible

On Fri, Jul 23, 2021 at 06:07:44PM +0200, Arnd Bergmann wrote:

> There is also a problem with locking: In the case that both firmware and
> kernel have to access registers within a syscon area, you may need to
> have a semaphore to protect an atomic sequence of accesses, but since
> the interface only provides a single register load/store, there is no way for
> a kernel driver to serialize against a firmware-internal driver.

The standard solution to this for the read/modify/write case would be to
expose an explicit update_bits() operation (some hardware does this for
concurrency and/or bus bandwidth/latency reasons), though that doesn't
help with larger or multi-register sequences (and to be clear as I've
been saying I don't think we should do this at all).

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