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Date:   Fri, 23 Jul 2021 17:41:58 +0100
From:   Mark Brown <broonie@kernel.org>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     =?iso-8859-1?Q?Cl=E9ment_L=E9ger?= <clement.leger@bootlin.com>,
        Lee Jones <lee.jones@linaro.org>,
        Rob Herring <robh+dt@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        DTML <devicetree@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peng Fan <peng.fan@nxp.com>,
        Sudeep Holla <sudeep.holla@arm.com>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>
Subject: Re: [PATCH 2/3] syscon: add support for "syscon-smc" compatible
Message-ID: <20210723164158.GJ5221@sirena.org.uk>
References: <20210723135239.388325-1-clement.leger@bootlin.com>
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 <CAK8P3a1stoM2zRpkEBOLSFRxPy6Tabn6NgO-1sVRL4uq3wudaQ@mail.gmail.com>
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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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