linux-kernel - Re: [PATCH 1/3] regmap: add regmap using ARM SMCCC

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Date:   Fri, 23 Jul 2021 15:43:18 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Clément Léger <clement.leger@...tlin.com>
Cc:     Lee Jones <lee.jones@...aro.org>, Rob Herring <robh+dt@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Peng Fan <peng.fan@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>
Subject: Re: [PATCH 1/3] regmap: add regmap using ARM SMCCC

On Fri, Jul 23, 2021 at 03:52:37PM +0200, Clément Léger wrote:

> When running under secure monitor control, some controllers can be placed in
> secure world and their access is thus not possible from normal world. However,
> these controllers frequently contain registers than are needed by the normal
> world for a few specific operations.

> This patch adds a regmap where registers are accessed using SMCs. The secure
> monitor is then responsible to allow or deny access to the requested registers.

I can't see any SMC specification for this interface?  Frankly I have
some very substantial concerns about the use case for this over exposing
the functionality of whatever device the SMC is gating access to through
SMC interfaces specific to that functionality.  Exposing raw access to a
(presumed?) subset of whatever device functionality feels like the wrong
abstraction level to be working at and like an invitation to system
integrators to do things that are going to get them into trouble down
the line.

If the end user really is just twiddling a few bits here and there I'd
expect those functionality specific services to be pretty simple to do,
slightly more effort on the secure monitor side but a lot safer.  If
there is a use case for passing through an entire device for some reason
(ran out of controllers or something?) then I think we probably want an
abstraction at the bus level so we don't need to add custom support to
every device that we want to pass through and it's clear what's going on.

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