linux-kernel - Re: [PATCH 0/9] Dynamic DT device nodes

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Message-ID: <CAHp75VdYBB_FaMr-uKswdvDBdobTYZkiE6ncoALuG+YYVoMwyw@mail.gmail.com>
Date:   Thu, 7 Oct 2021 10:04:41 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Zev Weiss <zev@...ilderbeest.net>
Cc:     OpenBMC Maillist <openbmc@...ts.ozlabs.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jeremy Kerr <jk@...econstruct.com.au>,
        Joel Stanley <joel@....id.au>,
        Rob Herring <robh+dt@...nel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Andrew Jeffery <andrew@...id.au>,
        Frank Rowand <frowand.list@...il.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andy Shevchenko <andy@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Francis Laniel <laniel_francis@...vacyrequired.com>,
        Kees Cook <keescook@...omium.org>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Daniel Axtens <dja@...ens.net>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        "moderated list:ARM/ASPEED MACHINE SUPPORT" 
        <linux-aspeed@...ts.ozlabs.org>
Subject: Re: [PATCH 0/9] Dynamic DT device nodes

On Thu, Oct 7, 2021 at 3:10 AM Zev Weiss <zev@...ilderbeest.net> wrote:
> This patch series is in some ways kind of a v2 for the "Dynamic
> aspeed-smc flash chips via 'reserved' DT status" series I posted
> previously [0], but takes a fairly different approach suggested by Rob
> Herring [1] and doesn't actually touch the aspeed-smc driver or
> anything in the MTD subsystem, so I haven't marked it as such.
>
> To recap a bit of the context from that series, in OpenBMC there's a
> need for certain devices (described by device-tree nodes) to be able
> to be attached and detached at runtime (for example the SPI flash for
> the host's firmware, which is shared between the BMC and the host but
> can only be accessed by one or the other at a time).

This seems quite dangerous. Why do you need that? Why can't device
tree overlays be used?

> To provide that
> ability, this series adds support for a new common device-tree
> property, a boolean "dynamic" that indicates that the device may come
> and go at runtime.  When present on a node, the sysfs file for that
> node's "status" property is made writable, allowing userspace to do
> things like:
>
>   $ echo okay > /sys/firmware/devicetree/.../status
>   $ echo reserved > /sys/firmware/devicetree/.../status
>
> to activate and deactivate a dynamic device.
>
> Because it leans on the OF_DYNAMIC machinery internally, this
> functionality will only work on busses that register for OF reconfig
> notifications and handle them appropriately (presently platform, i2c,
> and spi).  This series does not attempt to solve the "dynamic devices
> further down the tree" problem [2]; my hope is that handling for OF
> reconfig notifications can be extended to other families of devices
> (e.g. individual MTD spi-nor flash chips) in the future.

What about ACPI and software nodes?
How will all this affect the host?

> [0] https://lore.kernel.org/openbmc/20210929115409.21254-1-zev@bewilderbeest.net/
> [1] https://lore.kernel.org/openbmc/CAL_JsqJH+b5oFuSP+KBLBsN5QTA6xASuqXJWXUaDkHhugXPpnQ@mail.gmail.com/
> [2] https://lore.kernel.org/openbmc/20210929220038.GS17315@packtop/
> [3] https://git.kernel.org/pub/scm/linux/kernel/git/robh/linux.git/commit/?id=6663ae07d995f5fbe2988a19858b2f87e68cf929


-- 
With Best Regards,
Andy Shevchenko