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Date:   Thu, 7 Oct 2021 12:31:39 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Zev Weiss <zev@bewilderbeest.net>
Cc:     Andy Shevchenko <andy.shevchenko@gmail.com>,
        OpenBMC Maillist <openbmc@lists.ozlabs.org>,
        Jeremy Kerr <jk@codeconstruct.com.au>,
        Joel Stanley <joel@jms.id.au>,
        Rob Herring <robh+dt@kernel.org>,
        devicetree <devicetree@vger.kernel.org>,
        Andrew Jeffery <andrew@aj.id.au>,
        Frank Rowand <frowand.list@gmail.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Andy Shevchenko <andy@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Francis Laniel <laniel_francis@privacyrequired.com>,
        Kees Cook <keescook@chromium.org>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Daniel Axtens <dja@axtens.net>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= <kw@linux.com>,
        Heiner Kallweit <hkallweit1@gmail.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arm Mailing List <linux-arm-kernel@lists.infradead.org>,
        "moderated list:ARM/ASPEED MACHINE SUPPORT" 
        <linux-aspeed@lists.ozlabs.org>
Subject: Re: [PATCH 0/9] Dynamic DT device nodes
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References: <20211007000954.30621-1-zev@bewilderbeest.net>
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On Thu, Oct 07, 2021 at 02:05:41AM -0700, Zev Weiss wrote:
> On Thu, Oct 07, 2021 at 12:04:41AM PDT, Andy Shevchenko wrote:
> > On Thu, Oct 7, 2021 at 3:10 AM Zev Weiss <zev@bewilderbeest.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?
> 
> Sometimes the host needs access to the flash (it's the host's firmware,
> after all), sometimes the BMC needs access to it (e.g. to perform an
> out-of-band update to the host's firmware).  To achieve the latter, the
> flash needs to be attached to the BMC, but that requires some careful
> coordination with the host to arbitrate which one actually has access to it
> (that coordination is handled by userspace, which then tells the kernel
> explicitly when the flash should be attached and detached).
> 
> What seems dangerous?
> 
> > Why can't device tree overlays be used?
> 
> I'm hoping to stay closer to mainline.  The OpenBMC kernel has a documented
> policy strongly encouraging upstream-first development:
> https://github.com/openbmc/docs/blob/master/kernel-development.md
> 
> I doubt Joel (the OpenBMC kernel maintainer) would be eager to start
> carrying the DT overlay patches; I'd likewise strongly prefer to avoid
> carrying them myself as additional downstream patches.  Hence the attempt at
> getting a solution to the problem upstream.

Then why not work to get device tree overlays to be merged properly?
Don't work on a half-of-a-solution when the real solution is already
here.

thanks,

greg k-h