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

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Message-ID: <YV8VGeMreR6NJad4@hatter.bewilderbeest.net>
Date:   Thu, 7 Oct 2021 08:41:13 -0700
From:   Zev Weiss <zev@...ilderbeest.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        OpenBMC Maillist <openbmc@...ts.ozlabs.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 07, 2021 at 03:31:39AM PDT, Greg Kroah-Hartman wrote:
>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@...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?
>>
>> 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.
>

I had been under the impression that the overlay patches had very dim 
prospects of ever being accepted and that this might be a more tractable 
alternative, but apparently was mistaken -- I'll look into what the 
outstanding issues were with that and perhaps take a stab at addressing 
them.


Zev