linux-kernel - Re: [RFT PATCH v3 12/27] of/address: Add infrastructure to declare MMIO as non-posted

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Date:   Thu, 11 Mar 2021 17:48:07 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Hector Martin <marcan@...can.st>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Marc Zyngier <maz@...nel.org>, Olof Johansson <olof@...om.net>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Mark Kettenis <mark.kettenis@...all.nl>,
        Tony Lindgren <tony@...mide.com>,
        Mohamed Mediouni <mohamed.mediouni@...amail.com>,
        Stan Skowronek <stan@...ellium.com>,
        Alexander Graf <graf@...zon.com>,
        Will Deacon <will@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Corbet <corbet@....net>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoph Hellwig <hch@...radead.org>,
        "David S. Miller" <davem@...emloft.net>,
        DTML <devicetree@...r.kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        "open list:GENERIC INCLUDE/ASM HEADER FILES" 
        <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFT PATCH v3 12/27] of/address: Add infrastructure to declare
 MMIO as non-posted

On Thu, Mar 11, 2021 at 5:10 PM Rob Herring <robh@...nel.org> wrote:
> On Thu, Mar 11, 2021 at 2:12 AM Arnd Bergmann <arnd@...nel.org> wrote:
> > On Wed, Mar 10, 2021 at 6:01 PM Rob Herring <robh@...nel.org> wrote:
> > Ok, makes sense.
> >
> > Conceptually, I'd like to then see a check that verifies that the
> > property is only set for nodes whose parent also has it set, since
> > that is how AXI defines it: A bus can wait for the ack from its
> > child node, or it can acknowledge the write to its parent early.
> > However, this breaks down as soon as a bus does the early ack:
> > all its children by definition use posted writes (as seen by the
> > CPU), even if they wait for stores that come from other masters.
> >
> > Does this make sense to you?
>
> BTW, I don't think it's clear in this thread, but the current
> definition proposed for the spec[1] and schema is 'nonposted-mmio' is
> specific to 'simple-bus'. I like this restriction and we can expand
> where 'nonposted-mmio' is allowed later if needed.

That sounds ok, as long as we can express everything for the mac
at the moment. Do we need to explicitly add a description to allow
the property in the root node in addition to simple-bus to be able
to enforce the rule about parent buses also having it?

       Arnd