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From:   Rob Herring <robh@kernel.org>
Date:   Thu, 11 Mar 2021 11:10:20 -0700
Message-ID: <CAL_JsqK200WcxD3PP1FToc5w2dyF3b6TYnf2oNd9Mpz77g68og@mail.gmail.com>
Subject: Re: [RFT PATCH v3 12/27] of/address: Add infrastructure to declare
 MMIO as non-posted
To:     Arnd Bergmann <arnd@kernel.org>
Cc:     Hector Martin <marcan@marcan.st>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        Marc Zyngier <maz@kernel.org>, Olof Johansson <olof@lixom.net>,
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On Thu, Mar 11, 2021 at 9:48 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Thu, Mar 11, 2021 at 5:10 PM Rob Herring <robh@kernel.org> wrote:
> > On Thu, Mar 11, 2021 at 2:12 AM Arnd Bergmann <arnd@kernel.org> wrote:
> > > On Wed, Mar 10, 2021 at 6:01 PM Rob Herring <robh@kernel.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?

IMO it should not be allowed in the root node. That's a failure to
define a bus node. Also, would that mean your memory has to be
non-posted!?

Rob