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 14:35:05 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Hector Martin <marcan@...can.st>
Cc:     Rob Herring <robh@...nel.org>,
        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 1:11 PM Hector Martin <marcan@...can.st> wrote:
> On 11/03/2021 18.12, Arnd Bergmann wrote:
> > On Wed, Mar 10, 2021 at 6:01 PM Rob Herring <robh@...nel.org> wrote:
> >> On Wed, Mar 10, 2021 at 1:27 AM Hector Martin <marcan@...can.st> wrote:
> >>> Works for me; then let's just make it non-recursive.
> >>>
> >>> Do you think we can get rid of the Apple-only optimization if we do
> >>> this? It would mean only looking at the parent during address
> >>> resolution, not recursing all the way to the top, so presumably the
> >>> performance impact would be quite minimal.
> >
> > Works for me.
>
> Incidentally, even though it would now be unused, I'd like to keep the
> apple,arm-platform compatible at this point; we've already been pretty
> close to a use case for it, and I don't want to have to fall back to a
> list of SoC compatibles if we ever need another quirk for all Apple ARM
> SoCs (or break backwards compat). It doesn't really hurt to have it in
> the binding and devicetrees, right?

Yes, keeping the compatible string is a good idea regardless.

> >> Yeah, that should be fine. I'd keep an IS_ENABLED() config check
> >> though. Then I'll also know if anyone else needs this.
> >
> > 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?
>
> Makes sense. This shouldn't really be something the kernel concerns
> itself with at runtime, just something for the dts linting, right?
>
> I assume this isn't representable in json-schema, so it would presumably
> need some ad-hoc validation code.

Agreed, having a check in either dtc or expressed in the json scheme
is better than a runtime check. I assume Rob would know how to best
add such a check.

     Arnd