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 09:07:39 -0700
From:   Rob Herring <robh@...nel.org>
To:     Arnd Bergmann <arnd@...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 2:12 AM Arnd Bergmann <arnd@...nel.org> 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:
> > >
> > > On 10/03/2021 07.06, Rob Herring wrote:
> > > >> My main concern here is that this creates an inconsistency in the device
> > > >> tree representation that only works because PCI drivers happen not to
> > > >> use these code paths. Logically, having "nonposted-mmio" above the PCI
> > > >> controller would imply that it applies to that bus too. Sure, it doesn't
> > > >> matter for Linux since it is ignored, but this creates an implicit
> > > >> exception that PCI buses always use posted modes.
> > > >
> > > > We could be stricter that "nonposted-mmio" must be in the immediate
> > > > parent. That's kind of in line with how addressing already works.
> > > > Every level has to have 'ranges' to be an MMIO address, and the
> > > > address cell size is set by the immediate parent.
> > > >
> > > >> Then if a device comes along that due to some twisted fabric logic needs
> > > >> nonposted nGnRnE mappings for PCIe (even though the actual PCIe ops will
> > > >> end up posted at the bus anyway)... how do we represent that? Declare
> > > >> that another "nonposted-mmio" on the PCIe bus means "no, really, use
> > > >> nonposted mmio for this"?
> > > >
> > > > If we're strict, yes. The PCI host bridge would have to have "nonposted-mmio".
> > >
> > > 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.
>
> > 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?

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.

It's possible to express in json-schema, but I think it wouldn't be
pretty. Json-schema is not great for expressing inter-property
constraints and it gets worse if we're talking inter-node constraints.
I'd like to define a way to do python snippets of code for something
like this, but that's way down on the wish list.

Rob

[1] https://github.com/devicetree-org/devicetree-specification/pull/40