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

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Message-ID: <CACRpkdZd_PU-W37szfGL7J2RYWhZzXdX342vt93H7mWXdh5iHA@mail.gmail.com>
Date:   Tue, 9 Mar 2021 12:14:23 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Arnd Bergmann <arnd@...nel.org>, 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>,
        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 Mon, Mar 8, 2021 at 10:13 PM Rob Herring <robh@...nel.org> wrote:
> On Mon, Mar 08, 2021 at 09:29:54PM +0100, Arnd Bergmann wrote:

> > This is obviously more work for the drivers, but at least it keeps
> > the common code free of the hack while also allowing drivers to
> > use ioremap_np() intentionally on other platforms.
>
> I don't agree. The problem is within the interconnect. The device and
> its driver are unaware of this.

If it is possible that a driver needs to use posted access on one
SoC and nonposted on another SoC then clearly the nature
of the access need to be part of the memory access abstraction,
obviously ioremap() one way or another.

Having the driver conditionally use different ioremap_*
functions depending on SoC seems awkward. We had different
execution paths for OF and ACPI drivers and have been working
hard to create fwnode to abstract this away for drivers used with
both abstractions for example. If we can hide it from drivers
from day 1 I think we can save maintenance costs in the long
run.

Given that the Apple silicon through it's heritage from Samsung
S3C (the genealogy is unclear to me) already share drivers with
this platform, this seems to already be the case so it's not a
theoretical use case.

The core argument here seems to be "will this become common
practice or is it an Apple-ism?"

That is a question to someone who is deep down there
synthesizing SoCs. It appears the market for custom silicon
laptops has just begun. There are people that can answer this
question but I doubt that we have access to them or that they
would tell us. What is an educated guess? It seems Arnds
position is that it's an Apple-ism and I kind of trust him on this.
At the same time I know that in emerging markets, what
copycats are likely to do is say "give me exactly what Apple
has, exactly that thing".

Just my €0.01
Linus Walleij