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

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Message-ID: <7c56c08f-9382-5db4-647a-1afae79c84de@marcan.st>
Date:   Sat, 6 Mar 2021 02:19:23 +0900
From:   Hector Martin <marcan@...can.st>
To:     Arnd Bergmann <arnd@...nel.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        Marc Zyngier <maz@...nel.org>, Rob Herring <robh@...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>,
        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>,
        devicetree <devicetree@...r.kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Linux Documentation List <linux-doc@...r.kernel.org>,
        Linux Samsung SOC <linux-samsung-soc@...r.kernel.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFT PATCH v3 12/27] of/address: Add infrastructure to declare
 MMIO as non-posted

On 06/03/2021 01.43, Arnd Bergmann wrote:
> - setting ioremap() on PCI buses non-posted only makes them
>    only slower but not more reliable, because the non-posted flag
>    on the bus is discarded by the PCI host bridge.

Note that this doesn't work here *anyway*. The fabric is picky in both 
directions: thou shalt use nGnRnE for on-SoC MMIO and nGnRE for PCIe 
windows, or else, SError.

Since these devices can support *any* PCI device via Thunderbolt, making 
PCI drivers be the oddball ones needing special APIs would mean hundreds 
of changes needed - the vast majority of PCI drivers in the kernel use 
plain ioremap variants that don't have any flags to look at.

-- 
Hector Martin (marcan@...can.st)
Public Key: https://mrcn.st/pub