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

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Message-ID: <CACRpkdYSFGF1crqDnwB_UbEXV8q5xqx7n8VHCyKYjCpy1PMK8A@mail.gmail.com>
Date:   Tue, 9 Mar 2021 16:40:09 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Rob Herring <robh@...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 Tue, Mar 9, 2021 at 1:41 PM Arnd Bergmann <arnd@...nel.org> wrote:

> - A driver writer may want to choose between posted and
>   nonposted mmio based on performance considerations:
>   if writes are never serialized, posted writes should always
>   be faster. However, if the driver uses a spinlock to serialize
>   writes, then a nonposted write is likely faster than a posted
>   write followed by a read that serializes the spin_unlock.
>   In this case we want the driver to explicitly pick one over
>   the other, and not have rely on bus specific magic.

OK then I am all for having drivers explicitly choose access
method. Openness to speed optimization is a well established
Linux kernel design principle.

Yours,
Linus Walleij