linux-kernel - Re: [PATCH V7 2/4] asm-generic/io.h: Add big-endian MMIO accessors

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Message-Id: <77978246-11c0-4cea-8a22-26536a78eef1@app.fastmail.com>
Date: Wed, 24 Sep 2025 11:46:09 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Manikanta Guntupalli" <manikanta.guntupalli@....com>,
 "git (AMD-Xilinx)" <git@....com>, "Michal Simek" <michal.simek@....com>,
 "Alexandre Belloni" <alexandre.belloni@...tlin.com>,
 "Frank Li" <Frank.Li@....com>, "Rob Herring" <robh@...nel.org>,
 "krzk+dt@...nel.org" <krzk+dt@...nel.org>,
 "Conor Dooley" <conor+dt@...nel.org>,
 Przemysław Gaj <pgaj@...ence.com>,
 "Wolfram Sang" <wsa+renesas@...g-engineering.com>,
 "tommaso.merciai.xr@...renesas.com" <tommaso.merciai.xr@...renesas.com>,
 "quic_msavaliy@...cinc.com" <quic_msavaliy@...cinc.com>, "S-k,
 Shyam-sundar" <Shyam-sundar.S-k@....com>,
 "Sakari Ailus" <sakari.ailus@...ux.intel.com>,
 "'billy_tsai@...eedtech.com'" <billy_tsai@...eedtech.com>,
 "Kees Cook" <kees@...nel.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 "Jarkko Nikula" <jarkko.nikula@...ux.intel.com>,
 "Jorge Marques" <jorge.marques@...log.com>,
 "linux-i3c@...ts.infradead.org" <linux-i3c@...ts.infradead.org>,
 "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 Linux-Arch <linux-arch@...r.kernel.org>,
 "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Cc: "Pandey, Radhey Shyam" <radhey.shyam.pandey@....com>,
 "Goud, Srinivas" <srinivas.goud@....com>,
 "Datta, Shubhrajyoti" <shubhrajyoti.datta@....com>,
 "manion05gk@...il.com" <manion05gk@...il.com>
Subject: Re: [PATCH V7 2/4] asm-generic/io.h: Add big-endian MMIO accessors

On Wed, Sep 24, 2025, at 10:59, Guntupalli, Manikanta wrote:
>> From: Arnd Bergmann <arnd@...db.de>
>> Sent: Wednesday, September 24, 2025 12:08 AM
>> To: Guntupalli, Manikanta <manikanta.guntupalli@....com>; git (AMD-Xilinx)
>
> From both description and implementation, {read,write}{b,w,l,q}() 
> access little-endian memory and return the result in native endianness:
...
> This works on little-endian CPUs, but on big-endian CPUs the 
> {read,write}{b,w,l,q}() helpers already return data in big-endian 
> format.

> The extra byte swap in io{read,write}*be() therefore corrupts 
> the data, producing little-endian values when big-endian is expected.

> The newly introduced {read,write}{w,l,q}_be() helpers directly access 
> big-endian IO memory and return results in native endianness, avoiding 
> this mismatch.

No, in both your {read,write}{w,l,q}_be() and the existing io{read,write}*be()
helpers, big-endian platforms end up with an even number of swaps
(0 or 2), while little-endian platforms have exactly one swap.

The only exception is your {read,write}s{w,l,q}_be() string
helpers that are wrong because they have an extra swap and
are broken on FIFO registers in little-endian kernels.

     Arnd