linux-kernel - Re: [PATCH v2 1/4] rust: io: Add big-endian read and write functions

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Message-Id: <DG7DF76OVGTS.WMT6W8XPG4LK@kernel.org>
Date: Thu, 05 Feb 2026 23:11:10 +0100
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Link Mauve" <linkmauve@...kmauve.fr>
Cc: "Daniel Almeida" <daniel.almeida@...labora.com>, "Gary Guo"
 <gary@...yguo.net>, <rust-for-linux@...r.kernel.org>, "Madhavan Srinivasan"
 <maddy@...ux.ibm.com>, "Michael Ellerman" <mpe@...erman.id.au>, "Nicholas
 Piggin" <npiggin@...il.com>, "Christophe Leroy (CS GROUP)"
 <chleroy@...nel.org>, "Srinivas Kandagatla" <srini@...nel.org>, "Miguel
 Ojeda" <ojeda@...nel.org>, "Boqun Feng" <boqun@...nel.org>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Benno Lossin"
 <lossin@...nel.org>, "Andreas Hindborg" <a.hindborg@...nel.org>, "Alice
 Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>, "Ard
 Biesheuvel" <ardb@...nel.org>, "Martin K. Petersen"
 <martin.petersen@...cle.com>, "Eric Biggers" <ebiggers@...gle.com>, "Greg
 Kroah-Hartman" <gregkh@...uxfoundation.org>, "Lyude Paul"
 <lyude@...hat.com>, "Asahi Lina" <lina+kernel@...hilina.net>, "Viresh
 Kumar" <viresh.kumar@...aro.org>, "Lorenzo Stoakes"
 <lorenzo.stoakes@...cle.com>, "Tamir Duberstein" <tamird@...nel.org>,
 "FUJITA Tomonori" <fujita.tomonori@...il.com>,
 <linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>,
 <officialTechflashYT@...il.com>, "Ash Logan" <ash@...quark.com>, "Roberto
 Van Eeden" <rw-r-r-0644@...tonmail.com>,
 Jonathan Neuschäfer <j.neuschaefer@....net>
Subject: Re: [PATCH v2 1/4] rust: io: Add big-endian read and write
 functions

(Cc: Rob, Saravana)

On Thu Feb 5, 2026 at 10:20 PM CET, Link Mauve wrote:
> On Thu, Feb 05, 2026 at 08:05:08PM +0100, Danilo Krummrich wrote:
>> On Thu Feb 5, 2026 at 6:28 PM CET, Daniel Almeida wrote:
>> >> On 5 Feb 2026, at 12:16, Gary Guo <gary@...yguo.net> wrote:
>> >> I think we should have everything default to little endian, and have wrapper
>> >> types that do big endian which require expicit construction, similar to
>> >> RelaxedMmio in Alex's series.
>> >
>> > Ah yes, the RelaxedMmio pattern is definitely a good one. I agree that we
>> > should head in this direction.
>> 
>> I strongly disagree.
>> 
>> This is a great pattern for relaxed ordering because:
>> 
>>   (1) We need both strict and relaxed ordering.
>> 
>>   (2) Relaxed ordering is rare, hence it doesn't hurt to write e.g.
>> 
>> 	io.relaxed().write()
>> 
>>   (3) If you by accident just write
>> 
>> 	io.write()
>> 
>>       i.e. forget to call relaxed() it s not a bug, nothing bad happens.
>> 
>> Whereas for endianness it is a bad pattern because:
>> 
>>   (1) Devices are either little-endian or big-endian. Hence, having to write
>> 
>> 	io.big_endian().write()
>> 
>>       is excessive, we always want big-endian for a big-endian device.
>> 
>>   (2) It is error prone, if you forget to call big_endian() first, it is a bug.
>> 
>>   (3) It is unergonomic in combination with relaxed ordering.
>> 
>> 	io.big_endian().relaxed().write()
>> 
>>       (Does the other way around work as well? :)
>> 
>> It makes much more sense to define once when we request the I/O memory whether
>> the device is litte-endian or big-endian.
>> 
>> This could be done with different request functions, a const generic or a
>> function argument, but it should be done at request time.
>
> Could this ever be done in the device tree?  I understand this would
> mean having to change all drivers and all device trees that do big
> endian, but it seems to be the natural location for this information.  I
> have no idea how to structure that though.

I think that's a good idea, for newly supported devices we could probably do
that. For existing ones that might not work. IIRC, there is an expectation that
driver should still work with older device trees.