linux-kernel - Re: [PATCH v4 1/6] nova-core: bitfield: Move bitfield-specific code from register! into new macro

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Message-Id: <DD0ZTZM8S84H.1YDWSY7DF14LM@kernel.org>
Date: Wed, 24 Sep 2025 13:28:18 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Greg KH" <gregkh@...uxfoundation.org>
Cc: "Benno Lossin" <lossin@...nel.org>, "Joel Fernandes"
 <joelagnelf@...dia.com>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
 <acourbot@...dia.com>, "Alistair Popple" <apopple@...dia.com>, "Miguel
 Ojeda" <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Boqun
 Feng" <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 <bjorn3_gh@...tonmail.com>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 "David Airlie" <airlied@...il.com>, "Simona Vetter" <simona@...ll.ch>,
 "Maarten Lankhorst" <maarten.lankhorst@...ux.intel.com>, "Maxime Ripard"
 <mripard@...nel.org>, "Thomas Zimmermann" <tzimmermann@...e.de>, "John
 Hubbard" <jhubbard@...dia.com>, "Timur Tabi" <ttabi@...dia.com>,
 <joel@...lfernandes.org>, "Elle Rhumsaa" <elle@...thered-steel.dev>, "Yury
 Norov" <yury.norov@...il.com>, "Daniel Almeida"
 <daniel.almeida@...labora.com>, <nouveau@...ts.freedesktop.org>
Subject: Re: [PATCH v4 1/6] nova-core: bitfield: Move bitfield-specific code
 from register! into new macro

On Wed Sep 24, 2025 at 12:52 PM CEST, Greg KH wrote:
> Ok, great, but right now it's not doing that from what I am seeing when
> reading the code.  Shouldn't IoMem::new() take that as an argument?

That's correct, neither IoMem nor pci::Bar do consider it yet; it's on the list
of things that still need to be done.

> But, that feels odd as our current iomem api in C doesn't care about
> endian issues at all because it "assumes" that the caller has already
> handle this properly and all that the caller "wants" is to write/read to
> some memory chunk and not twiddle bits.

Yet it seems to be the correct place to deal with it. As mentioned below, regmap
could just become part of an I/O backend implementation to do exactly that.

>> (Actually, we could even implement an I/O backend that uses regmap.)
>
> That would probably be best to do eventually as most platform drivers
> use regmap today as it's the sanest api we have at the moment.

I agree it's what we should do eventually.

>> So, I think the register!() stuff is rather orthogonal.
>
> I think it's very relevant as people seem to just be "assuming" that all
> the world (hardware and cpus) are little-endian, while in reality, they
> are anything but.  As proof, the code that uses this register!() logic
> today totally ignores endian issues and just assumes that it is both
> running on a little-endian system, AND the hardware is little-endian.
>
> As a crazy example, look at the USB host controllers that at runtime,
> have to be queried to determine what endian they are running on and the
> kernel drivers have to handle this "on the fly".  Yes, one can argue
> that the hardware developers who came up with that should be forced to
> write the drivers as penance for such sins, but in the end, it's us that
> has to deal with it...
>
> So ignoring it will get us quite a ways forward with controlling sane
> hardware on sane systems, but when s390 finally realizes they can be
> writing their drivers in rust, we are going to have to have these
> conversations again :)

I think it's not really that anyone is ignoring it (intentionally). It's two
different things that should be addressed here; yet they are related:

  (1) Implementation of an abstract representation of a register that drivers
      can interact with.

  (2) The I/O layer that lays out the raw data on the physcial bus.

The register!() macro intends to provide an abstract representation of a
register for drivers to interact with. Think of it as an abstract box, where the
memory layout does not matter at all -- could be anything.

Theoretically, this abstraction could even store every single field of a
register in its own u32 or u64, etc. Of course, that's a waste of memory, which
is why we're using this bitfield thing instead.

The only thing that matters is that there is a contract between the struct
representing a register (generated by the register!() macro) and the I/O backend
layer that lays out the raw value on the bus.

This works attempts to address (1), whereas you are (rightfully) asking for (2).
And I think the answer for (2) simply is, we still have to address it.