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

Danilo Krummrich
2025-09-24T11:28:25Z

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.