Re: [PATCH] samples: rust: fix endianness issue in rust_driver_pci

Re: [PATCH] samples: rust: fix endianness issue in rust_driver_pci

[Marko Turk]

On Sat, Nov 01, 2025 at 10:46:54PM +0100, Marko Turk wrote:
> QEMU PCI test device specifies all registers as little endian. OFFSET
> register is converted properly, but the COUNT register is not.
> 
> Apply the same conversion to the COUNT register also.
> 
> Signed-off-by: Marko Turk <mt@markoturk.info>
> Fixes: 685376d18e9a ("samples: rust: add Rust PCI sample driver")

Hi,

Can someone take a look?

Thanks.

Marko

Re: [PATCH] samples: rust: fix endianness issue in rust_driver_pci

[Miguel Ojeda]

On Wed, Nov 12, 2025 at 9:47 AM Marko Turk <mt@markoturk.info> wrote:
>
> On Sat, Nov 01, 2025 at 10:46:54PM +0100, Marko Turk wrote:
> > QEMU PCI test device specifies all registers as little endian. OFFSET
> > register is converted properly, but the COUNT register is not.
> >
> > Apply the same conversion to the COUNT register also.
> >
> > Signed-off-by: Marko Turk <mt@markoturk.info>
> > Fixes: 685376d18e9a ("samples: rust: add Rust PCI sample driver")
>
> Can someone take a look?

Your message was in my spam folder -- that may be affecting who saw it.

From https://www.qemu.org/docs/master/specs/pci-testdev.html:

    "All registers are little endian."

So this seems right. A couple tags:

Cc: stable@vger.kernel.org
Link: https://www.qemu.org/docs/master/specs/pci-testdev.html

Cc'ing Dirk, since he tested the sample originally.

Thanks!

Cheers,
Miguel

Re: [PATCH] samples: rust: fix endianness issue in rust_driver_pci

[Miguel Ojeda]

On Wed, Nov 12, 2025 at 10:37 AM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> Cc'ing Dirk, since he tested the sample originally.

i.e. a new Tested-by is appreciated if you have the time since you
tested with QEMU originally, even if this is a no-op in x86 (where I
think you tested).

I guess we could potentially consider something like returning a
wrapper to force us to explicitly pick either LE or BE to prevent
things like this.

Cheers,
Miguel

Re: [PATCH] samples: rust: fix endianness issue in rust_driver_pci

[Danilo Krummrich]

On Wed Nov 12, 2025 at 8:56 PM AEDT, Miguel Ojeda wrote:
> I guess we could potentially consider something like returning a
> wrapper to force us to explicitly pick either LE or BE to prevent
> things like this.

In general, the I/O backend (e.g. MMIO, I2C, etc.) is supposed to take care
of endianness.

In the case of MMIO we (currently) always assume that the device is
little-endian; this is also what the C functions used to implement the MMIO
backend (e.g. readl()) assume, i.e. they always convert from little-endian to
CPU endianness.

I don't think we should do anything else; the cases where we need to deal with
big-endian devices for MMIO should be extremely rare -- rare enough that there
aren't even corresponding *_be() implementations for all architectures.

Given that, I think the proper fix is to drop the existing u32::from_le() call
(that ended up in the sample driver by accident) and properly document the
little-endian assumption of the MMIO backend.

(Currently, this simply is the Io structure, but there is a patch series [1]
from Zhi splitting things up, so we can start supporting arbitrary I/O
backends.)

@Marko: Two separate patches for this would be very welcome. :)

[1] https://lore.kernel.org/all/20251110204119.18351-1-zhiw@nvidia.com/

Re: [PATCH] samples: rust: fix endianness issue in rust_driver_pci

[Miguel Ojeda]

On Thu, Nov 13, 2025 at 9:22 AM Danilo Krummrich <dakr@kernel.org> wrote:
>
> I don't think we should do anything else; the cases where we need to deal with
> big-endian devices for MMIO should be extremely rare -- rare enough that there
> aren't even corresponding *_be() implementations for all architectures.
>
> Given that, I think the proper fix is to drop the existing u32::from_le() call
> (that ended up in the sample driver by accident) and properly document the
> little-endian assumption of the MMIO backend.

Sounds good then, thanks!

Cheers,
Miguel

Re: [PATCH] samples: rust: fix endianness issue in rust_driver_pci

[Danilo Krummrich]

On Thu Nov 13, 2025 at 7:22 PM AEDT, Danilo Krummrich wrote:
> On Wed Nov 12, 2025 at 8:56 PM AEDT, Miguel Ojeda wrote:
>> I guess we could potentially consider something like returning a
>> wrapper to force us to explicitly pick either LE or BE to prevent
>> things like this.
>
> In general, the I/O backend (e.g. MMIO, I2C, etc.) is supposed to take care
> of endianness.
>
> In the case of MMIO we (currently) always assume that the device is
> little-endian; this is also what the C functions used to implement the MMIO
> backend (e.g. readl()) assume, i.e. they always convert from little-endian to
> CPU endianness.

I.e. if we'd support a big-endian architecture in Rust

	u32::from_le(bar.read32())

would be a bug.

> I don't think we should do anything else; the cases where we need to deal with
> big-endian devices for MMIO should be extremely rare -- rare enough that there
> aren't even corresponding *_be() implementations for all architectures.
>
> Given that, I think the proper fix is to drop the existing u32::from_le() call
> (that ended up in the sample driver by accident) and properly document the
> little-endian assumption of the MMIO backend.
>
> (Currently, this simply is the Io structure, but there is a patch series [1]
> from Zhi splitting things up, so we can start supporting arbitrary I/O
> backends.)
>
> @Marko: Two separate patches for this would be very welcome. :)
>
> [1] https://lore.kernel.org/all/20251110204119.18351-1-zhiw@nvidia.com/

Re: [PATCH] samples: rust: fix endianness issue in rust_driver_pci

[Dirk Behme]

On 12/11/2025 10:37, Miguel Ojeda wrote:
> On Wed, Nov 12, 2025 at 9:47 AM Marko Turk <mt@markoturk.info> wrote:
>>
>> On Sat, Nov 01, 2025 at 10:46:54PM +0100, Marko Turk wrote:
>>> QEMU PCI test device specifies all registers as little endian. OFFSET
>>> register is converted properly, but the COUNT register is not.
>>>
>>> Apply the same conversion to the COUNT register also.
>>>
>>> Signed-off-by: Marko Turk <mt@markoturk.info>
>>> Fixes: 685376d18e9a ("samples: rust: add Rust PCI sample driver")
>>
>> Can someone take a look?
> 
> Your message was in my spam folder -- that may be affecting who saw it.
> 
>>From https://www.qemu.org/docs/master/specs/pci-testdev.html:
> 
>      "All registers are little endian."
> 
> So this seems right. A couple tags:
> 
> Cc: stable@vger.kernel.org
> Link: https://www.qemu.org/docs/master/specs/pci-testdev.html
> 
> Cc'ing Dirk, since he tested the sample originally.


Hmm, I can't find the initial patch in my Inbox. Even

https://lore.kernel.org/rust-for-linux/aRRJPZVkCv2i7kt2@vps.markoturk.info/

doesn't seem to have it?

Thanks

Dirk

Re: [PATCH] samples: rust: fix endianness issue in rust_driver_pci

[Marko Turk]

On Wed, Nov 12, 2025 at 11:16:40AM +0100, Dirk Behme wrote:
> On 12/11/2025 10:37, Miguel Ojeda wrote:
> > On Wed, Nov 12, 2025 at 9:47 AM Marko Turk <mt@markoturk.info> wrote:
> > > 
> > > On Sat, Nov 01, 2025 at 10:46:54PM +0100, Marko Turk wrote:
> > > > QEMU PCI test device specifies all registers as little endian. OFFSET
> > > > register is converted properly, but the COUNT register is not.
> > > > 
> > > > Apply the same conversion to the COUNT register also.
> > > > 
> > > > Signed-off-by: Marko Turk <mt@markoturk.info>
> > > > Fixes: 685376d18e9a ("samples: rust: add Rust PCI sample driver")
> > > 
> > > Can someone take a look?
> > 
> > Your message was in my spam folder -- that may be affecting who saw it.
> > 
> > > From https://www.qemu.org/docs/master/specs/pci-testdev.html:
> > 
> >      "All registers are little endian."
> > 
> > So this seems right. A couple tags:
> > 
> > Cc: stable@vger.kernel.org
> > Link: https://www.qemu.org/docs/master/specs/pci-testdev.html
> > 
> > Cc'ing Dirk, since he tested the sample originally.
> 
> 
> Hmm, I can't find the initial patch in my Inbox. Even
> 
> https://lore.kernel.org/rust-for-linux/aRRJPZVkCv2i7kt2@vps.markoturk.info/
> 
> doesn't seem to have it?

Initially I didn't send to rust-for-linux mailing list. It's here on lkml:
https://lore.kernel.org/lkml/20251101214629.10718-1-mt@markoturk.info/

Marko

Re: [PATCH] samples: rust: fix endianness issue in rust_driver_pci

[Miguel Ojeda]

On Wed, Nov 12, 2025 at 11:17 AM Dirk Behme <dirk.behme@de.bosch.com> wrote:
>
> Hmm, I can't find the initial patch in my Inbox. Even
>
> https://lore.kernel.org/rust-for-linux/aRRJPZVkCv2i7kt2@vps.markoturk.info/
>
> doesn't seem to have it?

It was only sent to the linux-pci list -- for situations like this,
you can click in the "[not found]" message at the bottom, and then on

    Message-ID: <20251101214629.10718-1-mt@markoturk.info>
    found in another inbox:

    ../../linux-pci/20251101214629.10718-1-mt@markoturk.info/

where that last line is a link to the other list that you can click to find it.

I hope that helps!

Cheers,
Miguel

Re: [PATCH] samples: rust: fix endianness issue in rust_driver_pci

[Dirk Behme]

On 12/11/2025 11:24, Miguel Ojeda wrote:
> On Wed, Nov 12, 2025 at 11:17 AM Dirk Behme <dirk.behme@de.bosch.com> wrote:
>>
>> Hmm, I can't find the initial patch in my Inbox. Even
>>
>> https://lore.kernel.org/rust-for-linux/aRRJPZVkCv2i7kt2@vps.markoturk.info/
>>
>> doesn't seem to have it?
> 
> It was only sent to the linux-pci list -- for situations like this,
> you can click in the "[not found]" message at the bottom, and then on
> 
>      Message-ID: <20251101214629.10718-1-mt@markoturk.info>
>      found in another inbox:
> 
>      ../../linux-pci/20251101214629.10718-1-mt@markoturk.info/
> 
> where that last line is a link to the other list that you can click to find it.
> 
> I hope that helps!


Yes, thanks, that helps a lot!

And yes, the change itself looks good. I gave this a try with x86 QEMU 
and as expected (as mentioned it is a no-op on x86) I get with and 
without this patch

rust_driver_pci 0000:00:04.0: pci-testdev data-match count: 1

With this:

Tested-by: Dirk Behme <dirk.behme@de.bosch.com>

Thanks,

Dirk