On Thu, Apr 17, 2025 at 12:05:02PM +0200, Boris Brezillon wrote:
> drm_panthor_gpu_info::shader_present is currently automatically offset
> by 4 byte to meet Arm's 32-bit/64-bit field alignment rules, but those
> constraints don't stand on 32-bit x86 and cause a mismatch when running
> an x86 binary in a user emulated environment like FEX. It's also
> generally agreed that uAPIs should explicitly pad their struct fields,
> which we originally intended to do, but a mistake slipped through during
> the submission process, leading drm_panthor_gpu_info::shader_present to
> be misaligned.
>
> This uAPI change doesn't break any of the existing users of panthor
> which are either arm32 or arm64 where the 64-bit alignment of
> u64 fields is already enforced a the compiler level.
>
> Fixes: 0f25e493a246 ("drm/panthor: Add uAPI")
> Signed-off-by: Boris Brezillon <boris.brezil...@collabora.com>
> ---
> include/uapi/drm/panthor_drm.h | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/include/uapi/drm/panthor_drm.h b/include/uapi/drm/panthor_drm.h
> index 97e2c4510e69..1379a2d4548c 100644
> --- a/include/uapi/drm/panthor_drm.h
> +++ b/include/uapi/drm/panthor_drm.h
> @@ -293,6 +293,18 @@ struct drm_panthor_gpu_info {
> /** @as_present: Bitmask encoding the number of address-space exposed
> by the MMU. */
> __u32 as_present;
>
> + /**
> + * @garbage: Unused field that's not even zero-checked.
By whom? Kernel provides it and initializes it to zero, so I guess that's why
it's not checked.
> + *
> + * This originates from a missing padding that leaked in the initial
> driver submission
> + * and was only found when testing the driver in a 32-bit x86
> environment, where
> + * u64 field alignment rules are relaxed compared to aarch32.
> + *
> + * This field can't be repurposed, because it's never been checked by
> the driver and
> + * userspace is not guaranteed to zero it out.
The direction is the other way around, it's provided by kernel. I would loosen
the restriction
on repurposing, maybe when 32bit x86 is no longer such a hot market we can
reuse it for some
other chicken bits.
With the comment updated,
Acked-by: Liviu Dudau <liviu.du...@arm.com>
Best regards,
Liviu
> + */
> + __u32 garbage;
> +
> /** @shader_present: Bitmask encoding the shader cores exposed by the
> GPU. */
> __u64 shader_present;
>
> --
> 2.49.0
>
--
====================
| I would like to |
| fix the world, |
| but they're not |
| giving me the |
\ source code! /
---------------
¯\_(ツ)_/¯
