Re: [PATCH] drm/tegra: Remove gratuitous pad field

["Terje Bergström" <tbergstrom@nvidia.com>]

On 09.04.2014 15:39, Thierry Reding wrote:
> From: Thierry Reding <treding@nvidia.com>
> 
> The version of the drm_tegra_submit structure that was merged all the
> way back in 3.10 contains a pad field that was originally intended to
> properly pad the following __u64 field. Unfortunately it seems like a
> different field was dropped during review that caused this padding to
> become unnecessary, but the pad field wasn't removed at that time.
> 
> One possible side-effect of this is that since the __u64 following the
> pad is now no longer properly aligned, the compiler may (or may not)
> introduce padding itself, which results in no predictable ABI.
> 
> Rectify this by removing the pad field so that all fields are again
> naturally aligned. Technically this is breaking existing userspace ABI,
> but given that there aren't any (released) userspace drivers that make
> use of this yet, the fallout should be minimal.
> 
> Signed-off-by: Thierry Reding <treding@nvidia.com>
> ---
>  include/uapi/drm/tegra_drm.h | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/include/uapi/drm/tegra_drm.h b/include/uapi/drm/tegra_drm.h
> index b042b48495d9..b75482112428 100644
> --- a/include/uapi/drm/tegra_drm.h
> +++ b/include/uapi/drm/tegra_drm.h
> @@ -120,7 +120,6 @@ struct drm_tegra_submit {
>  	__u32 num_waitchks;
>  	__u32 waitchk_mask;
>  	__u32 timeout;
> -	__u32 pad;
>  	__u64 syncpts;
>  	__u64 cmdbufs;
>  	__u64 relocs;
> 

The padding is hilarious. I added it to remove the possibility for
compiler to add implicit padding, but it does completely the opposite.

If we'd care about binary compatibility, we could have also just added
__packed to the definition. But as this is in staging, I don't think
that's necessary.

Acked-By: tbergstrom@nvidia.com <tbergstrom@nvidia.com>


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