[PATCH] drm/panic: Add a u64 divide by 10 for arm32

[Alice Ryhl]

On Fri, Jun 27, 2025 at 11:41 AM Jocelyn Falempe <jfalempe at redhat.com> wrote:
>
> On 32bits ARM, u64 divided by a constant is not optimized to a
> multiply by inverse by the compiler [1].
> So do the multiply by inverse explicitly for this architecture.
>
> Link: https://github.com/llvm/llvm-project/issues/37280 [1]
> Reported-by: Andrei Lalaev <andrey.lalaev at gmail.com>
> Closes: https://lore.kernel.org/dri-devel/c0a2771c-f3f5-4d4c-aa82-d673b3c5cb46@gmail.com/
> Fixes: 675008f196ca ("drm/panic: Use a decimal fifo to avoid u64 by u64 divide")
> Signed-off-by: Jocelyn Falempe <jfalempe at redhat.com>

Not to block this change, but I think this really ought to be fixed in
the compiler. We should not have to do this kind of thing to divide by
10.

>  drivers/gpu/drm/drm_panic_qr.rs | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_panic_qr.rs b/drivers/gpu/drm/drm_panic_qr.rs
> index dd55b1cb764d..82acecd505d3 100644
> --- a/drivers/gpu/drm/drm_panic_qr.rs
> +++ b/drivers/gpu/drm/drm_panic_qr.rs
> @@ -381,6 +381,24 @@ struct DecFifo {
>      len: usize,
>  }
>
> +/// On arm32 architecture, dividing an u64 by a constant will generate a call
> +/// to __aeabi_uldivmod which is not present in the kernel.
> +/// So use the multiply by inverse method for this architecture.
> +#[cfg(target_arch = "arm")]
> +fn div10(val: u64) -> u64
> +{

Please run rustfmt on your patch.

> +    let val_h = val >> 32;
> +    let val_l = val & 0xFFFFFFFF;
> +    let b_h: u64 = 0x66666666;
> +    let b_l: u64 = 0x66666667;
> +
> +    let tmp1 = val_h * b_l + ((val_l * b_l) >> 32);
> +    let tmp2 = val_l * b_h + (tmp1 & 0xffffffff);
> +    let tmp3 = val_h * b_h + (tmp1 >> 32) + (tmp2 >> 32);
> +
> +    tmp3 >> 2
> +}
> +
>  impl DecFifo {
>      fn push(&mut self, data: u64, len: usize) {
>          let mut chunk = data;
> @@ -389,7 +407,11 @@ fn push(&mut self, data: u64, len: usize) {
>          }
>          for i in 0..len {
>              self.decimals[i] = (chunk % 10) as u8;
> -            chunk /= 10;
> +            if cfg!(target_arch = "arm") {
> +                chunk = div10(chunk);
> +            } else {
> +                chunk /= 10;
> +            }

I would get rid of this conditional and declare another div10 function
that just does input/10 on other arches.

Alice