[PATCH] drm/panic: Use a decimal fifo to avoid u64 by u64 divide

[Andrei Lalaev]

On 18.04.25 18:48, Jocelyn Falempe wrote:
> On 32bits ARM, u64/u64 is not supported [1], so change the algorithm
> to use a simple fifo with decimal digits as u8 instead.
> This is slower but should compile on all architecture.
> 
> Link: https://lore.kernel.org/dri-devel/CANiq72ke45eOwckMhWHvmwxc03dxr4rnxxKvx+HvWdBLopZfrQ@mail.gmail.com/ [1]
> Signed-off-by: Jocelyn Falempe <jfalempe at redhat.com>
> ---
>  drivers/gpu/drm/drm_panic_qr.rs | 71 ++++++++++++++++++++++-----------
>  1 file changed, 48 insertions(+), 23 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_panic_qr.rs b/drivers/gpu/drm/drm_panic_qr.rs
> index 6025a705530e..dd55b1cb764d 100644
> --- a/drivers/gpu/drm/drm_panic_qr.rs
> +++ b/drivers/gpu/drm/drm_panic_qr.rs
> @@ -366,8 +366,48 @@ fn iter(&self) -> SegmentIterator<'_> {
>          SegmentIterator {
>              segment: self,
>              offset: 0,
> -            carry: 0,
> -            carry_len: 0,
> +            decfifo: Default::default(),
> +        }
> +    }
> +}
> +
> +/// Max fifo size is 17 (max push) + 2 (max remaining)
> +const MAX_FIFO_SIZE: usize = 19;
> +
> +/// A simple Decimal digit FIFO
> +#[derive(Default)]
> +struct DecFifo {
> +    decimals: [u8; MAX_FIFO_SIZE],
> +    len: usize,
> +}
> +
> +impl DecFifo {
> +    fn push(&mut self, data: u64, len: usize) {
> +        let mut chunk = data;
> +        for i in (0..self.len).rev() {
> +            self.decimals[i + len] = self.decimals[i];
> +        }
> +        for i in 0..len {
> +            self.decimals[i] = (chunk % 10) as u8;
> +            chunk /= 10;
> +        }
> +        self.len += len;
> +    }
> +
> +    /// Pop 3 decimal digits from the FIFO
> +    fn pop3(&mut self) -> Option<(u16, usize)> {
> +        if self.len == 0 {
> +            None
> +        } else {
> +            let poplen = 3.min(self.len);
> +            self.len -= poplen;
> +            let mut out = 0;
> +            let mut exp = 1;
> +            for i in 0..poplen {
> +                out += self.decimals[self.len + i] as u16 * exp;
> +                exp *= 10;
> +            }
> +            Some((out, NUM_CHARS_BITS[poplen]))
>          }
>      }
>  }
> @@ -375,8 +415,7 @@ fn iter(&self) -> SegmentIterator<'_> {
>  struct SegmentIterator<'a> {
>      segment: &'a Segment<'a>,
>      offset: usize,
> -    carry: u64,
> -    carry_len: usize,
> +    decfifo: DecFifo,
>  }
>  
>  impl Iterator for SegmentIterator<'_> {
> @@ -394,31 +433,17 @@ fn next(&mut self) -> Option<Self::Item> {
>                  }
>              }
>              Segment::Numeric(data) => {
> -                if self.carry_len < 3 && self.offset < data.len() {
> -                    // If there are less than 3 decimal digits in the carry,
> -                    // take the next 7 bytes of input, and add them to the carry.
> +                if self.decfifo.len < 3 && self.offset < data.len() {
> +                    // If there are less than 3 decimal digits in the fifo,
> +                    // take the next 7 bytes of input, and push them to the fifo.
>                      let mut buf = [0u8; 8];
>                      let len = 7.min(data.len() - self.offset);
>                      buf[..len].copy_from_slice(&data[self.offset..self.offset + len]);
>                      let chunk = u64::from_le_bytes(buf);
> -                    let pow = u64::pow(10, BYTES_TO_DIGITS[len] as u32);
> -                    self.carry = chunk + self.carry * pow;
> +                    self.decfifo.push(chunk, BYTES_TO_DIGITS[len]);
>                      self.offset += len;
> -                    self.carry_len += BYTES_TO_DIGITS[len];
> -                }
> -                match self.carry_len {
> -                    0 => None,
> -                    len => {
> -                        // take the next 3 decimal digits of the carry
> -                        // and return 10bits of numeric data.
> -                        let out_len = 3.min(len);
> -                        self.carry_len -= out_len;
> -                        let pow = u64::pow(10, self.carry_len as u32);
> -                        let out = (self.carry / pow) as u16;
> -                        self.carry %= pow;
> -                        Some((out, NUM_CHARS_BITS[out_len]))
> -                    }
>                  }
> +                self.decfifo.pop3()
>              }
>          }
>      }
> 
> base-commit: 74757ad1c105c8fc00b4cac0b7918fe3262cdb18

Hi Jocelyn,

Apologies for reviving this old thread, but I'm still encountering
the same issue with the latest master (78f4e737a53e).

When compiling this module for ARM32 (multi_v7_defconfig),
I get the following error:

    ld.lld: error: undefined symbol: __aeabi_uldivmod
    >>> referenced by drm_panic_qr.rs:392 (drivers/gpu/drm/drm_panic_qr.rs:392)
    >>>               drivers/gpu/drm/drm_panic_qr.o:(<drm_panic_qr::SegmentIterator as core::iter::traits::iterator::Iterator>::next) in archive vmlinux.a
    >>> referenced by drm_panic_qr.rs:392 (drivers/gpu/drm/drm_panic_qr.rs:392)
    >>>               drivers/gpu/drm/drm_panic_qr.o:(<drm_panic_qr::SegmentIterator as core::iter::traits::iterator::Iterator>::next) in archive vmlinux.a
    >>> referenced by drm_panic_qr.rs:392 (drivers/gpu/drm/drm_panic_qr.rs:392)
    >>>               drivers/gpu/drm/drm_panic_qr.o:(<drm_panic_qr::SegmentIterator as core::iter::traits::iterator::Iterator>::next) in archive vmlinux.a
    >>> referenced 14 more times
    >>> did you mean: __aeabi_uidivmod
    >>> defined in: vmlinux.a(arch/arm/lib/lib1funcs.o)

Since no one else has reported this in two months, I’m wondering
if this might be a configuration issue on my end.

Thanks a lot!

-- 
Best regards,
Andrei Lalaev