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

[Jocelyn Falempe <jfalempe@redhat.com>]

On 25/06/2025 00:18, Jocelyn Falempe wrote:
> On 24/06/2025 20:55, Andrei Lalaev wrote:
>> 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@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.
> 
> Ok, that's surprising, the lines 391 and 392 are:
> 
> self.decimals[i] = (chunk % 10) as u8;
> chunk /= 10;
> 
> So the compiler should be smart enough to do that without using a division.
> I will try to reproduce, and see if I can fix that.

I reproduced the issues, it looks like clang doesn't do the optimization 
on ARM32:

https://github.com/llvm/llvm-project/issues/37280

So I've made a quick test with the following changes, and it builds:


diff --git a/drivers/gpu/drm/drm_panic_qr.rs 
b/drivers/gpu/drm/drm_panic_qr.rs
index dd55b1cb764d..57bd3c6465bb 100644
--- a/drivers/gpu/drm/drm_panic_qr.rs
+++ b/drivers/gpu/drm/drm_panic_qr.rs
@@ -381,6 +381,20 @@ struct DecFifo {
      len: usize,
  }

+fn div10(val: u64) -> u64
+{
+    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 +403,7 @@ fn push(&mut self, data: u64, len: usize) {
          }
          for i in 0..len {
              self.decimals[i] = (chunk % 10) as u8;
-            chunk /= 10;
+            chunk = div10(chunk);
          }
          self.len += len;
      }


Best regards,

-- 

Jocelyn

> 
> Best regards,
> 
>>
>> Thanks a lot!
>>
>