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

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]
Reported-by: Miguel Ojeda <ojeda@kernel.org>
Closes: https://lore.kernel.org/dri-devel/CANiq72ke45eOwckMhWHvmwxc03dxr4rnxxKvx+HvWdBLopZfrQ@mail.gmail.com/
Fixes: ccb8ce5268 ("ARM: 9441/1: rust: Enable Rust support for ARMv7")
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250418165059.560503-1-jfalempe@redhat.com
Acked-by: Javier Martinez Canillas <javierm@redhat.com>

drivers/gpu/drm/drm_panic_qr.rs

@@ -366,8 +366,48 @@ fn iter(&self) -> SegmentIterator<'_> {
         SegmentIterator {
             segment: self,
             offset: 0,
-            carry: 0,
+            decfifo: Default::default(),
-            carry_len: 0,
+        }
+    }
+}
+
+/// 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,
+    decfifo: DecFifo,
-    carry_len: usize,
 }
 
 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 self.decfifo.len < 3 && self.offset < data.len() {
-                    // If there are less than 3 decimal digits in the carry,
+                    // If there are less than 3 decimal digits in the fifo,
-                    // take the next 7 bytes of input, and add them to the carry.
+                    // 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.decfifo.push(chunk, BYTES_TO_DIGITS[len]);
-                    self.carry = chunk + self.carry * pow;
                     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()
             }
         }
     }