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([2a01:e0a:c:37e0:8998:e0cf:68cc:1b62]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-453823c463asm51664655e9.39.2025.06.26.07.19.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 26 Jun 2025 07:19:02 -0700 (PDT) Message-ID: <80d092a3-0210-48d8-97da-f9175260a5fc@redhat.com> Date: Thu, 26 Jun 2025 16:19:00 +0200 Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] drm/panic: Use a decimal fifo to avoid u64 by u64 divide From: Jocelyn Falempe To: Andrei Lalaev Cc: Miguel Ojeda , Christian Schrefl , Arnd Bergmann , Russell King , Simona Vetter , Paolo Bonzini , David Airlie , Maxime Ripard , Maarten Lankhorst , Javier Martinez Canillas , Thomas Zimmermann , Linux ARM , dri-devel@lists.freedesktop.org, rust-for-linux References: <20250418165059.560503-1-jfalempe@redhat.com> In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: uph1eJm_nNrEBtAR_aA2c_EAdoMsoutR2tVaqyQDU-0_1750947544 X-Mimecast-Originator: redhat.com Content-Language: en-US, fr Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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 >>> --- >>>   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 { >>>                   } >>>               } >>>               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: >> (> 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: >> (> 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: >> (> 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! >> >