Commit 9af8f2b4 authored by Jocelyn Falempe's avatar Jocelyn Falempe
Browse files

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

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: default avatarAndrei Lalaev <andrey.lalaev@gmail.com>
Closes: https://lore.kernel.org/dri-devel/c0a2771c-f3f5-4d4c-aa82-d673b3c5cb46@gmail.com/


Fixes: 675008f1 ("drm/panic: Use a decimal fifo to avoid u64 by u64 divide")
Reviewed-by: default avatarAlice Ryhl <aliceryhl@google.com>
Signed-off-by: default avatarJocelyn Falempe <jfalempe@redhat.com>
parent 36007727

drivers/gpu/drm/drm_panic_qr.rs

+21 −1
Original line number Diff line number Diff line
@@ -381,6 +381,26 @@ 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.

fn div10(val: u64) -> u64 {

    if cfg!(target_arch = "arm") {

        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

    } else {

        val / 10

    }

}



impl DecFifo {

    fn push(&mut self, data: u64, len: usize) {

        let mut chunk = data;
@@ -389,7 +409,7 @@ 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;

    }