linux-kernel - Re: [PATCH v2] RAS/AMD/ATL: Remove bitwise_xor

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Message-ID: <aSQeD-RSZxeuPj_h@google.com>
Date: Mon, 24 Nov 2025 16:57:51 +0800
From: Kuan-Wei Chiu <visitorckw@...il.com>
To: Nikolay Borisov <nik.borisov@...e.com>
Cc: linux-edac@...r.kernel.org, linux-kernel@...r.kernel.org, bp@...en8.de,
	Yazen.Ghannam@....com
Subject: Re: [PATCH v2] RAS/AMD/ATL: Remove bitwise_xor_bits

Hi Nikolay,

On Mon, Nov 24, 2025 at 10:40:11AM +0200, Nikolay Borisov wrote:
> Both LLVM/GCC support a __builtin_parity function which is functionally
> equivalent to the custom bitwise_xor_bits() one. Let's simplify the code by
> relying on the built-in. No functional changes.

IIRC in some cases, if the compiler decides not to inline
__builtin_parity(), it generates a libgcc function call like
__paritysi2(). Since the kernel currently lacks this symbol, this could
lead to a build failure at link time. Although the compiler inlines it
in most cases, I am not sure if using __builtin_parity() here is a good
idea.

Regards,
Kuan-Wei

> 
> Signed-off-by: Nikolay Borisov <nik.borisov@...e.com>
> ---
> 
> Changes since v1:
> 
> * Reworded the commit message
> 
>  drivers/ras/amd/atl/umc.c | 22 +++++-----------------
>  1 file changed, 5 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/ras/amd/atl/umc.c b/drivers/ras/amd/atl/umc.c
> index 6e072b7667e9..7ff4a5a1c5da 100644
> --- a/drivers/ras/amd/atl/umc.c
> +++ b/drivers/ras/amd/atl/umc.c
> @@ -49,18 +49,6 @@ static u8 get_coh_st_inst_id_mi300(struct atl_err *err)
>  	return i;
>  }
> 
> -/* XOR the bits in @val. */
> -static u16 bitwise_xor_bits(u16 val)
> -{
> -	u16 tmp = 0;
> -	u8 i;
> -
> -	for (i = 0; i < 16; i++)
> -		tmp ^= (val >> i) & 0x1;
> -
> -	return tmp;
> -}
> -
>  struct xor_bits {
>  	bool	xor_enable;
>  	u16	col_xor;
> @@ -250,17 +238,17 @@ static unsigned long convert_dram_to_norm_addr_mi300(unsigned long addr)
>  		if (!addr_hash.bank[i].xor_enable)
>  			continue;
> 
> -		temp  = bitwise_xor_bits(col & addr_hash.bank[i].col_xor);
> -		temp ^= bitwise_xor_bits(row & addr_hash.bank[i].row_xor);
> +		temp  = (u16)__builtin_parity(col & addr_hash.bank[i].col_xor);
> +		temp ^= (u16)__builtin_parity(row & addr_hash.bank[i].row_xor);
>  		bank ^= temp << i;
>  	}
> 
>  	/* Calculate hash for PC bit. */
>  	if (addr_hash.pc.xor_enable) {
> -		temp  = bitwise_xor_bits(col  & addr_hash.pc.col_xor);
> -		temp ^= bitwise_xor_bits(row  & addr_hash.pc.row_xor);
> +		temp  = (u16)__builtin_parity(col & addr_hash.pc.col_xor);
> +		temp ^= (u16)__builtin_parity(row & addr_hash.pc.row_xor);
>  		/* Bits SID[1:0] act as Bank[5:4] for PC hash, so apply them here. */
> -		temp ^= bitwise_xor_bits((bank | sid << NUM_BANK_BITS) & addr_hash.bank_xor);
> +		temp ^= (u16)__builtin_parity((bank | sid << NUM_BANK_BITS) & addr_hash.bank_xor);
>  		pc   ^= temp;
>  	}
> 
> --
> 2.52.0
> 
>