linux-kernel - Re: [PATCH v2 2/2] bitfield.h: split up __BF_FIELD

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Date:   Wed, 8 Jul 2020 21:11:05 -0500
From:   Alex Elder <elder@...aro.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        "David S . Miller" <davem@...emloft.net>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>, oss-drivers@...ronome.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH v2 2/2] bitfield.h: split up __BF_FIELD_CHECK macro

On 7/8/20 6:04 PM, Nick Desaulniers wrote:
> This macro has a few expansion sites that pass literal 0s as parameters.
> Split these up so that we do the precise checks where we care about
> them.
> 
> Suggested-by: Alex Elder <elder@...aro.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>

I do like this better.  It makes it more obvious when only
the mask is being verified (in FIELD_FIT() for example).
I also appreciate that you distinguished the register
checks from the value checks.
- field masks must be (2^x - 1) << y, x > 0, and constant
- values must be representable within a field mask
- registers must be able to represent the field mask

Reviewed-by: Alex Elder <elder@...aro.org>

> ---
> Changes V1-V2:
> * New patch in v2.
> * Rebased on 0001.
> 
>  .../netronome/nfp/nfpcore/nfp_nsp_eth.c       | 11 ++++----
>  include/linux/bitfield.h                      | 26 +++++++++++++------
>  2 files changed, 24 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/net/ethernet/netronome/nfp/nfpcore/nfp_nsp_eth.c b/drivers/net/ethernet/netronome/nfp/nfpcore/nfp_nsp_eth.c
> index 311a5be25acb..938fc733fccb 100644
> --- a/drivers/net/ethernet/netronome/nfp/nfpcore/nfp_nsp_eth.c
> +++ b/drivers/net/ethernet/netronome/nfp/nfpcore/nfp_nsp_eth.c
> @@ -492,11 +492,12 @@ nfp_eth_set_bit_config(struct nfp_nsp *nsp, unsigned int raw_idx,
>  	return 0;
>  }
>  
> -#define NFP_ETH_SET_BIT_CONFIG(nsp, raw_idx, mask, val, ctrl_bit)	\
> -	({								\
> -		__BF_FIELD_CHECK(mask, 0ULL, val, "NFP_ETH_SET_BIT_CONFIG: "); \
> -		nfp_eth_set_bit_config(nsp, raw_idx, mask, __bf_shf(mask), \
> -				       val, ctrl_bit);			\
> +#define NFP_ETH_SET_BIT_CONFIG(nsp, raw_idx, mask, val, ctrl_bit)		\
> +	({									\
> +		__BF_FIELD_CHECK_MASK(mask, "NFP_ETH_SET_BIT_CONFIG: ");	\
> +		__BF_FIELD_CHECK_VAL(mask, val, "NFP_ETH_SET_BIT_CONFIG: ");	\
> +		nfp_eth_set_bit_config(nsp, raw_idx, mask, __bf_shf(mask),	\
> +				       val, ctrl_bit);				\
>  	})
>  
>  /**
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index 4e035aca6f7e..79651867beb3 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -41,18 +41,26 @@
>  
>  #define __bf_shf(x) (__builtin_ffsll(x) - 1)
>  
> -#define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)			\
> +#define __BF_FIELD_CHECK_MASK(_mask, _pfx)				\
>  	({								\
>  		BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),		\
>  				 _pfx "mask is not constant");		\
>  		BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero");	\
> +		__BUILD_BUG_ON_NOT_POWER_OF_2((_mask) +			\
> +					      (1ULL << __bf_shf(_mask))); \
> +	})
> +
> +#define __BF_FIELD_CHECK_VAL(_mask, _val, _pfx)				\
> +	({								\
>  		BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?		\
>  				 ~((_mask) >> __bf_shf(_mask)) & (_val) : 0, \
>  				 _pfx "value too large for the field"); \
> -		BUILD_BUG_ON_MSG((_mask) > (typeof(_reg))~0ull,		\
> +	})
> +
> +#define __BF_FIELD_CHECK_REG(_mask, _reg, _pfx)				\
> +	({								\
> +		BUILD_BUG_ON_MSG((_mask) > (typeof(_reg))~0ULL,		\
>  				 _pfx "type of reg too small for mask"); \
> -		__BUILD_BUG_ON_NOT_POWER_OF_2((_mask) +			\
> -					      (1ULL << __bf_shf(_mask))); \
>  	})
>  
>  /**
> @@ -64,7 +72,7 @@
>   */
>  #define FIELD_MAX(_mask)						\
>  	({								\
> -		__BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_MAX: ");	\
> +		__BF_FIELD_CHECK_MASK(_mask, "FIELD_MAX: ");		\
>  		(typeof(_mask))((_mask) >> __bf_shf(_mask));		\
>  	})
>  
> @@ -77,7 +85,7 @@
>   */
>  #define FIELD_FIT(_mask, _val)						\
>  	({								\
> -		__BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_FIT: ");	\
> +		__BF_FIELD_CHECK_MASK(_mask, "FIELD_FIT: ");		\
>  		!((((typeof(_mask))_val) << __bf_shf(_mask)) & ~(_mask)); \
>  	})
>  
> @@ -91,7 +99,8 @@
>   */
>  #define FIELD_PREP(_mask, _val)						\
>  	({								\
> -		__BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ");	\
> +		__BF_FIELD_CHECK_MASK(_mask, "FIELD_PREP: ");		\
> +		__BF_FIELD_CHECK_VAL(_mask, _val, "FIELD_PREP: ");	\
>  		((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask);	\
>  	})
>  
> @@ -105,7 +114,8 @@
>   */
>  #define FIELD_GET(_mask, _reg)						\
>  	({								\
> -		__BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: ");	\
> +		__BF_FIELD_CHECK_MASK(_mask, "FIELD_GET: ");		\
> +		__BF_FIELD_CHECK_REG(_mask, _reg,  "FIELD_GET: ");	\
>  		(typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask));	\
>  	})
>  
>