Subject: Re: [RFCv2 01/16] add basic register-field manipulation macros

From: Daniel Borkmann <[email-masked]>
Date: Mon, 29 Aug 2016 16:34:25 +0200

On 08/26/2016 08:06 PM, Jakub Kicinski wrote:
> Common approach to accessing register fields is to define
> structures or sets of macros containing mask and shift pair.
> Operations on the register are then performed as follows:
>
>   field = (reg >> shift) & mask;
>
>   reg &= ~(mask << shift);
>   reg |= (field & mask) << shift;
>
> Defining shift and mask separately is tedious.  Ivo van Doorn
> came up with an idea of computing them at compilation time
> based on a single shifted mask (later refined by Felix) which
> can be used like this:
>
>   #define REG_FIELD 0x000ff000
>
>   field = FIELD_GET(REG_FIELD, reg);
>
>   reg &= ~REG_FIELD;
>   reg |= FIELD_PREP(REG_FIELD, field);
>
> FIELD_{GET,PREP} macros take care of finding out what the
> appropriate shift is based on compilation time ffs operation.
>
> GENMASK can be used to define registers (which is usually
> less error-prone and easier to match with datasheets).
>
> This approach is the most convenient I've seen so to limit code
> multiplication let's move the macros to a global header file.
> Attempts to use static inlines instead of macros failed due
> to false positive triggering of BUILD_BUG_ON()s, especially with
> GCC < 6.0.
>
> Signed-off-by: Jakub Kicinski <[email-masked]>
[...]
> + * Bitfield access macros
> + *
> + * FIELD_{GET,PREP} macros take as first parameter shifted mask
> + * from which they extract the base mask and shift amount.
> + * Mask must be a compilation time constant.
> + *
> + * Example:
> + *
> + *  #define REG_FIELD_A  GENMASK(6, 0)
> + *  #define REG_FIELD_B  BIT(7)
> + *  #define REG_FIELD_C  GENMASK(15, 8)
> + *  #define REG_FIELD_D  GENMASK(31, 16)
> + *
> + * Get:
> + *  a = FIELD_GET(REG_FIELD_A, reg);
> + *  b = FIELD_GET(REG_FIELD_B, reg);
> + *
> + * Set:
> + *  reg = FIELD_PREP(REG_FIELD_A, 1) |
> + *	  FIELD_PREP(REG_FIELD_B, 0) |
> + *	  FIELD_PREP(REG_FIELD_C, c) |
> + *	  FIELD_PREP(REG_FIELD_D, 0x40);
> + *
> + * Modify:
> + *  reg &= ~REG_FIELD_C;
> + *  reg |= FIELD_PREP(REG_FIELD_C, c);
> + */
> +
> +#define _bf_shf(x) (__builtin_ffsll(x) - 1)
> +
> +#define _BF_FIELD_CHECK(_mask, _reg, _val, _pfx)			\

Nit: if possible, please always use "__" instead of "_" as prefix, which is
more common coding style in the kernel.

> +	({								\
> +		BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),		\
> +				 _pfx "mask is not constant");		\
> +		BUILD_BUG_ON_MSG(!(_mask), _pfx "mask is zero");	\
> +		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,		\
> +				 _pfx "type of reg too small for mask"); \
> +		__BUILD_BUG_ON_NOT_POWER_OF_2((_mask) +			\
> +					      (1ULL << _bf_shf(_mask))); \
> +	})
> +
> +/**
> + * FIELD_PREP() - prepare a bitfield element
> + * @_mask: shifted mask defining the field's length and position
> + * @_val:  value to put in the field
> + *
> + * FIELD_PREP() masks and shifts up the value.  The result should
> + * be combined with other fields of the bitfield using logical OR.
> + */
> +#define FIELD_PREP(_mask, _val)						\
> +	({								\
> +		_BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ");	\
> +		((typeof(_mask))(_val) << _bf_shf(_mask)) & (_mask);	\
> +	})
> +
> +/**
> + * FIELD_GET() - extract a bitfield element
> + * @_mask: shifted mask defining the field's length and position
> + * @_reg:  32bit value of entire bitfield
> + *
> + * FIELD_GET() extracts the field specified by @_mask from the
> + * bitfield passed in as @_reg by masking and shifting it down.
> + */
> +#define FIELD_GET(_mask, _reg)						\
> +	({								\
> +		_BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: ");	\
> +		(typeof(_mask))(((_reg) & (_mask)) >> _bf_shf(_mask));	\
> +	})

No strong opinion, but FIELD_PREP() sounds a bit weird. Maybe rather a
FIELD_GEN() (aka "generate") and FIELD_GET() pair?

> +#endif
> diff --git a/include/linux/bug.h b/include/linux/bug.h
> index e51b0709e78d..292d6a10b0c2 100644
> --- a/include/linux/bug.h
> +++ b/include/linux/bug.h
> @@ -13,6 +13,7 @@ enum bug_trap_type {
>   struct pt_regs;
>
>   #ifdef __CHECKER__
> +#define __BUILD_BUG_ON_NOT_POWER_OF_2(n) (0)
>   #define BUILD_BUG_ON_NOT_POWER_OF_2(n) (0)
>   #define BUILD_BUG_ON_ZERO(e) (0)
>   #define BUILD_BUG_ON_NULL(e) ((void*)0)
> @@ -24,6 +25,8 @@ struct pt_regs;
>   #else /* __CHECKER__ */
>
>   /* Force a compilation error if a constant expression is not a power of 2 */
> +#define __BUILD_BUG_ON_NOT_POWER_OF_2(n)	\
> +	BUILD_BUG_ON(((n) & ((n) - 1)) != 0)

Is there a reason BUILD_BUG_ON_NOT_POWER_OF_2(n) cannot be reused?

Because the (n) == 0 check would trigger (although it shouldn't ...)?

>   #define BUILD_BUG_ON_NOT_POWER_OF_2(n)			\
>   	BUILD_BUG_ON((n) == 0 || (((n) & ((n) - 1)) != 0))
>
>