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

From: Jakub Kicinski <[email-masked]>
Date: Mon, 29 Aug 2016 17:07:39 +0200

On Mon, 29 Aug 2016 16:34:25 +0200, Daniel Borkmann wrote:
> 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.

I went with single underscore, because my understanding was:
 - no underscore - safe, "user-facing" API;
 - two underscores - internal, make sure you know how to use it;
 - single underscore - library internals, shouldn't be touched.

I don't expect anyone to invoke those macros, the underscore is
there to avoid collisions. 

> > +	({								\
> > +		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?

FWIW PREP was suggested by Linus:

https://lkml.org/lkml/2016/8/17/384

> > +#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 ...)?

It would, I'm doing:
  mask + lowest bit of mask
which will result in:
  highest bit of mask << 1
which in turn will overflow for masks with highest bit set.