diff mbox

[RFCv2,01/16] add basic register-field manipulation macros

Message ID 1472234775-29453-2-git-send-email-jakub.kicinski@netronome.com
State RFC, archived
Delegated to: David Miller
Headers show

Commit Message

Jakub Kicinski Aug. 26, 2016, 6:06 p.m. UTC 
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 <jakub.kicinski@netronome.com>
---
 include/linux/bitfield.h | 93 ++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/bug.h      |  3 ++
 2 files changed, 96 insertions(+)
 create mode 100644 include/linux/bitfield.h

Comments

Daniel Borkmann Aug. 29, 2016, 2:34 p.m. UTC | #1 
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 <jakub.kicinski@netronome.com>
[...]
> + * 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))
>
>
Jakub Kicinski Aug. 29, 2016, 3:07 p.m. UTC | #2 
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 <jakub.kicinski@netronome.com>  
> [...]
> > + * 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.
Daniel Borkmann Aug. 29, 2016, 3:40 p.m. UTC | #3 
On 08/29/2016 05:07 PM, Jakub Kicinski wrote:
> 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 <jakub.kicinski@netronome.com>
>> [...]
>>> + * 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.

That convention would be new to me, at least I haven't seen it much (see
also recent comment on the act_tunnel set). Still think two underscores
is generally preferred (unless this is somewhere documented otherwise).

> 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

Hmm, ok, fair enough.

>>> +#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.

Ahh, right.
diff mbox

Patch

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
new file mode 100644
index 000000000000..32ca8863e66d
--- /dev/null
+++ b/include/linux/bitfield.h
@@ -0,0 +1,93 @@ 
+/*
+ * Copyright (C) 2014 Felix Fietkau <nbd@nbd.name>
+ * Copyright (C) 2004 - 2009 Ivo van Doorn <IvDoorn@gmail.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2
+ * as published by the Free Software Foundation
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#ifndef _LINUX_BITFIELD_H
+#define _LINUX_BITFIELD_H
+
+#include <linux/bug.h>
+
+/*
+ * 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)			\
+	({								\
+		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));	\
+	})
+
+#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)
 #define BUILD_BUG_ON_NOT_POWER_OF_2(n)			\
 	BUILD_BUG_ON((n) == 0 || (((n) & ((n) - 1)) != 0))