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

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

On 08/26/2016 08:06 PM, Jakub Kicinski wrote:

quoted

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 <redacted>  

[...]

quoted

+ * 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. 

quoted

+	({								\
+		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

quoted

+#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.