target/linux/generic/backport-6.1/200-v6.3-bitfield-add-FIELD_PREP_CONST.patch

   1 From e2192de59e457aef8d1f055a452131f0b3e5d097 Mon Sep 17 00:00:00 2001
   2 From: Johannes Berg <johannes.berg@intel.com>
   3 Date: Wed, 18 Jan 2023 14:26:53 +0100
   4 Subject: [PATCH] bitfield: add FIELD_PREP_CONST()
   5
   6 Neither FIELD_PREP() nor *_encode_bits() can be used
   7 in constant contexts (such as initializers), but we
   8 don't want to define shift constants for all masks
   9 just for use in initializers, and having checks that
  10 the values fit is also useful.
  11
  12 Therefore, add FIELD_PREP_CONST() which is a smaller
  13 version of FIELD_PREP() that can only take constant
  14 arguments and has less friendly (but not less strict)
  15 error checks, and expands to a constant value.
  16
  17 Signed-off-by: Johannes Berg <johannes.berg@intel.com>
  18 Link: https://lore.kernel.org/r/20230118142652.53f20593504b.Iaeea0aee77a6493d70e573b4aa55c91c00e01e4b@changeid
  19 Signed-off-by: Johannes Berg <johannes.berg@intel.com>
  20 ---
  21  include/linux/bitfield.h | 26 ++++++++++++++++++++++++++
  22  1 file changed, 26 insertions(+)
  23
  24 --- a/include/linux/bitfield.h
  25 +++ b/include/linux/bitfield.h
  26 @@ -115,6 +115,32 @@
  27                 ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask);   \
  28         })
  29
  30 +#define __BF_CHECK_POW2(n)     BUILD_BUG_ON_ZERO(((n) & ((n) - 1)) != 0)
  31 +
  32 +/**
  33 + * FIELD_PREP_CONST() - prepare a constant bitfield element
  34 + * @_mask: shifted mask defining the field's length and position
  35 + * @_val:  value to put in the field
  36 + *
  37 + * FIELD_PREP_CONST() masks and shifts up the value.  The result should
  38 + * be combined with other fields of the bitfield using logical OR.
  39 + *
  40 + * Unlike FIELD_PREP() this is a constant expression and can therefore
  41 + * be used in initializers. Error checking is less comfortable for this
  42 + * version, and non-constant masks cannot be used.
  43 + */
  44 +#define FIELD_PREP_CONST(_mask, _val)                                  \
  45 +       (                                                               \
  46 +               /* mask must be non-zero */                             \
  47 +               BUILD_BUG_ON_ZERO((_mask) == 0) +                       \
  48 +               /* check if value fits */                               \
  49 +               BUILD_BUG_ON_ZERO(~((_mask) >> __bf_shf(_mask)) & (_val)) + \
  50 +               /* check if mask is contiguous */                       \
  51 +               __BF_CHECK_POW2((_mask) + (1ULL << __bf_shf(_mask))) +  \
  52 +               /* and create the value */                              \
  53 +               (((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask))  \
  54 +       )
  55 +
  56  /**
  57   * FIELD_GET() - extract a bitfield element
  58   * @_mask: shifted mask defining the field's length and position