generic: 6.1: backport FIELD_PREP_CONST patch

Backport FIELD_PREP_CONST patch needed for at803x backport patches to
correctly compile and work.

This MACRO is needed to treat values derived from FIELD_PREP usage as
const to be used by switch case or other needed usage.

Signed-off-by: Christian Marangi <[email protected]>

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

@@ -0,0 +1,58 @@
+From e2192de59e457aef8d1f055a452131f0b3e5d097 Mon Sep 17 00:00:00 2001
+From: Johannes Berg <[email protected]>
+Date: Wed, 18 Jan 2023 14:26:53 +0100
+Subject: [PATCH] bitfield: add FIELD_PREP_CONST()
+
+Neither FIELD_PREP() nor *_encode_bits() can be used
+in constant contexts (such as initializers), but we
+don't want to define shift constants for all masks
+just for use in initializers, and having checks that
+the values fit is also useful.
+
+Therefore, add FIELD_PREP_CONST() which is a smaller
+version of FIELD_PREP() that can only take constant
+arguments and has less friendly (but not less strict)
+error checks, and expands to a constant value.
+
+Signed-off-by: Johannes Berg <[email protected]>
+Link: https://lore.kernel.org/r/20230118142652.53f20593504b.Iaeea0aee77a6493d70e573b4aa55c91c00e01e4b@changeid
+Signed-off-by: Johannes Berg <[email protected]>
+---
+ include/linux/bitfield.h | 26 ++++++++++++++++++++++++++
+ 1 file changed, 26 insertions(+)
+
+--- a/include/linux/bitfield.h
++++ b/include/linux/bitfield.h
+@@ -115,6 +115,32 @@
+ 		((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask);	\
+ 	})
+ 
++#define __BF_CHECK_POW2(n)	BUILD_BUG_ON_ZERO(((n) & ((n) - 1)) != 0)
++
++/**
++ * FIELD_PREP_CONST() - prepare a constant bitfield element
++ * @_mask: shifted mask defining the field's length and position
++ * @_val:  value to put in the field
++ *
++ * FIELD_PREP_CONST() masks and shifts up the value.  The result should
++ * be combined with other fields of the bitfield using logical OR.
++ *
++ * Unlike FIELD_PREP() this is a constant expression and can therefore
++ * be used in initializers. Error checking is less comfortable for this
++ * version, and non-constant masks cannot be used.
++ */
++#define FIELD_PREP_CONST(_mask, _val)					\
++	(								\
++		/* mask must be non-zero */				\
++		BUILD_BUG_ON_ZERO((_mask) == 0) +			\
++		/* check if value fits */				\
++		BUILD_BUG_ON_ZERO(~((_mask) >> __bf_shf(_mask)) & (_val)) + \
++		/* check if mask is contiguous */			\
++		__BF_CHECK_POW2((_mask) + (1ULL << __bf_shf(_mask))) +	\
++		/* and create the value */				\
++		(((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask))	\
++	)
++
+ /**
+  * FIELD_GET() - extract a bitfield element
+  * @_mask: shifted mask defining the field's length and position