From: David Laight <david.laight.linux@gmail.com>

Copied almost verbatim from the commit message that added the functions.

Signed-off-by: David Laight <david.laight.linux@gmail.com>
---
 include/linux/bitfield.h | 43 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index bfd80ebd25b1..9feb489a8da3 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -181,6 +181,49 @@ do {								\
 	*_reg_p = (*_reg_p & ~_mask) | ((_val << __bf_shf(_mask)) & _mask);	\
 })
 
+/*
+ * Primitives for manipulating bitfields both in host- and fixed-endian.
+ *
+ * * u32 le32_get_bits(__le32 val, u32 field) extracts the contents of the
+ *   bitfield specified by @field in little-endian 32bit object @val and
+ *   converts it to host-endian.
+ *
+ * * void le32p_replace_bits(__le32 *p, u32 v, u32 field) replaces
+ *   the contents of the bitfield specified by @field in little-endian
+ *   32bit object pointed to by @p with the value of @v.  New value is
+ *   given in host-endian and stored as little-endian.
+ *
+ * * __le32 le32_replace_bits(__le32 old, u32 v, u32 field) is equivalent to
+ *   ({__le32 tmp = old; le32p_replace_bits(&tmp, v, field); tmp;})
+ *   In other words, instead of modifying an object in memory, it takes
+ *   the initial value and returns the modified one.
+ *
+ * * __le32 le32_encode_bits(u32 v, u32 field) is equivalent to
+ *   le32_replace_bits(0, v, field).  In other words, it returns a little-endian
+ *   32bit object with the bitfield specified by @field containing the
+ *   value of @v and all bits outside that bitfield being zero.
+ *
+ * Such set of helpers is defined for each of little-, big- and host-endian
+ * types; e.g. u64_get_bits(val, field) will return the contents of the bitfield
+ * specified by @field in host-endian 64bit object @val, etc.  Of course, for
+ * host-endian no conversion is involved.
+ *
+ * Fields to access are specified as GENMASK() values - an N-bit field
+ * starting at bit #M is encoded as GENMASK(M + N - 1, M).  Note that
+ * bit numbers refer to endianness of the object we are working with -
+ * e.g. GENMASK(11, 0) in __be16 refers to the second byte and the lower
+ * 4 bits of the first byte.  In __le16 it would refer to the first byte
+ * and the lower 4 bits of the second byte, etc.
+ *
+ * Field specification must be a constant; __builtin_constant_p() doesn't
+ * have to be true for it, but compiler must be able to evaluate it at
+ * build time.  If it cannot or if the value does not encode any bitfield,
+ * the build will fail.
+ *
+ * If the value being stored in a bitfield is a constant that does not fit
+ * into that bitfield, a warning will be generated at compile time.
+ */
+
 extern void __compiletime_error("value doesn't fit into mask")
 __field_overflow(void);
 extern void __compiletime_error("bad bitfield mask")
-- 
2.39.5

On Tue,  9 Dec 2025 10:03:12 +0000 david.laight.linux@gmail.com wrote:
> + * * u32 le32_get_bits(__le32 val, u32 field) extracts the contents of the
> + *   bitfield specified by @field in little-endian 32bit object @val and
> + *   converts it to host-endian.

possibly also add declarations for these? So that ctags and co. sees
them?

On Wed, 10 Dec 2025 18:23:00 +0900
Jakub Kicinski <kuba@kernel.org> wrote:

> On Tue,  9 Dec 2025 10:03:12 +0000 david.laight.linux@gmail.com wrote:
> > + * * u32 le32_get_bits(__le32 val, u32 field) extracts the contents of the
> > + *   bitfield specified by @field in little-endian 32bit object @val and
> > + *   converts it to host-endian.  
> 
> possibly also add declarations for these? So that ctags and co. sees
> them?

The functions are bulk-generated using a #define, ctags is never going to
find definitions.

Adding kerneldoc comments is also painful.
I don't think it lets you use a single comment for multiple functions.

	David

On Wed, 10 Dec 2025 10:08:46 +0000 David Laight wrote:
> > possibly also add declarations for these? So that ctags and co. sees
> > them?  
> 
> The functions are bulk-generated using a #define, ctags is never going to
> find definitions.

I know. That's why I said declarations. But for code completions etc
decl is enough.

On Tue, Dec 09, 2025 at 10:03:12AM +0000, david.laight.linux@gmail.com wrote:

> Copied almost verbatim from the commit message that added the functions.

...

> +/*

Can it be a global DOC for being processed by kernel-doc?

> + * Primitives for manipulating bitfields both in host- and fixed-endian.
> + *
> + * * u32 le32_get_bits(__le32 val, u32 field) extracts the contents of the
> + *   bitfield specified by @field in little-endian 32bit object @val and
> + *   converts it to host-endian.
> + *
> + * * void le32p_replace_bits(__le32 *p, u32 v, u32 field) replaces
> + *   the contents of the bitfield specified by @field in little-endian
> + *   32bit object pointed to by @p with the value of @v.  New value is
> + *   given in host-endian and stored as little-endian.
> + *
> + * * __le32 le32_replace_bits(__le32 old, u32 v, u32 field) is equivalent to
> + *   ({__le32 tmp = old; le32p_replace_bits(&tmp, v, field); tmp;})
> + *   In other words, instead of modifying an object in memory, it takes
> + *   the initial value and returns the modified one.
> + *
> + * * __le32 le32_encode_bits(u32 v, u32 field) is equivalent to
> + *   le32_replace_bits(0, v, field).  In other words, it returns a little-endian
> + *   32bit object with the bitfield specified by @field containing the
> + *   value of @v and all bits outside that bitfield being zero.
> + *
> + * Such set of helpers is defined for each of little-, big- and host-endian
> + * types; e.g. u64_get_bits(val, field) will return the contents of the bitfield
> + * specified by @field in host-endian 64bit object @val, etc.  Of course, for
> + * host-endian no conversion is involved.
> + *
> + * Fields to access are specified as GENMASK() values - an N-bit field
> + * starting at bit #M is encoded as GENMASK(M + N - 1, M).  Note that
> + * bit numbers refer to endianness of the object we are working with -
> + * e.g. GENMASK(11, 0) in __be16 refers to the second byte and the lower
> + * 4 bits of the first byte.  In __le16 it would refer to the first byte
> + * and the lower 4 bits of the second byte, etc.
> + *
> + * Field specification must be a constant; __builtin_constant_p() doesn't
> + * have to be true for it, but compiler must be able to evaluate it at
> + * build time.  If it cannot or if the value does not encode any bitfield,
> + * the build will fail.
> + *
> + * If the value being stored in a bitfield is a constant that does not fit
> + * into that bitfield, a warning will be generated at compile time.
> + */

-- 
With Best Regards,
Andy Shevchenko