Re: [PATCH 8/9] bitfield: Add comment block for the host/fixed endian functions

Andy Shevchenko
2025-12-09T15:53:40Z

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