Re: [PATCH v7 0/5] bits: Fixed-type GENMASK_U*() and BIT_U*()

[Yury Norov <yury.norov@gmail.com>]

On Sat, Mar 22, 2025 at 06:23:11PM +0900, Vincent Mailhol via B4 Relay wrote:
> Introduce some fixed width variant of the GENMASK() and the BIT()
> macros in bits.h. Note that the main goal is not to get the correct
> type, but rather to enforce more checks at compile time. For example:

You say this, and then typecast both BIT and GENMASK. This may confuse
readers. Maybe add few words about promotion rules in C standard, or
just drop this note entirely? Doesn't require new submission, of
course.

>   GENMASK_U16(16, 0)
> 
> will raise a build bug.
> 
> This series is a continuation of:
> 
>   https://lore.kernel.org/intel-xe/20240208074521.577076-1-lucas.demarchi@intel.com
> 
> from Lucas De Marchi. Above series is one year old. I really think
> that this was a good idea and I do not want this series to die. So I
> am volunteering to revive it.
> 
> Meanwhile, many changes occurred in bits.h. The most significant
> change is that __GENMASK() was moved to the uapi headers. For this
> reason, a new GENMASK_TYPE() is introduced instead and the uapi
> __GENMASK() is left untouched.
> 
> Finally, I do not think it makes sense to expose the fixed width
> variants to the asm. The fixed width integers type are a C concept. So
> the GENMASK_U*() are only visible to the non-asm code. For asm, the
> long and long long variants seems sufficient.
> 
> This series does not modify the actual GENMASK(), GENMASK_ULL() and
> GENMASK_U128(). A consolidation of the existing genmasks will be
> proposed later on in a separate series.
> 
> As requested, here are the bloat-o-meter stats:
> 
>   $ ./scripts/bloat-o-meter vmlinux_before.o vmlinux_after.o 
>   add/remove: 0/0 grow/shrink: 0/0 up/down: 0/0 (0)
>   Function                                     old     new   delta
>   Total: Before=22723481, After=22723481, chg +0.00%
> 
> (done with GCC 12.4.1 on an x86_64 defconfig)
> 
> --
> 2.43.0
> 
> ---
> Changes from v6:
> 
>   - Split the series in two: this series leave any existing GENMASK*()
>     unmodified. The consolidation will be done in a separate series.
> 
>   - Collect some Reviewed-by tag.
> 
>   - Address miscellaneous nitpick on the code comments and the line
>     wrapping (details in each patch).
> 
>   - Link to v6: https://lore.kernel.org/r/20250308-fixed-type-genmasks-v6-0-f59315e73c29@wanadoo.fr
> 
> Changes from v5:
> 
>   - Update the cover letter message. I was still refering to
>     GENMASK_t() instead of GENMASK_TYPE().
> 
>   - Add a comment in the cover letter to explain that a common
>     GENMASK_TYPE() for C and asm wouldn't allow to generate the u128
>     variant.
> 
>   - Restore the comment saying that BUILD_BUG_ON() is not available in
>     asm code.
> 
>   - Add a FIXME message to highlight the absence of the asm GENMASK*()
>     unit tests.
> 
>   - Use git's histogram diff algorithm
> 
>   - Link to v5: https://lore.kernel.org/r/20250306-fixed-type-genmasks-v5-0-b443e9dcba63@wanadoo.fr
> 
> Changes from v4:
> 
>   - Rebase on https://github.com/norov/linux/tree/bitmap-for-next
> 
>   - Rename GENMASK_t() to GENMASK_TYPE()
> 
>   - First patch of v4 (the typo fix 'init128' -> 'int128') is removed
>     because it was resent separately in:
>     https://lore.kernel.org/all/20250305-fix_init128_typo-v1-1-cbe5b8e54e7d@wanadoo.fr
> 
>   - Replace the (t)~ULL(0) by type_max(t). This way, GENMASK_TYPE()
>     can now be used to generate GENMASK_U128().
> 
>   - Get rid of the unsigned int cast for the U8 and U16 variants.
> 
>   - Add the BIT_TYPE() helper macro.
> 
>   - Link to v4: https://lore.kernel.org/r/20250305-fixed-type-genmasks-v4-0-1873dcdf6723@wanadoo.fr
> 
> Changes from v3:
> 
>   - Rebase on v6.14-rc5
> 
>   - Fix a typo in GENMASK_U128() comment.
> 
>   - Split the asm and non-asm definition of 
> 
>   - Replace ~0ULL by ~ULL(0)
> 
>   - Since v3, __GENMASK() was moved to the uapi and people started
>     using directly. Introduce GENMASK_t() instead.
> 
>   - Link to v3: https://lore.kernel.org/intel-xe/20240208074521.577076-1-lucas.demarchi@intel.com
> 
> Changes from v2:
> 
>   - Document both in commit message and code about the strict type
>     checking and give examples how it´d break with invalid params.
> 
>   - Link to v2: https://lore.kernel.org/intel-xe/20240124050205.3646390-1-lucas.demarchi@intel.com
> 
> Link to v1: https://lore.kernel.org/intel-xe/20230509051403.2748545-1-lucas.demarchi@intel.com
> 
> ---
> Lucas De Marchi (3):
>       bits: introduce fixed-type BIT_U*()
>       drm/i915: Convert REG_GENMASK*() to fixed-width GENMASK_U*()
>       test_bits: add tests for GENMASK_U*()
> 
> Vincent Mailhol (2):
>       bits: introduce fixed-type GENMASK_U*()
>       test_bits: add tests for BIT_U*()
> 
>  drivers/gpu/drm/i915/i915_reg_defs.h | 108 ++++-------------------------------
>  include/linux/bitops.h               |   1 -
>  include/linux/bits.h                 |  57 +++++++++++++++++-
>  lib/test_bits.c                      |  30 ++++++++++
>  4 files changed, 96 insertions(+), 100 deletions(-)
> ---
> base-commit: e3f42c436d7e0cb432935fe3ae275dd8d9b60f71
> change-id: 20250228-fixed-type-genmasks-8d1a555f34e8
> 
> Best regards,
> -- 
> Vincent Mailhol <mailhol.vincent@wanadoo.fr>
>