[v6,0/7] bits: Fixed-type GENMASK_U*() and BIT_U*()

Message

Vincent Mailhol via B4 Relay March 7, 2025, 4:48 p.m. UTC

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:

  GENMASK_U16(16, 0)

will raise a build bug.

This series is a continuation of:

  https://lore.kernel.org/intel-xe/[e-mail]

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.

In v4 and onward, I introduce one big change: split the definition of
the asm and non-asm GENMASK(). I think this is controversial.
Especially, Yury commented that he did not want such split. So I
initially implemented a first draft in which both the asm and non-asm
version would rely on the same helper macro, i.e. adding this:

  #define __GENMASK_TYPE(t, w, h, l)			\
  	(((t)~_ULL(0) - ((t)1 << (l)) + 1) &		\
  	 ((t)~_ULL(0) >> (w - 1 - (h))))
    
to uapi/bits.h. And then, the different GENMASK()s would look like
this:

  #define __GENMASK(h, l) __GENMASK_TYPE(unsigned long, __BITS_PER_LONG, h, l)
    
and so on.
    
I implemented it, and the final result looks quite ugly. Not only do
we need to manually provide the width each time, the biggest concern
is that adding this to the uapi is asking for trouble. Who knows how
people are going to use this? And once it is in the uapi, there is
virtually no way back.

Adding to this, that macro can not even be generalized to u128
integers, whereas after the split, it is.

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. For asm, the long and long long variants seems sufficient.

And so, after implementing both, the asm and non-asm split seems way
more clean and I think this is the best compromise. Let me know what
you think :)

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: 4/2 up/down: 5/-4 (1)
  Function                                     old     new   delta
  intel_psr_invalidate                         666     668      +2
  mst_stream_compute_config                   1652    1653      +1
  intel_psr_flush                              977     978      +1
  intel_dp_compute_link_config                1327    1328      +1
  cfg80211_inform_bss_data                    5109    5108      -1
  intel_drrs_activate                          379     376      -3
  Total: Before=22723481, After=22723482, chg +0.00%

(done with GCC 12.4.1 on an x86_64 defconfig)

--
2.43.0

---
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/[e-mail]

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/[e-mail]

  - 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/[e-mail]

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/[e-mail]

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/[e-mail]

Link to v1: https://lore.kernel.org/intel-xe/[e-mail]

---
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 (3):
      bits: split the definition of the asm and non-asm GENMASK()
      test_bits: add tests for __GENMASK() and __GENMASK_ULL()
      test_bits: add tests for BIT_U*()

Yury Norov (1):
      bits: introduce fixed-type genmasks

 drivers/gpu/drm/i915/i915_reg_defs.h | 108 ++++-------------------------------
 include/linux/bitops.h               |   1 -
 include/linux/bits.h                 |  91 ++++++++++++++++++++---------
 lib/test_bits.c                      |  49 ++++++++++++++++
 4 files changed, 124 insertions(+), 125 deletions(-)
---
base-commit: 0312e94abe484b9ee58c32d2f8ba177e04955b35
change-id: 20250228-fixed-type-genmasks-8d1a555f34e8

Best regards,