Re: [PATCH 0/3] bits: Split asm and non-asm GENMASK*() and unify definitions

[Yury Norov]

+ Anshuman Khandual, Catalin Marinas, linux-arm-kernel@xxxxxxxxxxxxxxxxxxx

This series moves GENMASK_U128 out of uapi. ARM is the only proposed
user. Add ARM people for visibility.

Thanks,
Yury

On Sat, Mar 22, 2025 at 07:39:35PM +0900, Vincent Mailhol via B4 Relay wrote:
> This is a subset of below series:
> 
>   bits: Fixed-type GENMASK_U*() and BIT_U*()
>   Link: https://lore.kernel.org/r/20250308-fixed-type-genmasks-v6-0-f59315e73c29@xxxxxxxxxx
> 
> Yury suggested to split the above series in two steps:
> 
>   #1 Introduce the new fixed type GENMASK_U*()
>   #2 Consolidate the existing GENMASK*()
> 
> This new series is the resulting step #2 following the split.
> 
> And thus, this series consolidate all the non-asm GENMASK*() so that
> they now all depend on GENMASK_TYPE() which was introduced in step #1.
> 
> To do so, I had to split the definition of the asm and non-asm
> GENMASK(). I think this is controversial. 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) << (l)) &			\
>   	 ((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 looked 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 can.
> 
> And so, after implementing both, the asm and non-asm split seems way
> more clean and I think this is the best compromise.
> 
> Aside from the split, the asm's GENMASK() and GENMASK_ULL() are left
> untouched. While there are some strong incentives to also simplify
> these as pointed by David Laight in this thread:
> 
>   https://lore.kernel.org/all/20250309102312.4ff08576@pumpkin/
> 
> this series deliberately limit its scope to the non-asm variants.
> 
> 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
> 
> Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
> ---
> Vincent Mailhol (3):
>       bits: split the definition of the asm and non-asm GENMASK*()
>       bits: unify the non-asm GENMASK*()
>       test_bits: add tests for __GENMASK() and __GENMASK_ULL()
> 
>  include/linux/bits.h | 29 ++++++-----------------------
>  lib/test_bits.c      | 19 +++++++++++++++++++
>  2 files changed, 25 insertions(+), 23 deletions(-)
> ---
> base-commit: e3f42c436d7e0cb432935fe3ae275dd8d9b60f71
> change-id: 20250320-consolidate-genmask-6cd02abadf82
> prerequisite-change-id: 20250228-fixed-type-genmasks-8d1a555f34e8:v7
> prerequisite-patch-id: 572c05165229640db7dd8fe4d53e1a33ee5dd586
> prerequisite-patch-id: c16d122a487f83e2866a9a669259db097ef46a70
> prerequisite-patch-id: 35f115c0f1b327f1516cfc38b3076e07713df6cd
> prerequisite-patch-id: 5fe7058f6ea73b37df75d5c39ad69a4da928058d
> prerequisite-patch-id: 82fb628d052ce9f1efac7f3b61eafb2749f95847
> 
> Best regards,
> -- 
> Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
>