Re: [PATCH v5 0/7] bits: Fixed-type GENMASK()/BIT()

Andy Shevchenko
2025-03-06T13:02:50Z

On Thu, Mar 06, 2025 at 08:29:51PM +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:
>
> 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.
>
> In v4 an 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_t(t, w, h, l) \

I thought we agreed on renaming...

> (((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_t(unsigned long, __BITS_PER_LONG, h, l)

Ditto.

> 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.
>
> 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 a defconfig)

What defconfig? x86_64_defconfig?

--
With Best Regards,
Andy Shevchenko