Re: [PATCH] lib: count_zeros: fix 32/64-bit inconsistency in count_trailing_zeros()

Yury Norov
2026-03-13T18:15:11Z

On Fri, Mar 13, 2026 at 02:18:55PM -0300, Jason Gunthorpe wrote:
> On Thu, Mar 12, 2026 at 07:08:16PM -0400, Yury Norov wrote:
> > Based on 'sizeof(x) == 4' condition, in 32-bit case the function is wired
> > to ffs(), while in 64-bit case to __ffs(). The difference is substantial:
> > ffs(x) == __ffs(x) + 1. Also, ffs(0) == 0, while __ffs(0) is undefined.
> >
> > The 32-bit behaviour is inconsistent with the function description, so it
> > needs to get fixed.
> >
> > There are 9 individual users for the function in 6 different subsystems.
> > Some arches and drivers are 64-bit only:
> > - arch/loongarch/kvm/intc/eiointc.c;
> > - drivers/hv/mshv_vtl_main.c;
> > - kernel/liveupdate/kexec_handover.c;
> >
> > The others are:
> > - ib_umem_find_best_pgsz(): as per comment, __ffs() should be correct;
>
> So long as 32 bit works the same as 64 bit it is correct for ib

This is what the patch does, except that it doesn't account for the
word length. In you case, 'mask' is dma_addr_t, which is u32 or u64
depending ARCH_DMA_ADDR_T_64BIT.

This config is:

config ARCH_DMA_ADDR_T_64BIT
def_bool 64BIT || PHYS_ADDR_T_64BIT

And PHYS_ADDR_T_64BIT is simply def_bool 64BIT. So, at least now
dma_addr_t simply follows unsigned long, and thus, the patch is
correct. But IDK what's the history behind this configurations.

Anyways, the patch aligns 32-bit count_trailing_zeros() with the
64-bit one. If you OK with that, as you said, can you please send
an explicit ack?

Thanks,
Yury