vdso/math64: Add and use __iter_div64_u64_rem()

The vDSO basetimes for CLOCK_MONOTONIC and CLOCK_BOOTTIME are stored in
the scaled nanoseconds of tkr_mono, so normalising them requires a
division by NSEC_PER_SEC << shift.  That divisor does not fit the u32
parameter of __iter_div_u64_rem(), so update_vdso_time_data() open-codes
the same iterative division twice.

Repeated subtraction is the appropriate form at these two sites because
the quotient never exceeds one.  accumulate_nsecs_to_secs() keeps
xtime_nsec below one scaled second, and the offset added to it is a
normalised timespec64 fraction, so the dividend stays below twice the
divisor.  That bound holds on every architecture, which matters more here
than the cost of a division on any particular one.

Add __iter_div64_u64_rem(), the u64-divisor counterpart of
__iter_div_u64_rem(), and use it at both sites.  Return the quotient as a
u32 for consistency with the u32-divisor variant; the bound above leaves
no use for a wider type.

Store the remainder directly into the basetime, as the coarse clocks
already do.  The CLOCK_BOOTTIME copy of the CLOCK_MONOTONIC values then
takes both fields from the same place.

Folding the two open-coded loops into one inlined helper removes 16 bytes
of vsyscall.o text on x86-64 with gcc 13.

No functional change.

Suggested-by: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://patch.msgid.link/20261002153104.1347978-3-zhanxusheng@xiaomi.com

0 files changed, 0 insertions, 0 deletions