linux-kernel - Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation.

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Date:   Fri, 20 Dec 2019 19:24:22 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        vincenzo.frascino@....com, luto@...nel.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation.

Hi Thomas,

In do_hres(), I see:

		cycles = __arch_get_hw_counter(vd->clock_mode);
		ns = vdso_ts->nsec;
		last = vd->cycle_last;
		if (unlikely((s64)cycles < 0))
			return -1;

__arch_get_hw_counter() returns a u64 values. On the PPC, this is read 
from the timebase which is a 64 bits counter.

Why returning -1 if (s64)cycles < 0 ? Does it means we have to mask out 
the most significant bit when reading the HW counter ?

Christophe