linux-kernel - Re: [PATCH v2 6/7] xen/arm: introduce xen_read

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Date:	Tue, 10 Nov 2015 13:35:41 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org,
	Ian.Campbell@...rix.com
Subject: Re: [PATCH v2 6/7] xen/arm: introduce xen_read_wallclock

On Tuesday 10 November 2015 11:57:54 Stefano Stabellini wrote:
> +static void xen_read_wallclock(struct timespec64 *ts)
> +{
> +       u32 version;
> +       u64 delta;
> +       struct timespec64 now;
> +       struct shared_info *s = HYPERVISOR_shared_info;
> +       struct pvclock_wall_clock *wall_clock = &(s->wc);
> +
> +       /* get wallclock at system boot */
> +       do {
> +               version = wall_clock->version;
> +               rmb();          /* fetch version before time */
> +               now.tv_sec  = ((uint64_t)wall_clock->sec_hi << 32) | wall_clock->sec;
> +               now.tv_nsec = wall_clock->nsec;
> +               rmb();          /* fetch time before checking version */
> +       } while ((wall_clock->version & 1) || (version != wall_clock->version));
> +
> +       delta = arch_timer_read_counter() * (u64)NSEC_PER_SEC;
> +       do_div(delta, arch_timer_get_rate());     /* time since system boot */
> +       delta += now.tv_sec * (u64)NSEC_PER_SEC + now.tv_nsec;
> +
> +       now.tv_nsec = do_div(delta, NSEC_PER_SEC);
> +       now.tv_sec = delta;
> +
> +       set_normalized_timespec64(ts, now.tv_sec, now.tv_nsec);
> +
> +}

Instead of the two do_div(), I would do the entire calculation in
terms of nanoseconds and then call ns_to_timespec64() in the end
instead of set_normalized_timespec64(). That is just an optimization
though, your version looks correct as well.

	Arnd
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