linux-kernel - Re: [RFC 02/18] power/reset: vexpress: Use sched

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Message-ID: <CANcMJZALQ0khcqkdQA5Tnr6_9kfWK=UUmrUB4mCv2MEkmLv0sg@mail.gmail.com>
Date:	Mon, 23 Dec 2013 11:28:28 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Pawel Moll <pawel.moll@....com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Arnd Bergmann <arnd@...db.de>, Jon Medhurst <tixy@...aro.org>,
	arm@...nel.org, Olof Johansson <olof@...om.net>,
	Anton Vorontsov <anton@...msg.org>,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: [RFC 02/18] power/reset: vexpress: Use sched_clock as the time source

On Mon, Dec 23, 2013 at 8:23 AM, Pawel Moll <pawel.moll@....com> wrote:
> At this stage of system shutdown procedure the jiffies may
> not be updated anymore, so have to base on raw sched_clock
> values.
>
> Signed-off-by: Pawel Moll <pawel.moll@....com>
> Cc: Anton Vorontsov <anton@...msg.org>
> Cc: David Woodhouse <dwmw2@...radead.org>
> ---
>  drivers/power/reset/vexpress-poweroff.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/power/reset/vexpress-poweroff.c b/drivers/power/reset/vexpress-poweroff.c
> index 476aa49..d752233 100644
> --- a/drivers/power/reset/vexpress-poweroff.c
> +++ b/drivers/power/reset/vexpress-poweroff.c
> @@ -15,6 +15,7 @@
>  #include <linux/of.h>
>  #include <linux/of_device.h>
>  #include <linux/platform_device.h>
> +#include <linux/sched.h>
>  #include <linux/stat.h>
>  #include <linux/vexpress.h>
>
> @@ -27,12 +28,12 @@ static void vexpress_reset_do(struct device *dev, const char *what)
>                         vexpress_config_func_get_by_dev(dev);
>
>         if (func) {
> -               unsigned long timeout;
> +               unsigned long long timeout_ns;
>
>                 err = vexpress_config_write(func, 0, 0);
>
> -               timeout = jiffies + HZ;
> -               while (time_before(jiffies, timeout))
> +               timeout_ns = sched_clock() + 50000000;
> +               while (!err && time_before64(sched_clock(), timeout_ns))
>                         cpu_relax();
>         }

So this may not be a problem in this particular case, but sched_clock
could be backed by jiffies on some hardware, causing the same problem
to appear.

Might udelay/mdelay be a better fit for this sort of case (since
udelay may be counter backed, but may also be loop backed on hardware
without continuous counters)?

thanks
-john
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