linux-kernel - Re: LinuxPPS low-level IRQs timestamps & ldisc

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Message-ID: <20080601192613.GC16164@csclub.uwaterloo.ca>
Date:	Sun, 1 Jun 2008 15:26:13 -0400
From:	lsorense@...lub.uwaterloo.ca (Lennart Sorensen)
To:	Rodolfo Giometti <giometti@...eenne.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: LinuxPPS low-level IRQs timestamps & ldisc

On Sun, Jun 01, 2008 at 06:15:10PM +0200, Rodolfo Giometti wrote:
> Hello,
> 
> I write to both of you since my questions are related. :)
> 
> My last modifications to LinuxPPS are focused on adding a PPS ldisc
> and a low-level IRQs timestamps recording.
> 
> First I added a new dcd_change() ldisc method which can be used as
> follow:
> 
> static void pps_tty_dcd_change(struct tty_struct *tty, unsigned int status)
> {
>        int id = (int) tty->disc_data;
>        struct timespec __ts;
>        struct pps_ktime ts;
> 
>        /* First of all we get the time stamp... */
>        getnstimeofday(&__ts);
> 
>        /* ... and translate it to PPS time data struct */
>        ts.sec = __ts.tv_sec;
>        ts.nsec = __ts.tv_nsec;
> 
>        /* Now do the PPS event report */
>        pps_event(id, &ts,
>                        status ? PPS_CAPTUREASSERT : PPS_CAPTURECLEAR, tty);
> 
>        pr_debug("[STDev] PPS %s at %lu on source #%d\n",
>                        status ? "assert" : "clear", jiffies, id);
> }
> 
> However this solution gives very low precision timestamps so that's
> why I propose to add the following code to register IRQ timestamps as
> soon as possible (here the code for x86_32 architecture):
> 
> diff --git a/arch/x86/kernel/irq_32.c b/arch/x86/kernel/irq_32.c
> index d3fde94..a3b8457 100644
> --- a/arch/x86/kernel/irq_32.c
> +++ b/arch/x86/kernel/irq_32.c
> @@ -15,6 +15,7 @@
>  #include <linux/notifier.h>
>  #include <linux/cpu.h>
>  #include <linux/delay.h>
> +#include <linux/pps.h>
>  
>  #include <asm/apic.h>
>  #include <asm/uaccess.h>
> @@ -25,6 +26,11 @@ EXPORT_PER_CPU_SYMBOL(irq_stat);
>  DEFINE_PER_CPU(struct pt_regs *, irq_regs);
>  EXPORT_PER_CPU_SYMBOL(irq_regs);
>  
> +#ifdef CONFIG_PPS_IRQ_EVENTS
> +struct pps_ktime pps_irq_ts[NR_IRQS];
> +EXPORT_SYMBOL(pps_irq_ts);
> +#endif
> +
>  /*
>   * 'what should we do if we get a hw irq event on an illegal vector'.
>   * each architecture has to answer this themselves.
> @@ -76,6 +82,12 @@ fastcall unsigned int do_IRQ(struct pt_regs *regs)
>         union irq_ctx *curctx, *irqctx;
>         u32 *isp;
>  #endif
> +#ifdef CONFIG_PPS_IRQ_EVENTS
> +       struct timespec __ts;
> +
> +       /* Get IRQ timestamps as soon as possible for the PPS layer */
> +       getnstimeofday(&__ts);
> +#endif

How expensive is this call?  Would it be cheaper to check whether this
IRQ has anyone requesting the time info before doing it?  I suspect most
systems will have at most one IRQ that actually needs this done, so does
doing it for all IRQs make sense?

>         if (unlikely((unsigned)irq >= NR_IRQS)) {
>                 printk(KERN_EMERG "%s: cannot handle IRQ %d\n",
> @@ -83,6 +95,12 @@ fastcall unsigned int do_IRQ(struct pt_regs *regs)
>                 BUG();
>         }
>  
> +#ifdef CONFIG_PPS_IRQ_EVENTS
> +       /* Then, after sanity check, store the IRQ timestamp */
> +       pps_irq_ts[irq].sec = __ts.tv_sec;
> +       pps_irq_ts[irq].nsec = __ts.tv_nsec;
> +#endif
> +
>         old_regs = set_irq_regs(regs);
>         irq_enter();
>  #ifdef CONFIG_DEBUG_STACKOVERFLOW
> 
> This allow the following modifications to the dcd_change() method:
> 
> static void pps_tty_dcd_change(struct tty_struct *tty, unsigned int irq,
>                                unsigned int status)
> {
>        int id = (int) tty->disc_data;
> 
>        pps_event(id, &pps_irq_ts[irq],
>                        status ? PPS_CAPTUREASSERT : PPS_CAPTURECLEAR, tty);
> 
>        pr_debug("[IRQev] PPS %s at %lu on source #%d\n",
>                        status ? "assert" : "clear", jiffies, id);
> }
> 
> Note that in this case I need also the "irq" parameter.
> 
> This solution gives very high timestamps precision! Better that
> before! :D
> 
> What do you think about this solution? More precisely:
> 
> 1) The low-level IRQ patch can be acceptable?
> 
> 2) The new dcd_change() method is well implemented? Can I add the
> "irq" parameter or I can find it somewhere?
> 
> Thanks a lot for your suggestions,

I certainly like high precision time stamps.  Very useful for ntp.

-- 
Len Sorensen
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