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SubjectRe: LinuxPPS low-level IRQs timestamps & ldisc
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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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