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    SubjectRe: [PATCH v3] printk: add halt_delay parameter for printk delay in halt phase
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    On Mon, Jun 8, 2009 at 4:28 PM, Andrew Morton<akpm@linux-foundation.org> wrote:
    > On Mon, 8 Jun 2009 10:14:39 +0200 Ingo Molnar <mingo@elte.hu> wrote:
    >
    >>
    >> * Dave Young <hidave.darkstar@gmail.com> wrote:
    >>
    >> > Add a halt_delay module parameter in printk.c used to read the
    >> > printk messages in halt/poweroff/restart phase, delay each printk
    >> > messages by halt_delay milliseconds. It is useful for debugging if
    >> > there's no other way to dump kernel messages that time.
    >> >
    >> > The halt_delay max value is 65535, default value is 0, change it
    >> > by:
    >> >
    >> > echo xxx > /sys/module/printk/parameters/halt_delay
    >> >
    >> > Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
    >> > ---
    >> > Documentation/kernel-parameters.txt |    5 +++++
    >> > kernel/printk.c                     |   17 +++++++++++++++++
    >> > 2 files changed, 22 insertions(+)
    >> >
    >> > --- linux-2.6.orig/kernel/printk.c  2009-06-08 13:55:35.000000000 +0800
    >> > +++ linux-2.6/kernel/printk.c       2009-06-08 13:56:23.000000000 +0800
    >> > @@ -250,6 +250,22 @@ static inline void boot_delay_msec(void)
    >> >  }
    >> >  #endif
    >> >
    >> > +/* msecs delay after each halt/poweroff/restart phase printk,
    >> > + unsigned short is enough for delay in milliseconds */
    >> > +static unsigned short halt_delay;
    >> > +
    >> > +static inline void halt_delay_msec(void)
    >> > +{
    >> > +   if (unlikely(halt_delay == 0 || !(system_state == SYSTEM_HALT
    >> > +                           || system_state == SYSTEM_POWER_OFF
    >> > +                           || system_state == SYSTEM_RESTART)))
    >> > +           return;
    >>
    >> This is a tiny bit ugly (and goes into the vprintf path) but i can
    >> see no other way either - a system_state > SYSTEM_RUNNING check
    >> would needlessly include the suspend-to-disk state (which we dont
    >> want to include here).
    >>
    >> In theory we could turn system_state into a bitmask and have a
    >> print_delay_mask check instead of these flags ... but that is a far
    >> wider change and i'm not sure it's a net step forwards.
    >>
    >> I've applied your patch to tip:core/printk with small edits to the
    >> changelog - Linus & Andrew is Cc:ed, should they have any
    >> objections.
    >
    > Could we not put just a single delay in there, immediately prior to halting,
    > restarting or poweroffing?

    But, then prink messages will still flush too fast for us to see the detail.

    --
    Regards
    dave
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