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    SubjectRe: [PATCH v3] printk: add halt_delay parameter for printk delay in halt phase

    * Dave Young <hidave.darkstar@gmail.com> wrote:

    > On Mon, Jun 8, 2009 at 4:46 PM, Ingo Molnar<mingo@elte.hu> wrote:
    > >
    > > * Dave Young <hidave.darkstar@gmail.com> wrote:
    > >
    > >> On Mon, Jun 8, 2009 at 4:14 PM, 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).
    > >>
    > >> Can we move suspend-to-disk before halt state?
    > >
    > > Yes, we could do that - if all system_state uses are checked for
    > > side-effects - in particular comparisons. We have a few places that
    > > do 'if (system_state > SYSTEM_RUNNING)' - to designate 'shutdown
    > > state'. Now, if we have any use of > SYSTEM_SHUTDOWN that might
    > > break from such a reordering.
    > >
    > > I wouldnt expect such usage really, but it has to be checked. That
    > > would make this patch quite a bit cleaner.
    > >
    > > Mind sending a followup delta patch with this cleanup?
    >
    > I'm glad to check the system_state usage, then write a followup patch.

    I'd also suggest to add a comment to the constants, to make sure
    that when new system states are added, the ordering assumptions are
    not broken.

    Ingo
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