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    SubjectRe: PROBLEM: conflict between apm and system clock on Inspiron 8100
    Hi Stephen,

    I have timed the "cat /proc/apm" to take at least
    36ms on my inspiron 8100 using an external clock. I
    shut down my ntp daemon so the clock is free running.
    I then did "ntpdate -q clock". With no calls to apm
    the number returned is rather stable over a few
    minuites with multiple ntpdate calls. I then execute
    10 times "cat /proc/apm" and do "ntpdate -q clock"
    again. I take the difference and divide by 10. That
    gives me an average of about 36ms, or 3 to 4
    interrupts missed for each call.
    Last night I also ran though all of Dell's BIOSs. I
    installed each of the 8 or so of them on there web
    site. A few of them broke apm, but none fixed this
    problem. If we are sure the BIOS is the problem, I
    will continue to pester Dell. So just to verify, you
    are *not* disabling interrupts in the kernel for an
    apm call? I am still trying to understand the code.
    I have also tried kernel 2.4.2-2. I don't remember
    this problem with an earlier installation I had on
    this laptop. It turns out it is still a problem with
    that kernel version. I am currently using 2.4.18-10.

    Thanks,
    Mike

    --- Stephen Rothwell <sfr@canb.auug.org.au> wrote:
    > Hi Frank,
    >
    > On Wed, 28 Aug 2002 17:04:33 +0200
    > Frank.Otto@tc.pci.uni-heidelberg.de wrote:
    > >
    > > Alan Cox wrote:
    > > > On Mon, 2002-08-26 at 18:00, mike heffner wrote:
    > > > > Well, isn't that a nice feature. Is there a
    > > > > workaround for this hardware?
    > > >
    > > > A thinkpad ;)
    > >
    > > Unfortunately, that's not true -- I just got an
    > IBM Thinkpad R32
    > > which exhibits the same behaviour as Mike's Dell
    > Inspiron 8100,
    > > it's only a tad worse. When I have the
    > battstat_applet running (which
    > > checks the battery every second), kernel time runs
    > about 3% slow
    > > compared to the RTC (which seems to be half-way
    > accurate on my machine).
    >
    > Don't do that then. Why would you need to check the
    > battery status
    > every second? Check every 30 seconds. Does your
    > battery even update its
    > status more often than that?
    >
    > > The cause seems to be definitely APM. If I shut
    > off battstat_applet
    > > and apmd, kernel time and RTC are in sync. With
    > only apmd, I lose about
    > > 15 seconds per hour. With battstat_applet, I lose
    > 2 minutes per hour.
    > > With
    > > while true; do cat /proc/apm >/dev/null; done
    > > the system runs at about 1/4 of the right speed.
    > Using a kernel with ACPI
    > > eliminates the problem (of course, you lose almost
    > all power management
    > > functionality too).
    >
    > Interesting ... Howlong does "cat /proc/apm" take?
    > On my Thinkpad T22 I get:
    >
    > # time cat /proc/apm
    > 1.16 1.2 0x03 0x01 0x00 0x01 99% -1 ?
    >
    > real 0m0.009s
    > user 0m0.000s
    > sys 0m0.010s
    >
    > while ...
    >
    > # time ./tppow
    > Battery 0 present power units mW[h] design capacity
    > 38880 last full charge capacity 29260
    > status 0x0 rate 0 cap 29172 voltage 12485
    >
    > real 0m0.311s
    > user 0m0.100s
    > sys 0m0.000s
    >
    > tppow is a C implementation of the disassembled APCI
    > method for reading the
    > battery status. It does not disable interrupts but
    > does talk to the
    > embedded controller in the Thinkpad.
    >
    > > BTW, I have set CONFIG_APM_ALLOW_INT, and on
    > startup the kernel even says
    > > "IBM machine detected. Enabling interrupts during
    > APM calls." Doesn't
    > > seem to help, though.
    >
    > This just means that we enter the BIOS with
    > interrupts enabled, it doesn't
    > stop the BIOS from disabling interrupts ...
    >
    > --
    > Cheers,
    > Stephen Rothwell
    > sfr@canb.auug.org.au
    > http://www.canb.auug.org.au/~sfr/


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