Re: Linux 2.2.18: /proc/apm slows system time (was: Linux 2.2.19pre3)

From: Erik Mouw (J.A.K.Mouw@ITS.TUDelft.NL)
Date: Fri Dec 29 2000 - 07:42:52 EST


On Thu, Dec 28, 2000 at 02:53:37PM +0100, Matthias Andree wrote:
> Forget this all.
>
> I found the problem trigger, it's reading from /proc/apm, for a reason I
> cannot currently see.
>
> Current config, as far as it's APM-related:
> CONFIG_APM=y
> # CONFIG_APM_IGNORE_USER_SUSPEND is not set
> # CONFIG_APM_DO_ENABLE is not set
> # CONFIG_APM_CPU_IDLE is not set
> # CONFIG_APM_DISPLAY_BLANK is not set
> CONFIG_APM_RTC_IS_GMT=y
> CONFIG_APM_ALLOW_INTS=y
> # CONFIG_APM_REAL_MODE_POWER_OFF is not set
> # CONFIG_TOSHIBA is not set
>
> I had found out that my clock was slow while dnetc was running. I had a
> dummy loader that just did while(1) {} which did not slow my clock. Now, I
> straced that dnetc beast and found out that it reads /proc/apm quite
> often.

I got the same problem while running the GNOME battery_applet, which
checks /proc/apm every 2 seconds.

> I can have my clock almost halt with this one:
>
> while cat /proc/apm ; do : ; done
>
> If I leave this running for 15 s, my system time drifts back 11 s.

Same over here on an Asus M8300 notebook (Celeron 500, 128MB, Intel
440MX chipset). Output from /proc/apm:

  1.14 1.2 0x03 0x01 0x03 0x09 100% -1 ?

I also enabled CONFIG_APM_ALLOW_INTS to see if it makes any difference,
but apparently it doesn't.

However, reading from /proc/apm also triggers other weird problems:

- Sound hickups: mp3 output starts to sound "scratchy". problem
  disappears after restarting mp3 player or after a couple of reads
  from /proc/apm. This is with mpg123, xmms, and madplay.
- USB drop outs, especially for bulk devices like scanners or USB audio
  devices. For sound it usually takes a second or so to restart.
- Received characters dropped on serial line. I thought my serial port
  was broken, because a 16550 is supposed to have a receive buffer.

All these problems went away when I removed the GNOME battery_applet.

I got the same problems with my previous notebook (Asus P6300, PII 266,
112MB, Intel BX/ZX chipset). It might be a BIOS problem, because both
notebooks use a Phoenix BIOS. OTOH, I can create the same problems with
USB on my desktop (Asus P5A motherboard, K6 333, 160MB, Ali 1541
chipset, Award BIOS) when I run the GNOME battery_applet. So is this an
Asus problem, or a general APM problem?

Erik

-- 
J.A.K. (Erik) Mouw, Information and Communication Theory Group, Department
of Electrical Engineering, Faculty of Information Technology and Systems,
Delft University of Technology, PO BOX 5031,  2600 GA Delft, The Netherlands
Phone: +31-15-2783635  Fax: +31-15-2781843  Email: J.A.K.Mouw@its.tudelft.nl
WWW: http://www-ict.its.tudelft.nl/~erik/
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