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SubjectRe: Linux 2.2.18: /proc/apm slows system time (was: Linux 2.2.19pre3)
On Fri, 29 Dec 2000, Erik Mouw wrote:

> On Thu, Dec 28, 2000 at 02:53:37PM +0100, Matthias Andree wrote:
> However, reading from /proc/apm also triggers other weird problems:
>
> - Received characters dropped on serial line. I thought my serial port
> was broken, because a 16550 is supposed to have a receive buffer.

I don't know that the Linux driver sets the IRQ trigger to (you can have
the 16550 request interrupts after its 16-byte FIFO has 1, 4, 8 or 14
bytes ready for reading), but if it's set to 14 (to reduce the IRQ
frequency), you don't have much time at 115200 Bit/s, you have 1 Byte
every 87 ms then (it has 10 bit usually, 1 start + 1 stop bit), and
reading from /proc/apm stops my system clock for approx. 80 ... 90 ms -
then you still have IRQs with higher precedence and whooops, your buffer
overruns. Setting the trigger lower would help, but I never looked how
this will happen.

(I never run into this problem myself since I have 16750s here which
have at least 8 Bytes left when triggering, they have 64 Byte FIFO.)

> 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?

My problem shows up on a Gigabyte board with AMIBIOS, so it's certainly
not a Phoenix or Asus specific problem.

However, reading from /proc/apm triggers BIOS calls which involve
certain action, maybe switching to Real Mode and other things, and I
suspect that either IRQs are still disabled while the BIOS is called or
the BIOS plays bad games which Linux would have to compensate for. :-/

HTH,
Matthias
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