[Qemu-devel] i8254 - Cannot change Timer frequency

[adrien lebre <lebre.adrien@free.fr>]

Hi all,

I posted the following question on the QEMU general forum few weeks ago
(http://qemu-forum.ipi.fi/viewtopic.php?f=3&t=4638#p13814). Someone adviced me
to post it directly on the development mailing list. I already tried to look up
into the archive but I didn't find any answers. Since several people seems to be
concerned by this issue, I finally decided to do it.

Thanks in advance for your help.
Best Regards,
Adrien


PS: please find below the copy/paste from the QEMU forum.

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i8254 - Cannot change Timer frequency
by alebre on Wed Sep 17, 2008 4:26 pm

Hi all,

In the context of study, we have to develop a simple operating system from
scratch. To develop our ''kernel'' we are using QEMU.
For few days, we have fighted with the I8254 chip in order to create a timer for
our system.
Based on the code provided in the Simple OS
(http://sos.enix.org/lxr/source/hwcore/i8254.c), we are trying to change the
frequency.
Unfortunately, whatever the value of nb_tick, the frequency does not change.
Here, I just copied the related code. We tried by setting the nb_tick with
11931, 1193 and 119 directly in the following functions. Unfortunately, the time
frequency (according to the IRQ timer handler) is still the same .

Does anyone have an idea ?

Thanks in advance,
Adrien.

/** 82c54 clock frequency */
#define I8254_MAX_FREQ 1193180

/* Ports to communicate with the 82c54 */
#define I8254_TIMER0  0x40
#define I8254_TIMER1  0x41
#define I8254_TIMER2  0x42
#define I8254_CONTROL 0x43

...

int i8254_set_frequency(unsigned int freq){
   unsigned int nb_tick;

   if (freq <= 0)
      return -255;

   /* Compute counter value */
   nb_tick = I8254_MAX_FREQ / freq;

   /* Counter must be between 1 and 65536 */
   if (nb_tick > 65536)
      return -255;
   if (nb_tick <= 0)
     return -255;

   /* The i8254 interprets 0 to mean counter == 65536, because 65536
      cannot be coded on 16bits */
   if (nb_tick == 65536)
     nb_tick = 0;

   /* We want to configure timer0, we want to send both LSB+MSB to set
      timer0 freq (-> 0x30), and we configure timer0 in mode 2, ie as a
      rate generator (-> 0x4) ==> 0x34 */
   outb(0x34, I8254_CONTROL);

   /* Send LSB of counter first */
   outb((nb_tick & 0xFF), I8254_TIMER0);

   /* Send MSB of counter */
   outb((nb_tick >> 8) & 0xFF, I8254_TIMER0);

  return 1;
}


Re: i8254 - Cannot change Timer frequency
by alebre on Wed Sep 17, 2008 4:37 pm

What is the behavior of QEMU with regard to such instructions ?
Does QEMU emulate the I8254 chip or does it simply forward the instruction to
the host OS ? In that case, the host OS could simply does not consider such an
instruction ?

Adrien, still looking for some explanations....


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