No. I did not see such an OS. My concern
is: When VAL_SET_CNF is set, do the two registers all get updated? The spec
does provide information. From our tests, if you strictly follow the sequence attached
in my first mail, the comparator can keep increasing normally. With virtual
HPET, it is not true, because period is not considered to be set by only one
write. Our tests are carried out on ICH9.
This leads me to confusion. Which one is
the right behavior? Writing twice should guarantee the correctness, but is it a
must?
Haitao Shan
From: Keir Fraser [mailto:Keir.Fraser@xxxxxxxxxxxx]
Sent: 2007年12月26日 21:53
To: Shan, Haitao
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: Question on virtual
HPET's behavior
My reading of the (really bad)
specification doc is that in periodic mode writes to the comparator register
change the period only, unless VAL_SET_CNF is set in which case only the
comparator counter value is set. The Linux HPET code agrees with this interpretation
— it writes to the comparator register twice. Do you have an example of an OS
that assumes different HPET behaviour?
-- Keir
On 26/12/07 12:37, "Shan, Haitao" <haitao.shan@xxxxxxxxx>
wrote:
Here hpet.period is updated only when
timer 0 is in periodic mode and SET_VAL is not set. But from
HPET spec 1.0a, I can not find the reason. The spec provides a
recommended sequence to reset main counter and comparator.
If the software resets
the main counter, the value in the comparator’s value register needs to reset
as well. This
can be done by setting the n_VAL_SET_CNF bit. Again, to avoid race conditions, this
should be done
with the main counter halted. The following usage model is expected:
1) Software clears the
GLOBAL_ENABLE_CNF bit to prevent any interrupts
2) Software Clears the
main counter by writing a value of 00000000h to it.
3) Software sets the
TIMER0_VAL_SET_CNF bit.
4) Software writes the
new value in the TIMER0_COMPARATOR_VAL register
5) Software sets the
GLOBAL_ENABLE_CNF bit to enable interrupts.
What will the result is system software only writes the comparator once
after set SET_VAL like the above sequence?
Can you give me some hints? Thanks in advance!
