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To: Poul-Henning Kamp <phk@phk.freebsd.dk>
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Cc: freebsd-current@freebsd.org
Subject: Re: Evaluation of High Precision Event Timer Driver for userland
 timer facility
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Hi

Thank you for your comment.

I'm sorry that the reply is late. 

> 
> Basing this facility on the HPET almost guarantees that we cannot
> use it in FreeBSD, because the HPET is not available on more than
> a couple of our architectures.
>
Certainly the facility is not available non-PC platform, it
highly depends on board specs.

How do you think about introducing this facility as a kernel option.
As far as I think, this facility is a kind of device driver, this is 
not timer facility which is used in common.

Certainly, it may have no dramatic effect as you said.
Therefore,  I did not mean to introduce this facility into FreeBSD
by all means, certainly.

 
> Without userland access to the timekeeping hardware, it is difficult
> to avoid the system call overhead and once in the kernel anyway
> it is doubtful that splitting the code between userland and
> kernel really gives much of a payoff.
> 
> I am aware that Linux has a userland timestamping facility, but
> I am also aware of its numerous shortcomings.
> 
Sorry, I can not figure out shortcomings of this, would you tell me
 shortcomings of mmaped HPET time stamp facility?