2010-07-13 09:45:38

by Samuel Xu

Subject: Question on HPET timer reading from device driver?

HI:
I am a freshman for HPET timer. Previously, I use TSC counter, to use
rdtsc to get the time stamp. elapsed cycle can be counter from 2 time
stamps.
As a advanced replacement of TSC, I suppose HPET timer can be
conveniently to read time stamp from both kernel mode user and user
mode apps.
While I failed to find a way to get time stamp value of HPET in Linux
kenerl (I am doing a driver and want to know elapsed cycle from driver
code).

I tried hpet_readl() from my driver code, while hit compiling error.
Is it a correct API for kernel driver to invoke?
I also read the Linux Documentation of HPET, which contain a user mode
demo app to generate a periodic timer via HPET, while missing the time
stamp reading.

Any hint?

Thanks!

Samuel

2010-07-13 11:49:59

by Clemens Ladisch

Subject: Re: Question on HPET timer reading from device driver?

Samuel Xu wrote:
> While I failed to find a way to get time stamp value of HPET in Linux
> kenerl (I am doing a driver and want to know elapsed cycle from driver
> code).

The HPET is x86-specific any might not be available on every machine.

> I tried hpet_readl() from my driver code, while hit compiling error.

This function is intended for internal use by the kernel and not for
drivers, so it is not exported for modules.

You can hack your own accessor function by calling ioremap() on the HPET
address (0xfed00000 or 0xfefff000 or whatever) and reading from that
memory.

A more portable way to get a time stamp is getrawmonotonic().

> I also read the Linux Documentation of HPET, which contain a user mode
> demo app to generate a periodic timer via HPET, while missing the time
> stamp reading.

There is no ioctl for this; call mmap() on /dev/hpet and then read the
main counter value from offset 0xf0 of that memory.

A more portable way to get a time stamp is to use clock_gettime() with
CLOCK_MONOTONIC_RAW.


Regards,
Clemens

2010-07-13 13:45:11

by Samuel Xu

Subject: Re: Question on HPET timer reading from device driver?

Thanks Clemens!
Besides calling ioremap() on specific HPET address, is there more
elegant interface, "offically" for kernel mode driver accessing HPET?


2010/7/13 Clemens Ladisch <[email protected]>:
> Samuel Xu wrote:
>> While I failed to find a way to get time stamp value of HPET in Linux
>> kenerl (I am doing a driver and want to know elapsed cycle from driver
>> code).
>
> The HPET is x86-specific any might not be available on every machine.
>
>> I tried hpet_readl() from my driver code, while hit compiling error.
>
> This function is intended for internal use by the kernel and not for
> drivers, so it is not exported for modules.
>
> You can hack your own accessor function by calling ioremap() on the HPET
> address (0xfed00000 or 0xfefff000 or whatever) and reading from that
> memory.
>
> A more portable way to get a time stamp is getrawmonotonic().
>
>> I also read the Linux Documentation of HPET, which contain a user mode
>> demo app to generate a periodic timer via HPET, while missing the time
>> stamp reading.
>
> There is no ioctl for this; call mmap() on /dev/hpet and then read the
> main counter value from offset 0xf0 of that memory.
>
> A more portable way to get a time stamp is to use clock_gettime() with
> CLOCK_MONOTONIC_RAW.
>
>
> Regards,
> Clemens
>