linux-kernel - Re: Question on HPET timer reading from device driver?

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Date:	Tue, 13 Jul 2010 21:45:06 +0800
From:	Samuel Xu <samuel.xu.tech@...il.com>
To:	Clemens Ladisch <clemens@...isch.de>
Cc:	linux-kernel@...r.kernel.org
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 <clemens@...isch.de>:
> 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
>
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