On 03/06/2012 12:27 PM, Ralf Baechle wrote:
On Tue, Mar 06, 2012 at 03:01:50PM +0100, Florian Fainelli wrote:
hi all:
I have some questions about mips_hpt_frequency:
1. is mips_hpt_frequency == mips cpu frequency?
No, it is usually cpu frequency / 2.
The architecture specification leaves the counter clock rate up up to the
implementation and only says the clock rate is a function of the pipeline
clock. In all reality this means the counter is running at the full or
half frequency. Just don't build on it,
clock := pipeline_clock * next_weeks_lottery_number % 42
would by compliant ;-)
On some CPUs the frequency can even be selected through a configuration
bitstream at reset time so you can't always count on a fixed relation
between CPU clock and count rate.
Some older CPU manuals contain a confusing wording saying the counter
increments at half (or full) instruction issue rate. That just means the
pipeline clock, no reason to be confused.
If you have a v2 or later ISA, you can use 'rdhwr x, $3' to find the
ratio between the clock rate and the increment rate of the timer.
David Daney
