Re: some questions about mips timer

[David Daney <ddaney.cavm@gmail.com>]

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