Subject: Re: [PATCH v2 3/4] Implement clockevents driver for powerpc

From: Sergei Shtylyov <[email-masked]>
Date: Fri, 19 Oct 2007 15:49:37 +0400

Hello.

Paul Mackerras wrote:

>>>What problem do you see arising from this?

>>    Timers firing too early.

> Only if the minimum interrupt latency is less than 1 decrementer
> tick.  That seems pretty unlikely to me unless you have a very slow
> timebase frequency.

    Well, MPC8540 has 825 MHz CPU clock yet decrementor/timebase are clocked 
with 25 MHz clock if I don't mistake. That gives us 33 CPU clocks of available 
interrupt latency...

> In fact what we should program the decrementer to is:

> 	timeout - (is_booke? 0: 1) - min_interrupt_latency

    BTW, why not handle the decrementer difference right in set_dec() where we 
already have #ifdef'ed code?

> I was assuming that min_interrupt_latency (measured in timebase ticks)
> would be at least 1, but apparently some systems can have a timebase
> frequency as low as 1kHz, so we'll have to have an ifdef or something.

    IMHO it's better to have #ifdef based on the decremeter model and forget 
about the whole issue, rather than to #ifdef based on some bizarre system with 
slowish decremeter, isn't it?

> Paul.

WBR, Sergei