Re: Using sched_clock for mmio-trace

[Daniel Walker <dwalker@mvista.com>]

On Fri, 2007-02-16 at 11:30 -0500, Frank Ch. Eigler wrote:
> Jeff Muizelaar <jeff@infidigm.net> writes:
> 
> > I've built a tool with the goal of logging mmio writes and reads by
> > device drivers. See http://nouveau.freedesktop.org/wiki/MmioTrace.
> 
> FWIW, this is exactly a type of add-on trace patch that could be
> mooted by adoption of the ltt/systemtap "marker" facility.  With it,
> you would not need so much code (e.g. no new user-space tools at all,
> reuse of common tracing buffer logic, permanently placed hooks) and
> would probably get more utility.
> 
> > [...]  The question is, what [timer] api should I be using? I need
> > something that can be called from inside interrupt handlers, and
> > obviously the more accurate and the lower the overhead the better.
> 
> We in systemtap land have the same problem, and so far made do with
> slightly postprocessed per-cpu TSC values.

I've been working on a patch set (below), to expose the clocksources
used by generic time to multiple users . It would allow timestamps from
different clocks in a generic way. It's not merged, but I'd appreciate
any input either of you might have..

ftp://source.mvista.com/pub/dwalker/clocksource/

Daniel