linux-kernel - Re: Using sched_clock for mmio-trace

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Date:	Fri, 16 Feb 2007 23:36:57 -0500
From:	Jeff Muizelaar <jeff@...idigm.net>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	"Frank Ch. Eigler" <fche@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: Using sched_clock for mmio-trace

On Fri, Feb 16, 2007 at 02:47:45PM -0800, Daniel Walker wrote:
> > > Gets pretty ugly .. The clocksource interface already has a positive
> > > rating to describe the "best" clocks in the system, which is used to
> > > return the "best" clock .. Where the maintainers of the system give each
> > > clock a rating. I would imagine most people would just get the so called
> > > "best" clock which has the best rating..
> > > 
> > > I'm starting to think this long flags stringing effect could happen with
> > > negative flags also, but it's seems a lot less likely.
> > 
> > The amount of flag stringing should be the same.
> 
> I don't think so .. The common case with negative flags is no flags,
> then next would be CLOCKSOURCE_UNSTABLE. At most I would guess two
> flags .. The other direction your likely to have people using all flags
> most of the time. That's why I showed a function call with all the flags
> listed.

I think you still misunderstand me. The common case is still no flags.

clocksource_get_clock_must_have(0) would return clocks that are stable
and unstable. clocksource_get_clock_must_have(CLOCKSOURCE_STABLE) would
only return clocks that are stable, just like
clocksource_get_clock_masked(CLOCKSOURCE_UNSTABLE) only returns clocks
that are stable.

> > > > instead of
> > > > 
> > > > clocksource_get_clock_masked(CLOCKSOURCE_UNSTABLE)
> > > > clocksource_get_clock_masked(CLOCKSOURCE_PM_AFFECTED)
> > > > 
> > > > Especially awkward is the CLOCKSOURCE_64BIT flag, as there isn't really
> > > > anyway to specify that I want a 64bit timer, only a way to specify that
> > > > I don't.
> > > 
> > > I might add a way to get specific flags, but I still think the flags
> > > should be mostly negative features.
> > 
> > Yeah, the problem is that all of the features are negative except for
> > CLOCKSOURCE_64BIT, so you can't mask for it.
> 
> It's meant as a negative feature. So you can mask it if you can't handle
> the math .. The only 64bit clock I know off is the tsc, and it's got the
> highest rating of all clocks.

Ah ok, I see that now. Maybe CLOCKSOURCE_OVER_32BITS would be a better
name? It might convey the negativity better...

-Jeff
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