Date: Wed, 29 Jun 2011 22:56:46 +0200 From: Marius Strobl <marius@alchemy.franken.de> To: Jung-uk Kim <jkim@FreeBSD.org> Cc: freebsd-sparc64@FreeBSD.org Subject: Re: [RFC] Clean up sparc64 timecounters Message-ID: <20110629205646.GK14797@alchemy.franken.de> In-Reply-To: <201106281337.54901.jkim@FreeBSD.org> References: <201106281327.02537.jkim@FreeBSD.org> <201106281337.54901.jkim@FreeBSD.org>
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On Tue, Jun 28, 2011 at 01:37:52PM -0400, Jung-uk Kim wrote:
> On Tuesday 28 June 2011 01:26 pm, Jung-uk Kim wrote:
> > Can you please review the attached patch?
> >
> > sys/sparc64/pci/fire.c:
> > - Remove redundant timecounter masking from tc_get_timecount
> > method. - Remove an unnecessary macro for timecounter mask.
> > - Remove a redundant NULL assignment.
> >
> > sys/sparc64/pci/schizo.c:
> > - Remove redundant timecounter masking from tc_get_timecount
> > method. - Correct timecounter mask. Note this is a no-op because
> > the STX_CTRL_PERF_CNT_CNT0_SHIFT is actually zero.
> > - Remove a redundant NULL assignment.
I'm not sure whether you correctly understand how that timer works.
The hardware actually provides a pair of 32-bit timers which are
read via a single 64-bit register so the existing tc_counter_mask
is correct and your change is wrong. For the same reason the masking
and shifting in schizo_get_timecount() only happens to be unnecessary
in so far as we currently use the lower 32-bit counter and the
tc_get_timecount methods return u_int. If we'd either switch to
the upper 32-bit counter or the timecounter code would be enhanced
to support up to 64-bit counters it wouldn't be redundant. There's
actually a right-shift missing in schizo_get_timecount() though,
i.e. it should actually do:
return ((SCHIZO_CTRL_READ_8(sc, STX_CTRL_PERF_CNT) &
(STX_CTRL_PERF_CNT_MASK << STX_CTRL_PERF_CNT_CNT0_SHIFT) >>
STX_CTRL_PERF_CNT_CNT0_SHIFT);
The compiler should be smart enough to boil all that down to a
single 64-bit to 32-bit conversion when returning though.
For similar reasons I'd prefer to also keep the masking in
fire_get_timecount(), besides using the macro IMO is cleaner
than using ~0u(l).
>
> @@ -686,8 +684,7 @@ fire_attach(device_t dev)
> if (tc == NULL)
> panic("%s: could not malloc timecounter", __func__);
> tc->tc_get_timecount = fire_get_timecount;
> - tc->tc_poll_pps = NULL;
> - tc->tc_counter_mask = TC_COUNTER_MAX_MASK;
> + tc->tc_counter_mask = ~0ul;
> ^^^^
> ~0u
> if (OF_getprop(OF_peer(0), "clock-frequency", &prop,
> sizeof(prop)) == -1)
> panic("%s: could not determine clock frequency",
>
Well, if you really remove the masking from fire_get_timecount()
then you should actually also use ~0ul here for consistency as it's
an 64-bit counter.
Marius
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