On Mon, Jan 21, 2013 at 6:51 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:On 01/21/2013 02:54 PM, Matt Sealey wrote:Oh, it is just if I actually don't run setup_sched_clock on myOn Mon, Jan 21, 2013 at 4:36 PM, John Stultz <john.stultz@xxxxxxxxxx>As far as jiffies rating, from jiffies.c:
wrote:
On 01/21/2013 01:14 PM, Matt Sealey wrote:
.rating = 1, /* lowest valid rating*/
So I'm not sure what you mean by "the debug on the kernel log is telling me
it has a higher resolution".
platform, it gives a little message (with #define DEBUG 1 in
sched_clock.c) about who setup the last sched_clock. Since you only
get one chance, and I was fiddling with setup_sched_clock being probed
from multiple possible timers from device tree (i.MX3 has a crapload
of valid timers, which one you use right now is basically forced by
the not-quite-fully-DT-only code and some funky iomap tricks).
And what I got was, if I use the real hardware timer, it runs at 66MHz
and says it has 15ns resolution and wraps every 500 seconds or so. The
jiffies timer says it's 750MHz, with a 2ns resoluton.. you get the
drift. The generic reporting of how "good" the sched_clock source is
kind of glosses over the quality rating of the clock source and at
first glance (if you're not paying that much attention), it is a
little bit misleading..
So there are probably other folks who could better comment on sched_clock() or the delay timer (I'm guessing the delay() implementation is what you mean by that) design trade-offs.Yes, in the case I was remembering, the 60HZ was driven by the electricalWhile I have your attention, what would be the minimum "good" speed to
line.
run the sched_clock or delay timer implementation from? My rudimentary
scribblings in my notebook give me a value of "don't bother" with less
than 10KHz based on HZ=100, so I'm wondering if a direct 32.768KHz
clock would do (i.MX osc clock input if I can supply it to one of the
above myriad timers) since this would be low-power compared to a 66MHz
one (by a couple mA anyway). I also have a bunch of questions about
the delay timer requirements.. I might mail you personally.. or would
you prefer on-list?