On Tue, Oct 29, 2013 at 08:23:07AM -0400, Tom Rini wrote:
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On 10/29/2013 06:48 AM, Wolfgang Denk wrote:
Dear Måns Rullgård,
In message yw1x8uxc28y9.fsf@unicorn.mansr.com you wrote:
Something like this should be equivalent. That said, it looks suspiciously like it's meant to simply do a division and round up. If that is the case, +225 should be +249. It probably makes no difference for the values actually encountered.
Umm... this is the part which I do not understand.
The original code adds 90%; you add 90%, too. However, to round up, one usually adds only 50% ?
Adding 50% would round to nearest. For integer division to round up, you must add one less than the divisor.
Agreed. But do we want to round up? The original code used +90%, which is something else, too...
And I imagine it's unlikely the original author of the code is around anymore, or recalls exactly why. I'm pretty sure Matt just lifted the code from the vendor tree and since it wasn't throwing warnings didn't notice the floating point part.
Where are these 90% coming from? Are they in any way meaningful, or even critical?
My guess is that it was someone's approximation of 249 / 250. I don't know the hardware, so it's conceivable that it really should be this way, although it seems unlikely.
Are you able to test such a modificationon actual hardware?
I suspect Matt can, after Linaro Connect. I don't have one of these platforms handy but I think he still does.
Thankfully Tom reminded me of this because I lost some of the list traffic due to some local mail issues.
Although not explicitly mentioned in the TRM or any application notes I can find, the 90% appears to come from jitter compensation for the delta sigma fractional divider. I see some comments that imply this in various old vendor kernel tree clock implementations where they are rounding various pll constants. I can't be 100% sure without some insight from TI folks. Given that it's working for our known users, I'd like to preserve that until we get somebody that can shed some light on that.
As Tom guessed, I low-level cherry-picked a lot of pieces from long-lost authors in this area. I obviously missed this floating point math in the cleanup.
I tested this patch on my TI8148 EVM and it works as expected.
Acked-by: Matt Porter mporter@linaro.org
-Matt