Messages in this thread | | | Date | Mon, 23 Feb 2015 08:23:04 +0100 | From | Sascha Hauer <> | Subject | Re: [PATCH 0/3] clk: divider: three exactness fixes (and a rant) |
| |
On Sat, Feb 21, 2015 at 11:40:22AM +0100, Uwe Kleine-König wrote: > Hello, > > TLDR: only apply patch 1 and rip of CLK_DIVIDER_ROUND_CLOSEST. > > I stared at clk-divider.c for some time now given Sascha's failing test > case. I found a fix for the failure (which happens to be what Sascha > suspected). > > The other two patches fix problems only present when handling dividers > that have CLK_DIVIDER_ROUND_CLOSEST set. Note that these are still > heavily broken however. So having a 4bit-divider and a parent clk of > 10000 (as in Sascha's test case) requesting > > clk_set_rate(clk, 666) > > sets the rate to 625 (div=15) instead of 667 (div=16). The reason is the > choice of parent_rate in clk_divider_bestdiv's loop is wrong for > CLK_DIVIDER_ROUND_CLOSEST (with and without patch 1). A fix here is > non-trivial and for sure more than one rate must be tested here. This is > complicated by the fact that clk_round_rate might return a value bigger > than the requested rate which convinces me (once more) that it's a bad > idea to allow that. Even if this was fixed for .round_rate, > clk_divider_set_rate is still broken because it also uses > > div = DIV_ROUND_UP(parent_rate, rate); > > to calculate the (pretended) best divider to get near rate. > > Note this makes at least two reasons to remove support for > CLK_DIVIDER_ROUND_CLOSEST! > > Instead I'd favour creating a function > > clk_round_rate_nearest
Full ack. It's a clock consumer who wants to decide the rounding strategy, not the clock itself and for sure not a specific entity of the clock tree. CLK_DIVIDER_ROUND_CLOSEST should be dropped.
Sascha
-- Pengutronix e.K. | | Industrial Linux Solutions | http://www.pengutronix.de/ | Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |
| |