Re: [PATCH 0/3] clk: divider: three exactness fixes (and a rant)

[Uwe Kleine-König]

Hello Mike,

On Fri, Mar 06, 2015 at 10:57:30AM -0800, Mike Turquette wrote:
> Quoting Uwe Kleine-König (2015-02-21 02:40:22)
> > 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
> > 
> > as was suggested some time ago by Soren Brinkmann and me[1] that doesn't
> 
> Uwe,
> 
> Thanks for the fixes. I'm thinking of taking all three for 4.0. I also
> agree on clk_round_rate_nearest (along with a _ceil and _floor version
> as well). That's something we can do for 4.1 probably.
I'd say that we make round_rate the _floor version. I guess in most
cases that already does the right thing. Also I think 4.1 is very
ambitious, so my suggestion for 4.1 is:

 - add a WARN_ON_ONCE to clk_round_rate catching calls that return a
   value bigger than requested.
 - implement clk_round_rate_nearest using clk_round_rate and the
   assumption that it returns a value that is <= the requested rate.
   I think without that there are too many special cases to handle and
   probably not even a reliable way to determine the nearest rate.
 - while we're at it tightening the requirements for clk_round_rate
   let's also specify the expected rounding. I'd vote for the
   mathematical rounding, that is

   	clk_round_rate(someclk, 333)

   explicitly is allowed to return a rate bigger than 333 as long as it
   is less than 333.5.

At one point while developing patch 1 I had the dividers fixed for the
rounding issue. I think I still have that patch somewhere so can post it
as RFC.

> There are currently 3 users of CLK_DIVIDER_ROUND_CLOSEST:
> 
> Loongson
> QCOM
> ST
> 
> So now is probably the right time to remove the flag if we're going to
> do it.
"now" is before we have clk_round_rate_nearest, right?
So what do you want to do with these three users? Move them to the
default implementation?

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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