[PATCH v2 1/2] clk: change clk_ops' ->round_rate() prototype

[Boris Brezillon]

Mike, Stephen,

On Tue, 09 Jun 2015 20:02:54 -0700
Michael Turquette <mturquette at linaro.org> wrote:

> Quoting Jon Hunter (2015-06-05 01:46:09)
> > 
> > On 05/06/15 00:02, Paul Walmsley wrote:
> > > Hi folks
> > > 
> > > just a brief comment on this one:
> > > 
> > > On Thu, 30 Apr 2015, Boris Brezillon wrote:
> > > 
> > >> Clock rates are stored in an unsigned long field, but ->round_rate()
> > >> (which returns a rounded rate from a requested one) returns a long
> > >> value (errors are reported using negative error codes), which can lead
> > >> to long overflow if the clock rate exceed 2Ghz.
> > >>
> > >> Change ->round_rate() prototype to return 0 or an error code, and pass the
> > >> requested rate as a pointer so that it can be adjusted depending on
> > >> hardware capabilities.
> > > 
> > > ...
> > > 
> > >> diff --git a/Documentation/clk.txt b/Documentation/clk.txt
> > >> index 0e4f90a..fca8b7a 100644
> > >> --- a/Documentation/clk.txt
> > >> +++ b/Documentation/clk.txt
> > >> @@ -68,8 +68,8 @@ the operations defined in clk.h:
> > >>              int             (*is_enabled)(struct clk_hw *hw);
> > >>              unsigned long   (*recalc_rate)(struct clk_hw *hw,
> > >>                                              unsigned long parent_rate);
> > >> -            long            (*round_rate)(struct clk_hw *hw,
> > >> -                                            unsigned long rate,
> > >> +            int             (*round_rate)(struct clk_hw *hw,
> > >> +                                            unsigned long *rate,
> > >>                                              unsigned long *parent_rate);
> > >>              long            (*determine_rate)(struct clk_hw *hw,
> > >>                                              unsigned long rate,
> > > 
> > > I'd suggest that we should probably go straight to 64-bit rates.  There 
> > > are already plenty of clock sources that can generate rates higher than 
> > > 4GiHz.
> 
> Paul,
> 
> I agree. Changing the underlying struct clk.rate to 64 bits is on my
> radar.
> 
> > 
> > An alternative would be to introduce to a frequency "base" the default
> > could be Hz (for backwards compatibility), but for CPUs we probably only
> > care about MHz (or may be kHz) and so 32-bits would still suffice. Even
> > if CPUs cared about Hz they could still use Hz, but in that case they
> > probably don't care about GHz. Obviously, we don't want to break DT
> > compatibility but may be the frequency base could be defined in DT and
> > if it is missing then Hz is assumed. Just a thought ...
> 
> I was thinking of doing it the other way. E.g. use 64-bit rates
> throughout the clk framework and have a base unit of millihertz instead
> of hertz. 64 bits gives us a LOT of room to grow, and the current base
> of hertz completely ignores all applications with fractional hertz
> requirements.

Do you have examples of applications manipulating milliHz ?

Anyway, moving from Hz to milliHz is a lot more invasive than moving
from an unsigned long to an u64 field.
How about first storing the frequency on an u64 value (this can be part
of my patch adapting the ->determine_rate() prototype [1]), and then
plan for a big rework accounting for this unit change ?

Best Regards,

Boris

[1]http://patchwork.linux-mips.org/patch/10092/

-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com