[PATCH] arc: get rate from clk driver instead of reading device tree

[Vlad Zakharov]

Hi Vineet,

On Wed, 2017-02-22 at 10:49 -0800, Vineet Gupta wrote:
> On 02/22/2017 02:36 AM, Vlad Zakharov wrote:
> > We were reading clock rate directly from device tree "clock-frequency"
> > property of corresponding clock node in show_cpuinfo function.
> > 
> > Such approach is correct only in case cpu is always clocked by
> > "fixed-clock". If we use clock driver that allows rate to be changed
> > this won't work as rate may change during the time or even
> > "clock-frequency" property may not be presented at all.
> > 
> > So this commit replaces reading device tree with getting rate from clock
> > driver. This approach is much more flexible and will work for both fixed
> > and mutable clocks.
> 
> So except the clk_put() comment - this looks much better to me.
> 
> And while we are at it - one of my wish list has been to print the core clk in
> early boot - i.e. in setup_processor()->arc_cpu_mumbojumbo() call path which is
> different from the other case u fixed. Any ideas how to do that - clk framework
> will not have init by then - so we need to read out from FDT or some such !

We can either parse FDT or read from real hardware - but is that the point?
I thought the goal is to get rid of as much board-dependent code as possible, isn't it?

In fact there are two ways of getting frequencies before clk framework inits: 

  1) Parse FDT - this way is not platform-dependent, but if someone adds clock nodes without "clock-frequency" property
this is gonna fail. The second issue here is that device tree property is not believed to be accurate - actual value may
differ. So, there are a lot of disadvantages.

  2) Read values from real hardware: this is platform-dependent way and we will have add such "hacks" for each supported
platform even if clock driver is provided for such platform. And that's why I don't like this way.

Of course clk framework initializes a bit later. But maybe we can wait for this and add just a couple of code lines that
read rate using common clk API?

-- 
Best regards,
Vlad Zakharov <vzakhar at synopsys.com>