Hi Mark,

On Thu, 1 Dec 2011, Mark Brown wrote:

On Wed, Nov 30, 2011 at 11:39:59PM -0700, Paul Walmsley wrote:

quoted

Clock rate/parent-change notifiers are requirements for DVFS use-cases, 
and they must be paired with something like the 
clk_{allow,block}_rate_change() functions to work efficiently.  I intend 
to comment on this later; it's not a simple problem.  It might be worth 
noting that Tero and I implemented a simplified version of this for the 
N900.

I'm thinking that if we're going to have clk_{allow,block}_rate_change()
we may as well make that the main interface to enable rate changes - if
a device wants to change the clock rate it allows rate changes using
that interface rather than by disabling the clocks.  I've got devices
which can do glitch free updates of active clocks so having to disable
would be a real restriction, and cpufreq would have issues with actually
disabling the clock too I expect.

The intention behind the clk_{allow,block}_rate_change() proposal was to 
allow the current user of the clock to change its rate without having to 
call clk_{allow,block}_rate_change(), if that driver was the sole user of 
the clock.

So for example, if you had a driver that did:

c = clk_get(dev, clk_name);
clk_enable(c);
clk_set_rate(c, clk_rate);

and c was currently not enabled by any other driver on the system, and 
nothing else had called clk_block_rate_change(c), then the rate change 
would be allowed to proceed.  (modulo any notifier activity, etc.)  

So clk_{allow,block}_rate_change() was simply intended to allow or 
restrict other users of the same clock, not the current user.


- Paul