Re: [PATCH 05/11] OMAPDSS: add clk_prepare and clk_unprepare

[Rajendra Nayak <rnayak@ti.com>]

On Monday 25 June 2012 11:37 AM, Tomi Valkeinen wrote:
> Hi,
>
> On Fri, 2012-06-22 at 19:18 +0530, Rajendra Nayak wrote:
>> In preparation of OMAP moving to Common Clk Framework(CCF) add clk_prepare()
>> and clk_unprepare() for the omapdss clocks.
>
> You used clk_prepare and clk_unprepare instead of clk_prepare_enable and
> clk_disable_unprepare. I didn't check the dss driver yet, but my hunch
> is that the clocks are normally not enabled/disabled from atomic
> context.
>
> What does the prepare/unprepare actually do? Is there any benefit in
> delaying preparing, i.e. is there a difference between prepare right
> after clk_get, or prepare right before clk_enable? (And similarly for
> unprepare)

clk_prepare/unprepare are useful for clocks which need the 'enable'
logic to be implemented as a slow part (which can sleep) and a fast part
(which does not sleep). For all the dss clocks in question we don't need
a slow part and hence they do not have a .clk_prepare/unprepare
platform hook.

The framework however still does prepare usecounting (it has a prepare
count and an enable count, and prepare count is expected to be non-zero
while the clock is being enabled) and uses a mutex around to guard it.
So while the dss driver would do multiple clk_enable/disable while its
active, it seems fair to just prepare/unprepare the clocks once just
after clk_get() and before clk_put() in this particular case.

>
>   Tom
>