On 05/14/2012 04:08 AM, Peter De Schrijver wrote:
On Sun, May 13, 2012 at 06:31:42AM +0200, Stephen Warren wrote:
On 05/11/2012 08:58 PM, Saravana Kannan wrote:
On 05/09/2012 03:36 AM, Peter De Schrijver wrote:
On Wed, May 09, 2012 at 02:41:37AM +0200, Saravana Kannan wrote:
On 05/08/2012 10:15 AM, Turquette, Mike wrote:
On Mon, May 7, 2012 at 10:07 PM, zhoujie wu<zhoujiewu@xxxxxxxxx>
wrote:
Hi Mike,
Could you please explain more details about how to implement a
re-parenting operation as part of it's .set_rate implementation?
Sure.
As far as I know, we can not call clk_set_parent in .set_rate function
directly, since clk_set_rate and clk_set_parent are using the same
prepare_lock.
That is correct.
Any other interface can be used to implement it?
You have two options available to you.
1) __clk_reparent can be used from your .set_rate callback today to
reflect changes made to the tree topology. OMAP uses this in our PLL
.set_rate implementation: depending on the re-lock frequency the PLL
may switch parents dynamically. __clk_reparent does the
framework-level cleanup needed for this (that function is also used
when populating the clock tree with new clock nodes).
2) __clk_set_parent could be made non-static if you needed this (I've
been meaning to talk to Saravana about this since I think MSM needs
something like this).
Thanks!
I don't think I need (2). But I don't think I can use (1) as is either.
I can use (1) with some additional code in my set rate op.
While set rate is in progress, both the parents might need to stay
enabled for a short duration. So, in my internal set rate, I need to
check if my clock is prepared/enabled and call prepare/enable on the
"old parent", call __clk_reparent (which will reduce the ref count for
the old parents and increase it for the new parents), finish the
reparent in HW and then unprepare/disable the old parent if I have
prepared/enabled them earlier.
It might be beneficial to provide something like a
__clk_reparent_start(new_parent, *scratch_pointer) and
__clk_reparent_finish(*scratch_pointer) if it will be useful for more
than just MSM. Based on this email, I would guess that Tegra would want
something similar too.
We also need to reparent clocks using a pll if we want to change the
PLLs rate
while the users are active.
Peter,
Is this reparent permanent (as in, stays reparented when you return from
clk_set_rate()) or is it a reparenting for just a short duration inside
the set_rate ops?
I've seen both cases, and indeed the case sometimes depends on the
target rate of the clock.
For example, when the CPU clock changes, we basically do the following
within set_rate:
* Set CPU clock parent to some "backup" PLL
* Change the CPU PLL to the desired rate
* Set CPU clock parent to the CPU PLL
However, the lowest CPU clock rate we support is actually the rate that
the backup PLL runs at, so if we're targeting that rate, the CPU clock
set_rate /just/ does:
* Set CPU clock parent to some "backup" PLL
and leaves it there, until a different CPU clock rate is requested, at
which time the CPU clock will be re-parented back to the CPU PLL.
The backup PLL and rate are statically defined however. It's not chosen at
runtime.
Yeah, this is pretty much what MSM has been doing for a while for our
CPU clocks too. But in MSM we made the decision of not to control the
CPU clocks through the clock APIs. It makes your life so much more
easier. Really, the CPU clocks are very different from the rest of the
clocks -- the set rate operation of the clock needs the clock to be
running. We also never disable the CPU clock we are running on. Just
that it might not be as useful as it might appear. It was especially
hard when we didn't have clk_prepare/_unprepare().
Anyway, not saying this is the only way to proceed or using the clock
APIs to control the CPU clock is wrong. Just some food for thought.
Anyway, getting back to the point of this thread, you might look at the
other email I sent out on Friday [1]. Sorry I forgot to cc you on that.
I have a list of people I copy paste when sending clock patches. I can
add you to that list if you want (or remove anyone who is annoyed by it).
If you are doing the parent management inside the .set_rate ops, I
believe you will have a race condition with the current clock framework.
We need to update the clock framework to provide more APIs to implement
this correctly. I was confused by some code and pinged Mike about it.
Now that I understand what the code was trying to do, I think I might be
able to clean it up a bit and then fix up set rate support.
Thanks,
Saravana
[1] http://thread.gmane.org/gmane.linux.ports.arm.msm/2655
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