On 05/08/2012 10:15 AM, Turquette, Mike wrote:

quoted

On Mon, May 7, 2012 at 10:07 PM, zhoujie wu[off-list ref]  wrote:

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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.

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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.

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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.

Thinking more about this, I think this is how any clk op that might change
the parent should operate. I will try to write up an RFC patch for this
and send it out soon. I'm in a hurry, so will explain more in the RFC
patch or in a later email.

Thanks,
Saravana

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