Re: [PATCH] clk: prevent erronous parsing of children during rate change

From: Tero Kristo
Date: Tue Sep 30 2014 - 04:48:16 EST

On 09/30/2014 10:07 AM, Mike Turquette wrote:
Quoting Tero Kristo (2014-09-29 01:09:24)
On 09/27/2014 02:24 AM, Mike Turquette wrote:
Quoting Tero Kristo (2014-09-26 00:18:55)
On 09/26/2014 04:35 AM, Stephen Boyd wrote:
On 09/23/14 06:38, Tero Kristo wrote:
On 09/22/2014 10:18 PM, Stephen Boyd wrote:
On 08/21, Tero Kristo wrote:
/* Skip children who will be reparented to another clock */
if (child->new_parent && child->new_parent != clk)

Are we not hitting the new_parent check here? I don't understand
how we can be changing parents here unless the check is being
avoided, in which case I wonder why determine_rate isn't being

It depends how the clock underneath handles the situation. The error I
am seeing actually happens with a SoC specific compound clock (DPLL)
which integrates set_rate + mux functionality into a single clock
node. A call to the clk_set_rate changes the parent of this clock
(from bypass clock to reference clock), in addition to changing the
rate (tune the mul+div.) I looked at using the determine rate call
with this type but it breaks everything up... the parent gets changed
but not the clock rate, in addition to some other issues.

Ok. Is this omap3_noncore_dpll_set_rate()?


> Can we use determine_rate +
clk_set_parent_and_rate()? At least clk_set_parent_and_rate() would
allow us to do the mult+div and the parent in the same op call, although
I don't understand why setting the parent and then setting the rate is
not going to work.

Well, setting parent first, then rate later causes problems with the
DPLL ending up running with illegal (non-specified) rate, the M+N values
are most likely wrong if you just switch from bypass clock to reference
clock first without programming the M+N first.

I took a quick look and it still seems to me that the OMAP DPLLs are
still not modeled properly as mux clocks. Is this correct?

Yeah, they are not mux clocks, but rather a compound of mux + DPLL
multiplier/divider logic. Changing the DPLL to be a separate mux + DPLL
div/mult clock will still have overlapping usage of the DPLL_EN field,

I'm not talking about splitting up the clock into two separate clocks.
If memory serves the DPLL clock implementation "cheats" and hides the
bypass_clk info from the clock framework. To be explicit, from the
perspective of Linux clock framework DPLL clocks only have one parent.

In reality a typical DPLL should have at least 2 parents (and in some
cases starting with OMAP4, some of the DPLL output clocks should have a
second HSD parent). But the implementation does not reflect this.

No, this is not the DPLLs are modelled. Each DPLL has currently two parents, ref-clk and bypass-clk, which are both modelled as separate clock nodes, and the DPLL switches parents based on bypass/lock mode. The bypass clock is also usually a mux clock, which further selects separate bypass parent, resulting in 3 or more parents for a certain DPLL.

as the DPLL must be in bypass mode during M+N change. Or, should the
DPLL rate change only be allowed if the mux is in bypass setting?
Several drivers still depend on direct dpll clk_set_rate working
'properly' (there are some other issues currently present also which
have nothing to do with the mux behavior.)

This issue has been lingering for a long time and we can't use
determine_rate unless that clock has multiple parents. Simply hacking
knowledge of the parent bypass clock into the .set_rate callback is not

If you believe this _must_ be changed, I can take a look at this for
next merge window, but this will cause a DT data compatibility break if
nothing else (personally I don't care about this as I always rebuild DT
blob with kernel, but lots of other people seem to do.)

Well I guess the question is how long will we put up with the many small
headaches caused by incorrectly modeling the clock?

Well, its not kind of incorrectly modelled, it is just modelled in such way that clk_set_rate doesn't cope too well with it.

determine_rate and clk_set_parent_and_rate should be sufficient for the
OMAP DPLLs but only if they are correctly modeled in the framework.

Do we have implementation for clk_set_parent_and_rate someplace? I looked at rc7 and didn't find this. I think this would fix the issues I am seeing combined with determine_rate, if clk_set_rate would internally handle changing both rate + parent.





I'm interested in the other issues that you mentioned

Mostly these were side-effects from the illegal DPLL setup I guess, like
boot hang, failed drivers etc. I didn't really investigate this that
much as it is much more simpler just to use safe list iteration here.


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