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

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Date:	Fri, 26 Sep 2014 10:18:55 +0300
From:	Tero Kristo <t-kristo@...com>
To:	Stephen Boyd <sboyd@...eaurora.org>
CC:	<linux-kernel@...r.kernel.org>, <linux-omap@...r.kernel.org>,
	<tony@...mide.com>, <nm@...com>, <mturquette@...aro.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] clk: prevent erronous parsing of children during rate
 change

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)
>>>>                continue;
>>>
>>> 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
>>> used.
>>>
>>
>> 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()?

Yes.

 > 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'm interested in the other issues that you mentioned
> too.

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.

-Tero
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