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

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Date:	Fri, 26 Sep 2014 16:24:53 -0700
From:	Mike Turquette <mturquette@...aro.org>
To:	Tero Kristo <t-kristo@...com>,
	"Stephen Boyd" <sboyd@...eaurora.org>
Cc:	linux-kernel@...r.kernel.org, linux-omap@...r.kernel.org,
	tony@...mide.com, nm@...com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] clk: prevent erronous parsing of children during rate change

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)
> >>>>                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 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?

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
enough.

Regards,
Mike

> 
>   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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