Message-ID: <542D53C9.1090305@ti.com>
Date:	Thu, 2 Oct 2014 16:31:53 +0300
From:	Tero Kristo <t-kristo@...com>
To:	Mike Turquette <mturquette@...aro.org>,
	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

On 09/30/2014 10:03 PM, Mike Turquette wrote:
> Quoting Tero Kristo (2014-09-30 01:48:49)
>> 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)
>>>>>>>>>>                   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?
>>>>
>>>> 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.
>
> I stand corrected. I thought it was still done the old way where the
> machine-specific clock struct was holding the pointer to the ref_clk and
> bypass_clk. I'm glad that is not the case any more.
>
>>
>>>
>>>> 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
>>>>> enough.
>>>>
>>>> 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 made it hard for you to find it because I typo'd. It's not a clk api
> but a clk_op:
>
> int (*set_rate_and_parent)(struct clk_hw *hw, unsigned long rate,
>                             unsigned long parent_rate, u8 index);

Oh I see. I have been experimenting with this last couple of days and it 
looks like I got a working solution with determine_rate / 
set_rate_and_parent for the DPLLs now. Need to do some cleanup for the 
patches and will hopefully post those tomorrow, but I guess this will 
probably miss the next merge window seeing we are pretty late in -rc.

-Tero

>
> Regards,
> Mike
>
>>
>> -Tero
>>
>>>
>>> Regards,
>>> Mike
>>>
>>>>
>>>> -Tero
>>>>
>>>>>
>>>>> 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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