Date:	Mon, 08 Apr 2013 20:32:30 +0200
From:	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
To:	Mike Turquette <mturquette@...aro.org>
CC:	Guenter Roeck <linux@...ck-us.net>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	Rob Landley <rob@...dley.net>,
	Stephen Warren <swarren@...dia.com>,
	Thierry Reding <thierry.reding@...onic-design.de>,
	Dom Cobley <popcornmix@...il.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Arnd Bergmann <arnd@...db.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Rabeeh Khoury <rabeeh@...id-run.com>,
	Daniel Mack <zonque@...il.com>,
	Jean-Francois Moine <moinejf@...e.fr>,
	Lars-Peter Clausen <lars@...afoo.de>,
	devicetree-discuss@...ts.ozlabs.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [v5] clk: add si5351 i2c common clock driver

On 04/08/2013 07:36 PM, Mike Turquette wrote:
> Quoting Sebastian Hesselbarth (2013-04-08 08:38:44)
>> On Mon, Apr 8, 2013 at 4:54 PM, Guenter Roeck<linux@...ck-us.net>  wrote:
>>> On Mon, Apr 08, 2013 at 08:11:38AM +0200, Sebastian Hesselbarth wrote:
>>>> On 04/08/2013 02:17 AM, Guenter Roeck wrote:
>>>>> On Mon, Apr 08, 2013 at 01:49:24AM +0200, Sebastian Hesselbarth wrote:
>>>>>> On 04/08/2013 12:50 AM, Guenter Roeck wrote:
>>>>>>> On Fri, Apr 05, 2013 at 05:23:35AM -0000, Sebastian Hesselbarth wrote:
>>>>>>>> This patch adds a common clock driver for Silicon Labs Si5351a/b/c
>>>>>>>> i2c programmable clock generators. Currently, the driver supports
>>>>>>>> DT kernels only and VXCO feature of si5351b is not implemented. DT
>>>>>>>> bindings selectively allow to overwrite stored Si5351 configuration
>>>>>>>> which is very helpful for clock generators with empty eeprom
>>>>>>>> configuration. Corresponding device tree binding documentation is
>>>>>>>> also added.
>>>>>>>>
>>>>>>>> Signed-off-by: Sebastian Hesselbarth<sebastian.hesselbarth@...il.com>
>>>>>>>> Tested-by: Daniel Mack<zonque@...il.com>
>>>>>>>>
>>>>>>> [ ... ]
>>>>>>>
>>>>>>>> +static inline void _si5351_msynth_set_pll_master(
>>>>>>>> + struct si5351_driver_data *drvdata, unsigned char num, int is_master)
>>>>>>>> +{
>>>>>>>> + unsigned long flags;
>>>>>>>> +
>>>>>>>> + if (num>    8 ||
>>>>>>>> +     (drvdata->variant == SI5351_VARIANT_A3&&    num>    3))
>>>>>>>> +         return;
>>>>>>>> +
>>>>>>>> + flags = __clk_get_flags(drvdata->msynth[num].hw.clk);
>>>>>>>> + if (is_master)
>>>>>>>> +         flags |= CLK_SET_RATE_PARENT;
>>>>>>>> + else
>>>>>>>> +         flags&= ~CLK_SET_RATE_PARENT;
>>>>>>>> + __clk_set_flags(drvdata->msynth[num].hw.clk, flags);
>>>>>>>> +}
>>>>>>>> +
>>>>>>> Unless I am missing something, neither __clk_get_flags() nor the new
>>>>>>> __clk_set_flags is exported.
>>>>>>>
>>>>>>> Did you try to build and load the driver as module ?
>>>>>>
>>>>>> Well, good catch. I didn't try to build v5 as a module, but I guess it
>>>>>> will fail. But I consider this as something that has to be addressed in
>>>>>> clk framework itself, not in this patch. There will be other
>>>>>> clk-providers built as module in the future for sure.
>>>>>>
>>>>> Sure, but you provided the patch to make __clk_set_flags global. To avoid
>>>>> build failures, I would suggest to either submit a patch to export the
>>>>> missing functions, or to remove the ability to build the driver as module.
>>>>
>>>> Actually, I knew that __clk_set_flags patch will not be accepted
>>>> before posting it ;)
>>>>
>>> Ah, but part of that is to get you to think about it again, and to defend it if
>>> it is really needed. After all, "it can be abused" applies to pretty much every
>>> API.
>>
>> Guenter,
>>
>> I already thought about it a lot and I consider clk api broken in a way here.
>>
>>> Key question is if you _really_ need run-time flag modifications, or if you can
>>> live with initialization-time settings. If you really need it, you'll have to
>>> explain the reasons.
>>
>> The question is not if _I_ really need run-time flags but why the api allows to
>> perform run-time modifications of the clock hierarchy without allowing different
>> flags? There is clk_set_parent() so I guess clk api knows about run-time changes
>> already, but you cannot have different flags per parent. And with
>> __clk_set_flags()
>> rejected, you are not allowed to change the flags.
>>
>> I understand that some flags are permanent and required at registration, but
>> CLK_SET_PARENT_RATE is not. It is not limited by hardware but limited by api.
>> One way would be a more generic clk-mux with per parent flags, but for
>> the current
>> implementation, I cannot see how clk-mux can be exploited here.
>>
>
> There are a couple of ways to get past this issue.  One is removal of
> some of the flags.  I have never liked CLK_SET_RATE_PARENT, since I
> think the ability to propagate a rate-change request up to the parent
> should be enabled for all clocks.  This is in the interest of the driver
> author who does not care to know intimate details of the clock
> hierarchy.
>
> When I developed the rate-change propagation code I was likely too hasty
> in defining a per-clock flag for that behavior.  It might have been
> enough to simply compare the&best_parent_rate value from .round_rate
> and compare it against clk->parent->rate.  This means that no flag is
> necessary.
>
> This assumes that the .round_rate implementation has enough knowledge to
> know whether or not to propagate the rate-change request up to the
> parent.  This may not not true for the common divider type.
>
> I'll make some tests on removing this flag (and potentially other flags)
> to see how painful it is.

Mike,

another good thing would be to have implementation independent building 
blocks for clk drivers. I think clk-fixed-rate, clk-gate, clk-mux and
some clk-synth should be enough. The current core clk templates in the
api are almost all special implementations.

Sebastian
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