linux-kernel - Re: [v5] clk: add si5351 i2c common clock driver

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Date:	Mon, 8 Apr 2013 17:38:44 +0200
From:	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	Rob Landley <rob@...dley.net>,
	Mike Turquette <mturquette@...aro.org>,
	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 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.

I can live with it, because then dynamic muxing of clock hierarchy
within clk-si5351
is just not supported or will break function. Currently, there is no
support for dynamic
muxing, so everything is fine.

>> >On a side note, do you happen to know anyone working on drivers for Si5319 or
>> >Si5368 ?
>>
>> No.
>
> Too bad ... I may have to write that code myself then.

Well, if clk-si5351 will ever get in mainline kernel, feel free to use
it as a template ;)

Sebastian
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