Re: [PATCH v5 1/6] clk: add of_clk_get_parent_rate function

From: Ray Jui
Date: Fri Mar 06 2015 - 15:07:26 EST

Hi Mike,

On 3/6/2015 11:55 AM, Mike Turquette wrote:
> Quoting Sascha Hauer (2015-02-26 00:43:19)
>> On Wed, Feb 25, 2015 at 11:42:44PM -0800, Ray Jui wrote:
>>> On 2/25/2015 10:51 PM, Sascha Hauer wrote:
>>>> On Wed, Feb 25, 2015 at 10:13:15PM -0800, Ray Jui wrote:
>>>>> Hi Sascha,
>>>>> On 2/25/2015 9:54 PM, Sascha Hauer wrote:
>>>>>> Hi Ray,
>>>>>> On Wed, Feb 04, 2015 at 04:55:00PM -0800, Ray Jui wrote:
>>>>>>> Sometimes a clock needs to know the rate of its parent before itself is
>>>>>>> registered to the framework. An example is that a PLL may need to
>>>>>>> initialize itself to a specific VCO frequency, before registering to the
>>>>>>> framework. The parent rate needs to be known, for PLL multipliers and
>>>>>>> divisors to be configured properly.
>>>>>>> Introduce helper function of_clk_get_parent_rate, which can be used to
>>>>>>> obtain the parent rate of a clock, given a device node and index.
>>>>>> I can't see how this patch helps you. First it's not guaranteed that
>>>>>> the parent is already registered, what do you do in this case?
>>>>> In the case when clock parent is not found, as you can see from the
>>>>> code, it simply returns zero, just like other clk get rate APIs.
>>>> Yes, but what do you do with the 0 result then in your PLL initialization?
>>> As of the current code, it fails the PLL frequency initialization and
>>> bails out. Thinking about it more, it actually makes more sense to just
>>> warn and still go ahead to register the clock, in which case it will use
>>> whatever default frequency after chip power on reset or a frequency
>>> configured in the bootloader.
>>>>> I thought the order of clock registration is based on order of the clock
>>>>> nodes in device tree. It makes sense to me to declare the parent clock
>>>>> before a child clock, so it's guaranteed that the parent is registered
>>>>> before the child.
>>>> No, you can't rely on that. The order of the device nodes may happen to
>>>> define the order of clock initialization now, but that may change.
>>>> device nodes are usually ordered by bus addresses, not by intended
>>>> initialization order. Even if you reorder them everything must still
>>>> work.
>>> Okay I get your point that the order of device nodes may not be relied
>>> on for device initialization order. But then another mechanism should be
>>> deployed to give developers the option to decide on the clock
>>> initialization sequence. It can be optional but it should be there.
>>>>>> Then the clock framework doesn't require that you initialize the PLL
>>>>>> before registering. That can be done in the clk ops later.
>>>>> Sure it's not mandatory. But what's wrong with me choosing to initialize
>>>>> the PLL clock to a known frequency before registering it to the framework?
>>>> Appearantly you don't know the (input) frequency of the PLL when
>>>> registering it to the framework, so the question must be: What's wrong
>>>> with keeping it uninitialized?
>>>> If the PLL is unused then you don't care about it's initialization
>>>> status. If it happens to be enabled by a bootloader and still unused
>>>> at late_initcall time the clock framework will disable it so you
>>>> have a known state then. If a consumer for the PLL appears it's its
>>>> job to initialize it through the clk api.
>>>> Sascha
>>> Okay, what we need here is to initialize the PLL to a desired frequency,
>>> based on device tree settings (since it will be configured differently,
>>> among different boards). This is a PLL that 1) has limited options of
>>> frequencies which it can be configured to, and 2) has multiple child
>>> clocks, where is a more suitable place to initialize it to the desired
>>> frequency than right before registering it to the framework? I know a
>>> lot of people do it in the bootloader, but I thought we should be given
>>> the flexibility of configuring it in the kernel.
>>> When you say "consumers", do you mean 1) the device driver that uses the
>>> PLL; or 2) the device driver that use the child clock of the PLL? If
>>> it's case 1), then we don't really have a device driver that directly
>>> uses the PLL, and I thought that's quite normal, as most PLLs don't
>>> directly feed into any peripherals.
>> I meant 1) and 2). Before a consumer comes along the state of the PLL
>> doesn't matter. When a consumer shows up it has to call
>> clk_prepare_enable which (directly or indirectly) will enable your PLL.
>> Then it's still time to apply the default settings you found out during
>> probe of the PLL.
> My review comments are really for iproc_pll_setup() in patch #3, but the
> discussion is here so I'll respond to this thread.
> I think the root of this problem is that your pll clk_ops does not
> support .set_rate. That is why your clock driver hacks in a call to
> pll_set_rate in iproc_pll_setup.
> Due to the above shortcoming you also do not use the assigned-clock-rate
> infrastructure to set your pll rate at registration-time. There is no
> reason for your driver to re-invent this logic. iproc_pll_setup is
> fetching the clock-frequency property from DT and then trying to set
> that rate. Instead please use the generic code.
> The right way to handle this is to support a .set_rate callback (looks
> like you're 90% of the way there with pll_set_rate) and then use the
> assigned-clock-rates property to specify this from DT.
> Regards,
> Mike

Okay. It's good to know that "assigned-clock-rate" can be used and serve
exactly what we need here. I'll update my patch series to use that instead.

In this case, do you think I should still keep of_clk_get_parent_rate in
the patch series?



>> Sascha
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