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

[Sascha Hauer <s.hauer@pengutronix.de>]

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.

Sascha

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