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

Mike Turquette
2015-03-06T22:57:53Z

Quoting Ray Jui (2015-03-06 12:07:13)
> 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?

No. Without a user we should drop it, and I also do not like its use of
clk_lookup.

Thanks,
Mike

>
> Thanks,
>
> Ray
>
> >>
> >> Sascha
> >>
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