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 -- Pengutronix e.K. | | Industrial Linux Solutions | http://www.pengutronix.de/ | Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 | -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
