Message-ID: <4FBEA540.1010409@codeaurora.org>
Date:	Thu, 24 May 2012 14:16:48 -0700
From:	Saravana Kannan <skannan@...eaurora.org>
To:	Rob Herring <robherring2@...il.com>
CC:	Stephen Boyd <sboyd@...eaurora.org>,
	Shawn Guo <shawn.guo@...aro.org>,
	Mike Turquette <mturquette@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	"arm@...nel.org" <arm@...nel.org>,
	Shawn Guo <shawn.guo@...escale.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [GIT PULL] DT clk binding support

On 05/23/2012 06:59 AM, Rob Herring wrote:
> On 05/22/2012 08:38 PM, Saravana Kannan wrote:
>>
>> On Tue, May 22, 2012 6:52 am, Rob Herring wrote:
>>> On 05/21/2012 11:17 PM, Stephen Boyd wrote:
>>>> On 05/21/12 19:15, Shawn Guo wrote:
>>>>> On Mon, May 21, 2012 at 06:52:37PM -0500, Rob Herring wrote:
>>>>>> As Grant states: "This proposed binding is only about one thing:
>>>>>> attaching clock providers to clock consumers." This means you have to
>>>>>> have at least a single provider and a single consumer defined in the
>>>>>> DT.
>>>>>>
>>>>> I just read through Grant's comments over again.  I agree with the
>>>>> statement which implicitly requires the clk provider defined in DT.
>>>>> However, for some case, this provider in DT is just a skeleton which
>>>>> is backed by clock driver where the provider is actually defined.
>>>>>
>>>>> Looking at Grant's comment below, the second option is also to match
>>>>> the clock in driver just using name.  The only difference to my
>>>>> proposal is the name here is given by the argument of phandle pointing
>>>>> to that skeleton provider node.
>>>>>
>>>>> I'm fine with that.  So go ahead with your bindings.
>>>>>
>>>>
>>>> Can we do what the regulator framework has done and have a common
>>>> binding of<connection_name>-clk =<&phandle>? Something like:
>>>>
>>>>      core-clk =<&uart3_clk>
>>>>
>>>> and then have clk_get() use the of node of the device passed in to find
>>>> a property named %s-clk and find the clock with the matching phandle.
>>>
>>> Sigh... That is what we had in previous versions from over a year ago
>>> and we moved away from that approach. The current binding has been
>>> reviewed multiple times in the last 6 months...
>>>
>>> The current approach is aligned with how interrupts are handled (with
>>> the addition of a phandle). I think not having per clock property names
>>> is easier to parse and easier to document.
>>>
>>>> This looks like it's trying to cover both the end consumers (uart uses
>>>> uart3_clk) and the internal clock tree consumers (a crystal oscillator
>>>> connects to a PLL or a mux has multiple parents). We can certainly use
>>>> these bindings for muxes and internal parent-child relationships but I
>>>> would prefer we use different bindings for consumer bindings that match
>>>> what regulators do today.
>>>
>>> The binding supports either defining every last internal clock or just
>>> the leaf clocks. I took the former route on highbank since I don't have
>>> a lot of clocks. If I was doing imx or omap for example, I'd probably
>>> just define all the clock controller outputs.
>>>
>>
>> If only the leaf nodes are defined in DT, then how is the clock platform
>> driver implementer supposed to instantiate the rest of the tree and
>> connect it up with the partial list of clocks in DT? So, they have to
>> switch back and forth between DT and the .c file which defines the rest
>> and make sure the parent<->child names match?
>>
>> To me it looks that it might better to decouple the description of the
>> clock HW from the mapping of a clock leaf to a consumer device. If we just
>> use a string to identify the clock that's consumed by a device, we can
>> achieve this decoupling at a clean boundary -- clock consumers devices
>> (UART) vs clock producer devices (clock controller in the SoC, in a PMIC,
>> audio codec, etc).
>>
>> With the decoupling, we don't have the inconsistency of having some of the
>> clocks of a clock producer device incompletely defined in DT and the rest
>> of the clocks of the same clock producer device hard coded in the kernel.
>> So, you either put your entire clock tree in the SoC in the DT or put all
>> of it in the kernel but you aren't forced to put just some of them in the
>> DT just to get DT working. I see no benefit in defining only some of the
>> clocks in DT -- it just adds more confusion in the clock tree definition.
>> What am I missing?
>
> I fail to see what would need changing in the binding itself. The
> binding just describes connections. Whether that is a connection to a
> clock controller node to a device or a clock gate/mux/divider node to a
> device is really beyond the clock binding. This is really just policy.
> You are free to put no clocks in DT, all clocks, or a nexus of clocks.

With the current approach you are taking can you please give an example 
of how a random device described in DT would hook itself up with a leaf 
clock if that leaf clock is not described in DT? So that it can do a 
call a DT version of clk_get() to get the clock it cares for.

And no, there is a huge difference between binding a clock controller 
node (by which I mean the block that provides many clocks) to a device 
vs. binding a clock leaf to a device. The former is useless wrt to 
clk_get() and similar functions. The latter is very useful to handle that.

Thanks,
Saravana


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