Date:	Thu, 24 May 2012 20:33:15 -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 05/24/2012 02:54 PM, Rob Herring wrote:
> On 05/24/2012 04:16 PM, Saravana Kannan wrote:
>> On 05/23/2012 06:59 AM, Rob Herring wrote:
>>> On 05/22/2012 08:38 PM, Saravana Kannan wrote:
>
> snip
>
>>>> 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.
>
> No, because that's impossible with any binding.

So, this is really forcing everyone moving rest of their devices to DT 
to put part/all of the clocks in DT. Either that or deal with the "aux 
data" that's supposed to be temporary.

> The only way that would
> work is provide a string with a clock name and matching to the struct
> clk name string. That means putting linux clock names into the h/w
> description.

No, the name of the clocks stored in clk->name should be the name of the 
clock in HW.

> That is the wrong direction and not how bindings work.
> Defining bindings should not get tangled up with how the OS
> implementation is done.

So, I'm not asking to bind to the name of a clock as defined by the OS 
implementation. I'm just asking to bind to the name of the clock in HW 
instead of the name binding to an actual DT clock node. And I'm only 
asking this because we seem to want to give an option to NOT have the 
clocks in DT.

A developer might choose to abbreviate the clock name in DT and in 
clk->name field in the kernel, but there's nothing wrong with it. Nor 
can DT do anything about it for ANY (not just clock) DT device descriptions.

>> 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.
>
> The binding and clkdev changes support clk_get fully. Drivers don't have
> to change at all. There is not a DT version of clk_get that all drivers
> have to adopt. It's all handled within clk_get and should be transparent
> to drivers. The only thing that has to change is callers of clk_get_sys
> to use of_clk_get, but that's a small fraction of clocks.

I understand there are changes to have clk_get() work on clocks added 
through DT. But, you said earlier that it doesn't matter if a general DT 
device binds to a clock controller DT device or to an actual clock node 
(say, a leaf in the clock tree) defined in DT. I'm just pointing out 
that it's not true. If a general DT device binds to a clock controller 
DT device, there is no way to figure out what clock node/leaf from the 
clock controller is actually consumed by this general device.

-Saravana

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
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