Common clock: function clock and bus clock

Chao Xie xiechao.mail at
Wed Mar 12 22:15:20 EDT 2014

On Wed, Mar 12, 2014 at 6:14 PM, Grygorii Strashko
<grygorii.strashko at> wrote:
> On 03/12/2014 04:30 AM, Chao Xie wrote:
>> On Tue, Mar 11, 2014 at 10:48 AM, Haojian Zhuang
>> <haojian.zhuang at> wrote:
>>> On Tue, Mar 11, 2014 at 10:06 AM, Chao Xie <xiechao.mail at>
>>> wrote:
>>>> hi
>>>> I can not find any examples for handling function clock and bus clock
>>>> in drivers/clk/.
>>>> For a device, it will have a function clock and bus clock. function
>>>> clock will control the fucntionality of this device, while bus clock
>>>> will control the communication part to the bus.
>>>> For some SOCes, they do not export bus clock, so from the hardware it
>>>> seems that function clock is combined with bus clock, while for some
>>>> SOCes, they are not.
>>>> For most of the device driver, they will enable/disable function clock
>>>> and bus clock both. While for some devices, they may share bus clock,
>>>> and have different function clocks.
>>>> In fact, i want to define the APIs as below:
>>>> struct clk* soc_register_bus_clock(struct device *dev, char
>>>> **parent_name, ...);
>>>> This function will  create and register bus clock
>>>> struct clk* soc_register_fucntion_clock(struct device *dev, char
>>>> **parent_name, ...);
>>>> This function will create and register function clock
>>>> struct clk* soc_register_clk(struct device *dev, char **parent_name);
>>>> This function will create and register a device clock. Then this clock
>>>> is enable, it will enable bus clock and function clock both, and it is
>>>> similar for disabling.
>>>> struct clk* soc_register_composite_clk(struct device *dev, char
>>>> *bus_clk_name, char *function_clk_name);
>>>> This function will create and register a composite device clock. When
>>>> enable this clock, it will invoke clk_enable(bus_clk) and
>>>> clk_enable(function_clk) both. It is similar for disabling.
>>>> So for the device which has its own control register and bits for
>>>> function clock and bus clock. We can call  soc_register_clk.
>>>> For the device share bus clock with other devices, we create the clock
>>>> for the device by calling  soc_register_composite_clk.
>>>> 1. Now clk_enable can be preempted by same caller.
>>>> 2. For above devices, the bus clock only have enable/disable ops.
>>>> Based on above conditions, I think soc_register_composite_clk is fine.
>>>> Does anyone have same problems? Will above proposal break common clock
>>>> rules.
>>> I think it's too complex.
>>> If the bus clock and function clock is only used for gating, maybe you
>>> can set the bus clock as parent of function clock.
>> You can not do it. Because the function clock may have mux and div, it
>> can not depends on bus clock's rate.
>>> If both of them are complex, why not define two clocks in your device
>>> driver?
>> It is the problem. In fact for different SOCes, we can not know which
>> device will have such limitation.
>> Even for same device, for SOC-a, it has the limitation, for SOC-b, it does
>> not.
>> So the choice is make all the device drivers to grap two clocks. There
>> are too many of them, and for
>> the devices does not have such limitation, it is redundant.
>> So the point is we donot want to change the device driver, and we want
>> to make the device driver interfaces
>> maintain unchanged. .
> Clk pm domain - Is it what you want?
> drivers/base/power/clock_ops.c
> Usage example:
> arch/arm/mach-davinci/pm_domain.c
> arch/arm/mach-keystone/pm_domain.c
> Then you can simply use Runtime PM API to control clocks.
It is not related to clk pm domain. It is related to clock depenency.

For a device, it has bus clock and function clock. usally bus clock is
a gate type clock.

some devices share same bus clock, but have different function clock.
If we want to use the device, we have to enable the bus clock and
function clock both.

The problem is for some SOCes, there is no sharing of bus clock while
for others, it has, and this limitation can happen at all clocks in
clock domain.

So i do not want to make device driver be aware of it. I just let it
know there is only one clock, and keep the dependency in clock

> Regrads,
> -grygorii

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