Re: [PATCH v4 5/5] clk: dt: binding for basic gate clock

[Stephen Warren <swarren@wwwdotorg.org>]

On 08/29/2013 07:45 PM, Haojian Zhuang wrote:
> On 22 August 2013 13:53, Mike Turquette <mturquette@linaro.org> wrote:
>> Device Tree binding for the basic clock gate, plus the setup function to
>> register the clock.  Based on the existing fixed-clock binding.
>>
>> A different approach to this was proposed in 2012[1] and a similar
>> binding was proposed more recently[2] if anyone wants some extra
>> reading.

>> diff --git a/Documentation/devicetree/bindings/clock/gate-clock.txt b/Documentation/devicetree/bindings/clock/gate-clock.txt

>> +Binding for simple gate clock.
...
>> +       clock_foo: clock_foo@4a008100 {
>> +               compatible = "gate-clock";
>> +               #clock-cells = <0>;
>> +               clocks = <&clock_bar>;
>> +               reg = <0x4a008100 0x4>
>> +               bit-shift = <3>
> 
> There's some argument on my clock binding patch set of Hi3620.
> 
> I defined each clock as one clock node and some of them are sharing one
> register. Stephen attacked on this since it means multiple clock node sharing
> one register.

s/attacked/disagreed with/ I think:-)

> Now the same thing also exists in Mike's patch. Mike's patch could also
> support this behavior. And it's very common that one register is sharing among
> multiple clocks in every SoC. Which one should I follow?

I believe it's a matter of how the HW is structured.

If the HW truly has segregated register regions for each individual
clock, and is documented in a way that implies each individual clock is
a separate HW module, then it makes sense to describe each clock as a
separate DT node.

However, if the HW simply has a "clock module" that provides many
clocks, with inter-mingled registers all over the place, and is
documented as a single module that generates lots of clocks, then it
makes sense to describe that one module as a single DT node.

In other words, the DT representation should match the HW design and
documentation.

This is exactly why we have the #clock-cells property in DT; so that a
clock provider can provide multiple clocks if that's the way the HW is
designed.

>> diff --git a/drivers/clk/clk-gate.c b/drivers/clk/clk-gate.c

>> +void of_gate_clk_setup(struct device_node *node)
>> +{
>> +       struct clk *clk;
>> +       const char *clk_name = node->name;
>> +       void __iomem *reg;
>> +       const char *parent_name;
>> +       u8 clk_gate_flags = 0;
>> +       u32 bit_idx = 0;
>> +
>> +       of_property_read_string(node, "clock-output-names", &clk_name);
>> +
>> +       parent_name = of_clk_get_parent_name(node, 0);
>> +
>> +       reg = of_iomap(node, 0);
> 
> I suggest not using of_iomap for each clock node.
> 
> If each clock is one node, it means hundreds of clock node existing in
> device tree. Hundreds of mapping page only cost unnecessary mapping.

The page table entries will get re-used. I'm not familiar with the mm
code, but multiple of_iomap() for the exact same range probably just map
down to incrementing a refcount on some kernel data structure, so
actually has zero overhead?


> Maybe we can resolve it by this way.
> 
> DTS file:
>         clock register bank {

You need a compatible value here, in order to instantiate the top-level
driver for the "clock generator" HW block.

>                 reg = <{start} {size}>;
>                 #address-cells = <1>;
>                 #size-cells = <0>;             /* each clock only
> exists in one register */

You would need a ranges property here, to map the child reg properties
into the parent node's address space.

>                 clock node {
>                         compatible = "xxx";
>                         #clock-cells = <0>;
>                         clock-output-names = yyy";
>                         reg = <{offset}>;
>                         ... other properties ...
>                 };
>          };

That could be a reasonable solution; the existence of a single "clock
generator" HW block is clearly called out by the existing of the
top-level DT node, yet the internals of that node are free to be
whatever you want, since this is purely defined by the binding
definition for that top-level "clock generator" node.

That all said, I see almost zero value in having all these child nodes,
since the top-level compatible value implies every single detail about
the HW, so the list of clocks and their parameters could just as easily
be a table in the driver for the HW, in order to avoid having to parse
that data every boot just to end up with the same table...

The only exception would be if the SoC designer truly had composed the
top-level "clock generator" HW block out of many completely independent
re-usable clock IP blocks, and many SoCs existed that used those
individual clock blocks completely unchanged, without any SoC-specific
differences such as additional SoC-specific clock block types, so that
one could get greater re-use by parametrizing everything in DT. In my
(perhaps limited) experience of SoCS, this seems like an /extremely/
unlikely situation.