linux-kernel - Re: [PATCH 2/2] usb: dwc3: add clock and resets

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Date:   Thu, 29 Mar 2018 13:32:34 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Rob Herring <robh@...nel.org>,
        Felipe Balbi <felipe.balbi@...ux.intel.com>
Cc:     linux-usb@...r.kernel.org, Masami Hiramatsu <mhiramat@...nel.org>,
        Jassi Brar <jaswinder.singh@...aro.org>,
        Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>,
        devicetree@...r.kernel.org, Felipe Balbi <balbi@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH 2/2] usb: dwc3: add clock and resets

2018-03-19 7:37 GMT+09:00 Masahiro Yamada <yamada.masahiro@...ionext.com>:
> Hi Rob,
>
> 2018-03-18 21:52 GMT+09:00 Rob Herring <robh@...nel.org>:
>> On Thu, Mar 15, 2018 at 08:39:58PM +0900, Masahiro Yamada wrote:
>>> dwc3-of-simple.c only handles arbitrary number of clocks and resets.
>>> They are both generic enough to be put into the dwc3 core.  For simple
>>> cases, a nested node structure like follows:
>>>
>>>   dwc3-glue {
>>>           compatible = "foo,dwc3";
>>>           clocks = ...;
>>>           resets = ...;
>>>           ...
>>>
>>>           dwc3 {
>>>                   compatible = "snps,dwc3";
>>>                   ...
>>>           };
>>
>> I'm not a fan of how this was done.
>>
>>
>>>   }
>>>
>>> would be turned into a single node:
>>>
>>>   dwc3 {
>>>           compatible = "foo,dwc3", "snps,dwc3";
>>>           clocks = ...;
>>>           resets = ...;
>>>           ...
>>>   }
>>
>> And yes, this is what I'd prefer.
>
>
>
> Not only dwc3-of-simple.c, but all dwc3 nodes are
> written like this.
>
>
> omap_dwc3_1: omap_dwc3_1@...80000 {
>         compatible = "ti,dwc3";
>         reg = <0x48880000 0x10000>;
>         #address-cells = <1>;
>         #size-cells = <1>;
>         ranges;
>         ...
>
>         usb1: usb@...90000 {
>                 compatible = "snps,dwc3";
>                 reg = <0x48890000 0x17000>;
>                 ...
>         };
> };
>
>
> The glue layer initializes SoC-specific things,
> then populates the child "snps,dwc3".
>
>
> I think the following structure should work
> by handling EPROBE_DEFER properly.
>
> omap_dwc3_1: omap_dwc3_1@...80000 {
>         compatible = "ti,dwc3"; (should be "ti,dwc3-glue" or something)
>         reg = <0x48880000 0x10000>;
>         ...
> };
>
> usb1: usb@...90000 {
>         compatible = "snps,dwc3";
>         reg = <0x48890000 0x17000>;
>         ...
> };
>
>
>
>>>
>>> I inserted reset_control_deassert() and clk_enable() before the first
>>> register access, i.e. dwc3_cache_hwparams().
>>>
>>> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
>>> ---
>>>
>>>  Documentation/devicetree/bindings/usb/dwc3.txt |   2 +
>>>  drivers/usb/dwc3/core.c                        | 127 ++++++++++++++++++++++++-
>>>  drivers/usb/dwc3/core.h                        |   5 +
>>>  3 files changed, 132 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/Documentation/devicetree/bindings/usb/dwc3.txt b/Documentation/devicetree/bindings/usb/dwc3.txt
>>> index 44e8bab..67e9cfb 100644
>>> --- a/Documentation/devicetree/bindings/usb/dwc3.txt
>>> +++ b/Documentation/devicetree/bindings/usb/dwc3.txt
>>> @@ -9,12 +9,14 @@ Required properties:
>>>   - interrupts: Interrupts used by the dwc3 controller.
>>>
>>>  Optional properties:
>>> + - clocks: list of phandle and clock specifier pairs
>>
>> However, this should be specific as to how many clocks and their
>> function. This should be readily available to someone with access to
>> Synopsys datasheet. The number of clocks should generally be the same
>> across SoCs, it is just some SoCs either tie clocks together or don't
>> provide s/w control of some of the clocks.
>
>
> Make sense.
> You also implies this property is mandatory.
> The number of clocks should be available in the datasheet
> and no hardware can work with zero clock.
>
> However, making it mandatory breaks the binding
> since the existing DT files do not specify clocks at all
> in the "snps,dwc3" node.
>
>
>
> Anyway, our current situation:
>
> - We have the dwc3-core under the glue layer node
>   despite they are independent in the CPU address view
> - We add all sorts of clocks and resets in the glue layer node,
>   and nothing in the dwc3-core node.
>
> If these are design mistake, what should we do?
>
> Continue development based on it?
> If we fix it, how to change the course?
>

Any insight about this?


I think this is rather a general question.

If somebody upstreams a driver without clocks,
then later it turns out clocks are necessary,
adding required clocks would break existing platforms
since clk_get() fails.


-- 
Best Regards
Masahiro Yamada