Re: [PATCH v2 1/4] usb: dwc3: Add property snps,refclk-period-ns

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Hi,

On 12/19/2018 10:48 PM, Felipe Balbi wrote:
> Hi,
>
> Thinh Nguyen <thinh.nguyen@xxxxxxxxxxxx> writes:
>>>> On 12/18/2018 8:39 AM, Rob Herring wrote:
>>>>> On Fri, Dec 07, 2018 at 06:27:30PM -0800, Thinh Nguyen wrote:
>>>>>> This patch introduces property "snps,refclk-period-ns" to inform the
>>>>>> controller of the reference clock period. If the reference clock period
>>>>>> is different from the default Core Consultant setting, then this
>>>>>> property can be set to the reference clock period.
>>>>>>
>>>>>> This property does not control the reference clock rate. The controller
>>>>>> uses this value to perform internal timing calculations that are based
>>>>>> on the reference clock.
>>>>>>
>>>>>> Signed-off-by: Thinh Nguyen <thinhn@xxxxxxxxxxxx>
>>>>>> ---
>>>>>> Changes in v2:
>>>>>> - Split from "usb: dwc3: Add reference clock properties"
>>>>>> - Revise commit message and property description
>>>>>>
>>>>>>  Documentation/devicetree/bindings/usb/dwc3.txt | 2 ++
>>>>>>  1 file changed, 2 insertions(+)
>>>>>>
>>>>>> diff --git a/Documentation/devicetree/bindings/usb/dwc3.txt b/Documentation/devicetree/bindings/usb/dwc3.txt
>>>>>> index 8e5265e9f658..b7e67edff9b2 100644
>>>>>> --- a/Documentation/devicetree/bindings/usb/dwc3.txt
>>>>>> +++ b/Documentation/devicetree/bindings/usb/dwc3.txt
>>>>>> @@ -99,6 +99,8 @@ Optional properties:
>>>>>>                      this and tx-thr-num-pkt-prd to a valid, non-zero value
>>>>>>                      1-16 (DWC_usb31 programming guide section 1.2.3) to
>>>>>>                      enable periodic ESS TX threshold.
>>>>>> + - snps,refclk-period-ns: if set, this value informs the controller of the
>>>>>> +                    reference clock period in nanoseconds.
>>>>> Shouldn't you be able to retrieve the refclk frequency and then
>>>>> calculate the period?
>>>> The thing is we cannot determine the ref_clk frequency for some devices
>>>> that don't specify their clocks. So I think we should have an option to
>>>> inform the controller of the ref_clk period for those devices.
>>> Specifying the clock should be mandatory (if you want/need this
>>> feature). It just requires a fixed-clock node at a minimum.
>> Depending on the design of the controller, the ref_clk frequency is not
>> something that the OS can read/control. So we cannot make it mandatory
>> for every device to have a clock node.
> We can make it mandatory to everyone who wants to use the feature. It's
> no different than making snps,refclk-period-ns mandatory to everyone who
> wants to use the feature you're introducing.
>

But not every design has access to the clock with an actual address for
the OS to read. Only the developers will know the frequency of the
ref_clk, and they can inform the controller via this property.

We cannot force the developers to change their design requirement simply
to inform the controller of the ref_clk period.

Thinh




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