Hi,

Thinh Nguyen <thinh.nguyen@synopsys.com> 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@synopsys.com>
>>>>> ---
>>>>> 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.

-- 
balbi