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

[Rob Herring <robh@kernel.org>]

On Tue, Dec 18, 2018 at 6:22 PM Thinh Nguyen <thinh.nguyen@synopsys.com> wrote:
>
> Hi Rob,
>
> 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.

Rob