linux-kernel - Re: [PATCH v3 5/7] usb: dwc3: Add snps,ref-clock-frequency-hz property for ACPI

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Message-ID: <CAHp75VfGFNFqOU0BymNwh_FWKVw+rSyJJRDSKOinWY_Z39qaGA@mail.gmail.com>
Date:   Fri, 4 Feb 2022 20:38:50 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Sean Anderson <sean.anderson@...o.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        Robert Hancock <robert.hancock@...ian.com>,
        Baruch Siach <baruch@...s.co.il>,
        Felipe Balbi <balbi@...nel.org>,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Balaji Prakash J <bjagadee@...eaurora.org>
Subject: Re: [PATCH v3 5/7] usb: dwc3: Add snps,ref-clock-frequency-hz
 property for ACPI

On Fri, Feb 4, 2022 at 6:00 PM Sean Anderson <sean.anderson@...o.com> wrote:
> On 2/4/22 7:54 AM, Andy Shevchenko wrote:

...

> > On Thursday, January 27, 2022, Sean Anderson <sean.anderson@...o.com <mailto:sean.anderson@...o.com>> wrote:

> > Is it function or interface clock?
> >
> > We have standard property for the functional clock rate, I.e. “clock-frequency” (in Hz), can it be used here?
>
> I believe this is a "functional" clock. It it is a reference for the USB
> signals. I'm not sure exactly what the purpose of this clock is, since I
> do not have access to the databook for this IP. I considered using
> "clock-frequency", but I am concerned about ambiguity because there is a
> second "suspend" clock which is also a "functional" clock. The latter
> clock appears to be used when the PHY is shut down (and not necessarily
> corresponding to Linux's notion of a suspended device). If it is
> necessary in the future to configure that clock on ACPI platforms (e.g.
> to set GCTL.PWRDNSCALE) I think it is clear what the property name would
> be (snps,susp-clock-frequency-hz).

In order to have more or less unified APIs in the future I would
suggest using 'clock-frequency' for the "main" functional clock. For
example, 8250_dw uses it for the baud rate generator, while it also
utilizes auxiliary clock(s) on some platforms.

-- 
With Best Regards,
Andy Shevchenko