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

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Date:   Fri, 4 Feb 2022 11:00:25 -0500
From:   Sean Anderson <sean.anderson@...o.com>
To:     Andy Shevchenko <andy.shevchenko@...il.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

Hi Andy,

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:
> 
>     This property allows setting the reference clock frequency properly for
>     ACPI-based systems. It is not documented under dt-bindings, since it is
>     not intended for use on DT-based systems. DT-based systems should use
>     the clocks property instead.
> 
>     Frequency is preferred over period since it has greater precision when
>     used in 
> 
> 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).

--Sean

>     Signed-off-by: Sean Anderson <sean.anderson@...o.com <mailto:sean.anderson@...o.com>>
>     ---
> 
>     (no changes since v2)
> 
>     Changes in v2:
>     - New
> 
>      drivers/usb/dwc3/core.c | 6 ++++--
>      drivers/usb/dwc3/core.h | 4 +++-
>      2 files changed, 7 insertions(+), 3 deletions(-)
> 
>     diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
>     index 18adddfba3da..c1b045121672 100644
>     --- a/drivers/usb/dwc3/core.c
>     +++ b/drivers/usb/dwc3/core.c
>     @@ -353,8 +353,8 @@ static void dwc3_ref_clk_period(struct dwc3 *dwc)
>             unsigned long rate;
>             u32 reg;
> 
>     -       if (dwc->ref_clk) {
>     -               rate = clk_get_rate(dwc->ref_clk);
>     +       if (dwc->ref_clk || dwc->ref_clk_freq) {
>     +               rate = clk_get_rate(dwc->ref_clk) ?: dwc->ref_clk_freq;
>                     if (!rate)
>                             return;
>                     period = NSEC_PER_SEC / rate;
>     @@ -1497,6 +1497,8 @@ static void dwc3_get_properties(struct dwc3 *dwc)
>                                      &dwc->fladj);
>             device_property_read_u32(dev, "snps,ref-clock-period-ns",
>                                      &dwc->ref_clk_per);
>     +       device_property_read_u32(dev, "snps,ref-clock-frequency-hz",
>     +                                &dwc->ref_clk_freq);
> 
>             dwc->dis_metastability_quirk = device_property_read_bool(dev,
>                                     "snps,dis_metastability_quirk");
>     diff --git a/drivers/usb/dwc3/core.h b/drivers/usb/dwc3/core.h
>     index eb9c1efced05..00a792459fec 100644
>     --- a/drivers/usb/dwc3/core.h
>     +++ b/drivers/usb/dwc3/core.h
>     @@ -988,7 +988,8 @@ struct dwc3_scratchpad_array {
>       * @regs: base address for our registers
>       * @regs_size: address space size
>       * @fladj: frame length adjustment
>     - * @ref_clk_per: reference clock period configuration
>     + * @ref_clk_per: reference clock period; deprecated in favor of @ref_clk_freq
>     + * @ref_clk_freq: reference clock frequency to use if @ref_clk is missing
>       * @irq_gadget: peripheral controller's IRQ number
>       * @otg_irq: IRQ number for OTG IRQs
>       * @current_otg_role: current role of operation while using the OTG block
>     @@ -1171,6 +1172,7 @@ struct dwc3 {
> 
>             u32                     fladj;
>             u32                     ref_clk_per;
>     +       u32                     ref_clk_freq;
>             u32                     irq_gadget;
>             u32                     otg_irq;
>             u32                     current_otg_role;
>     -- 
>     2.25.1
> 
> 
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>