Re: [PATCH v2] usb: dwc3: add power down scale setting

[Thinh Nguyen <Thinh.Nguyen@synopsys.com>]

Li Jun wrote:
> Some SoC(e.g NXP imx8MQ) may have a wrong default power down scale
> setting so need init it to be the correct value, the power down
> scale setting description in DWC3 databook:
> 
> Power Down Scale (PwrDnScale)
> The USB3 suspend_clk input replaces pipe3_rx_pclk as a clock source to
> a small part of the USB3 core that operates when the SS PHY is in its
> lowest power (P3) state, and therefore does not provide a clock.
> The Power Down Scale field specifies how many suspend_clk periods fit
> into a 16 kHz clock period. When performing the division, round up the
> remainder.
> For example, when using an 8-bit/16-bit/32-bit PHY and 25-MHz Suspend
> clock,
> Power Down Scale = 25000 kHz/16 kHz = 13'd1563 (rounder up)
> 
> So use the suspend clock rate to calculate it.
> 
> Signed-off-by: Li Jun <jun.li@nxp.com>
> ---
> Changes for v2:
> - Add PwrDnScale field update condition: less than the calculated
>   value or more than 3x the calculated value.
> - Correct 16k from 16384 to 16000.
> - Delare variables in separate lines.
> 
>  drivers/usb/dwc3/core.c | 30 ++++++++++++++++++++++++++++++
>  drivers/usb/dwc3/core.h |  1 +
>  2 files changed, 31 insertions(+)
> 
> diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
> index e027c0420dc3..37c34690d11b 100644
> --- a/drivers/usb/dwc3/core.c
> +++ b/drivers/usb/dwc3/core.c
> @@ -1029,6 +1029,33 @@ static void dwc3_set_incr_burst_type(struct dwc3 *dwc)
>  	dwc3_writel(dwc->regs, DWC3_GSBUSCFG0, cfg);
>  }
>  
> +static void dwc3_set_power_down_clk_scale(struct dwc3 *dwc)
> +{
> +	u32 scale;
> +	u32 reg;
> +
> +	if (!dwc->susp_clk)
> +		return;
> +
> +	/*
> +	 * The power down scale field specifies how many suspend_clk
> +	 * periods fit into a 16KHz clock period. When performing
> +	 * the division, round up the remainder.
> +	 *
> +	 * Only update the power down scale when the default scale:
> +	 * Less than the calculated value from clk_get_rate(); or
> +	 * Ridiculously high that's more than 3x the calculated value.

We should describe why we want to do this. Maybe rephrase the line above
as below (or something equivalent):

"The power down scale value is calculated using the fastest frequency of
the suspend_clk. If it isn't fixed (but within the accuracy
requirement), the driver may not know the max rate of the suspend_clk.
Only update the power down scale if the default is less than the
calculated value from clk_get_rate() or if the default is questionably
high (3x or more) to be within the requirement."

The rest looks fine to me.

Reviewed-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>

Thanks,
Thinh

> +	 */
> +	scale = DIV_ROUND_UP(clk_get_rate(dwc->susp_clk), 16000);
> +	reg = dwc3_readl(dwc->regs, DWC3_GCTL);
> +	if ((reg & DWC3_GCTL_PWRDNSCALE_MASK) < DWC3_GCTL_PWRDNSCALE(scale) ||
> +	    (reg & DWC3_GCTL_PWRDNSCALE_MASK) > DWC3_GCTL_PWRDNSCALE(scale*3)) {
> +		reg &= ~(DWC3_GCTL_PWRDNSCALE_MASK);
> +		reg |= DWC3_GCTL_PWRDNSCALE(scale);
> +		dwc3_writel(dwc->regs, DWC3_GCTL, reg);
> +	}
> +}
> +
>  /**
>   * dwc3_core_init - Low-level initialization of DWC3 Core
>   * @dwc: Pointer to our controller context structure
> @@ -1105,6 +1132,9 @@ static int dwc3_core_init(struct dwc3 *dwc)
>  	if (ret)
>  		goto err1;
>  
> +	/* Set power down scale of suspend_clk */
> +	dwc3_set_power_down_clk_scale(dwc);
> +
>  	/* Adjust Frame Length */
>  	dwc3_frame_length_adjustment(dwc);
>  
> diff --git a/drivers/usb/dwc3/core.h b/drivers/usb/dwc3/core.h
> index 81c486b3941c..722808d8c0af 100644
> --- a/drivers/usb/dwc3/core.h
> +++ b/drivers/usb/dwc3/core.h
> @@ -231,6 +231,7 @@
>  
>  /* Global Configuration Register */
>  #define DWC3_GCTL_PWRDNSCALE(n)	((n) << 19)
> +#define DWC3_GCTL_PWRDNSCALE_MASK	GENMASK(31, 19)
>  #define DWC3_GCTL_U2RSTECN	BIT(16)
>  #define DWC3_GCTL_RAMCLKSEL(x)	(((x) & DWC3_GCTL_CLK_MASK) << 6)
>  #define DWC3_GCTL_CLK_BUS	(0)