linux-kernel - Re: [PATCH v3 4/7] usb: dwc3: Program GFLADJ

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Date:   Thu, 3 Feb 2022 03:10:05 +0000
From:   Thinh Nguyen <Thinh.Nguyen@...opsys.com>
To:     Sean Anderson <sean.anderson@...o.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
CC:     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 4/7] usb: dwc3: Program GFLADJ

Sean Anderson wrote:
> GUCTL.REFCLKPER can only account for clock frequencies with integer
> periods. To address this, program REFCLK_FLADJ with the relative error
> caused by period truncation. The formula given in the register reference
> has been rearranged to allow calculation based on rate (instead of
> period), and to allow for fixed-point arithmetic.
> 
> Additionally, calculate a value for 240MHZDECR. This configures a
> simulated 240Mhz clock using a counter with one fractional bit (PLS1).
> 
> This register is programmed only for versions >= 2.50a, since this is
> the check also used by commit db2be4e9e30c ("usb: dwc3: Add frame length
> adjustment quirk").
> 
> Signed-off-by: Sean Anderson <sean.anderson@...o.com>
> Reviewed-by: Robert Hancock <robert.hancock@...ian.com>
> Tested-by: Robert Hancock <robert.hancock@...ian.com>
> ---
> 
> Changes in v3:
> - Define each variable on its own line
> - Update comment to notes some things mentioned during review
> 
> Changes in v2:
> - Also program GFLADJ.240MHZDECR
> - Don't program GFLADJ if the version is < 2.50a
> 
>  drivers/usb/dwc3/core.c | 39 ++++++++++++++++++++++++++++++++++++++-
>  drivers/usb/dwc3/core.h |  3 +++
>  2 files changed, 41 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
> index 38fef5c74359..18adddfba3da 100644
> --- a/drivers/usb/dwc3/core.c
> +++ b/drivers/usb/dwc3/core.c
> @@ -348,6 +348,8 @@ static void dwc3_frame_length_adjustment(struct dwc3 *dwc)
>  static void dwc3_ref_clk_period(struct dwc3 *dwc)
>  {
>  	unsigned long period;
> +	unsigned long fladj;
> +	unsigned long decr;
>  	unsigned long rate;
>  	u32 reg;
>  
> @@ -358,6 +360,7 @@ static void dwc3_ref_clk_period(struct dwc3 *dwc)
>  		period = NSEC_PER_SEC / rate;
>  	} else if (dwc->ref_clk_per) {
>  		period = dwc->ref_clk_per;
> +		rate = NSEC_PER_SEC / period;
>  	} else {
>  		return;
>  	}
> @@ -366,9 +369,43 @@ static void dwc3_ref_clk_period(struct dwc3 *dwc)
>  	reg &= ~DWC3_GUCTL_REFCLKPER_MASK;
>  	reg |=  FIELD_PREP(DWC3_GUCTL_REFCLKPER_MASK, period);
>  	dwc3_writel(dwc->regs, DWC3_GUCTL, reg);
> +
> +	if (DWC3_VER_IS_PRIOR(DWC3, 250A))
> +		return;
> +
> +	/*
> +	 * The calculation below is
> +	 *
> +	 * 125000 * (NSEC_PER_SEC / (rate * period) - 1)
> +	 *
> +	 * but rearranged for fixed-point arithmetic. The division must be
> +	 * 64-bit because 125000 * NSEC_PER_SEC doesn't fit in 32 bits (and
> +	 * neither does rate * period).
> +	 *
> +	 * Note that rate * period ~= NSEC_PER_SECOND, minus the number of
> +	 * nanoseconds of error caused by the truncation which happened during
> +	 * the division when calculating rate or period (whichever one was
> +	 * derived from the other). We first calculate the relative error, then
> +	 * scale it to units of 8 ppm.
> +	 */
> +	fladj = div64_u64(125000ULL * NSEC_PER_SEC, (u64)rate * period);
> +	fladj -= 125000;
> +
> +	/*
> +	 * The documented 240MHz constant is scaled by 2 to get PLS1 as well.
> +	 */
> +	decr = 480000000 / rate;
> +
> +	reg = dwc3_readl(dwc->regs, DWC3_GFLADJ);
> +	reg &= ~DWC3_GFLADJ_REFCLK_FLADJ_MASK
> +	    &  ~DWC3_GFLADJ_240MHZDECR
> +	    &  ~DWC3_GFLADJ_240MHZDECR_PLS1;
> +	reg |= FIELD_PREP(DWC3_GFLADJ_REFCLK_FLADJ_MASK, fladj)
> +	    |  FIELD_PREP(DWC3_GFLADJ_240MHZDECR, decr >> 1)
> +	    |  FIELD_PREP(DWC3_GFLADJ_240MHZDECR_PLS1, decr & 1);
> +	dwc3_writel(dwc->regs, DWC3_GFLADJ, reg);
>  }
>  
> -
>  /**
>   * dwc3_free_one_event_buffer - Frees one event buffer
>   * @dwc: Pointer to our controller context structure
> diff --git a/drivers/usb/dwc3/core.h b/drivers/usb/dwc3/core.h
> index 45cfa7d9f27a..eb9c1efced05 100644
> --- a/drivers/usb/dwc3/core.h
> +++ b/drivers/usb/dwc3/core.h
> @@ -388,6 +388,9 @@
>  /* Global Frame Length Adjustment Register */
>  #define DWC3_GFLADJ_30MHZ_SDBND_SEL		BIT(7)
>  #define DWC3_GFLADJ_30MHZ_MASK			0x3f
> +#define DWC3_GFLADJ_REFCLK_FLADJ_MASK		GENMASK(21, 8)
> +#define DWC3_GFLADJ_240MHZDECR			GENMASK(30, 24)
> +#define DWC3_GFLADJ_240MHZDECR_PLS1		BIT(31)
>  
>  /* Global User Control Register*/
>  #define DWC3_GUCTL_REFCLKPER_MASK		0xffc00000

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

Thanks for the work,
Thinh