On Fri, 2022-01-14 at 18:39 -0500, 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.
>
> Signed-off-by: Sean Anderson <sean.anderson@xxxxxxxx>
> ---
>
> drivers/usb/dwc3/core.c | 25 +++++++++++++++++++++++--
> drivers/usb/dwc3/core.h | 1 +
> 2 files changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
> index 5214daceda86..48bb3e42cdd0 100644
> --- a/drivers/usb/dwc3/core.c
> +++ b/drivers/usb/dwc3/core.c
> @@ -348,7 +348,7 @@ static void dwc3_frame_length_adjustment(struct dwc3
> *dwc)
> static void dwc3_ref_clk_period(struct dwc3 *dwc)
> {
> u32 reg;
> - unsigned long rate, period;
> + unsigned long fladj, rate, period;
>
> if (dwc->ref_clk) {
> rate = clk_get_rate(dwc->ref_clk);
> @@ -357,16 +357,37 @@ 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;
> }
>
> + /*
> + * The calculation below is
> + *
> + * 125000 * (NSEC_PER_SEC / (rate * period) - 1)
> + *
> + * but rearranged for fixed-point arithmetic.
> + *
> + * 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 0.08%.
> + */
> + fladj = div64_u64(125000ULL * NSEC_PER_SEC, (u64)rate * period);
> + fladj -= 125000;
> +
> reg = dwc3_readl(dwc->regs, DWC3_GUCTL);
> reg &= ~DWC3_GUCTL_REFCLKPER_MASK;
> reg |= FIELD_PREP(DWC3_GUCTL_REFCLKPER_MASK, period);
> dwc3_writel(dwc->regs, DWC3_GUCTL, reg);
> -}
>
>
dwc3_frame_length_adjustment which also writes to the DWC3_GFLADJ register has
a conditional to skip it if DWC3_VER_IS_PRIOR(DWC3, 250A), not sure if it's
needed for this field or not?
> + reg = dwc3_readl(dwc->regs, DWC3_GFLADJ);
> + reg &= ~DWC3_GFLADJ_REFCLK_FLADJ_MASK;
> + reg |= FIELD_PREP(DWC3_GFLADJ_REFCLK_FLADJ_MASK, fladj);
> + dwc3_writel(dwc->regs, DWC3_GFLADJ, reg);
> +}
>
> /**
> * dwc3_free_one_event_buffer - Frees one event buffer
> diff --git a/drivers/usb/dwc3/core.h b/drivers/usb/dwc3/core.h
> index 32dfcf3a83d5..50c094af131d 100644
> --- a/drivers/usb/dwc3/core.h
> +++ b/drivers/usb/dwc3/core.h
> @@ -388,6 +388,7 @@
> /* 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)
>
> /* Global User Control Register*/
> #define DWC3_GUCTL_REFCLKPER_MASK 0xffc00000
--
Robert Hancock
Senior Hardware Designer, Calian Advanced Technologies
www.calian.com
