[PATCH 4/6] usb: dwc3: Handle fractional reference clocks

[Sean Anderson]

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);
-}
 
+	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
-- 
2.25.1