usb: dwc3: Program GFLADJ

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 db2be4e9 ("usb: dwc3: Add frame length
adjustment quirk").
Tested-by: Robert Hancock <robert.hancock@calian.com>
Reviewed-by: Robert Hancock <robert.hancock@calian.com>
Reviewed-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Link: https://lore.kernel.org/r/20220127200636.1456175-5-sean.anderson@seco.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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;
 	}
@@ -366,8 +369,42 @@ 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

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