On 1/14/22 8:09 PM, Robert Hancock wrote:
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?
That's a good question...
The oldest version I found with it is 2.60a [1].
[1] https://www.mouser.com/pdfdocs/silver-celeron-datasheet-vol-2.pdf
--Sean
