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SubjectRe: [PATCH 4/6] usb: dwc3: Handle fractional reference clocks
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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@seco.com>
>> ---
>>
>> 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

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