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From:   Sean Anderson <sean.anderson@seco.com>
Subject: Re: [PATCH v2 4/7] usb: dwc3: Program GFLADJ
To:     Thinh Nguyen <Thinh.Nguyen@synopsys.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>
Cc:     Felipe Balbi <balbi@kernel.org>,
        Balaji Prakash J <bjagadee@codeaurora.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Robert Hancock <robert.hancock@calian.com>,
        Baruch Siach <baruch@tkos.co.il>
References: <20220119002438.106079-1-sean.anderson@seco.com>
 <20220119002438.106079-5-sean.anderson@seco.com>
 <4696c5a4-5921-f7cb-196c-5ad956e696f9@synopsys.com>
Message-ID: <f528aeb5-6155-a75e-9d35-9bf473e0bbc7@seco.com>
Date:   Mon, 24 Jan 2022 18:06:00 -0500
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On 1/24/22 5:46 PM, Thinh Nguyen wrote:
> 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.
>> 
>> 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 db2be4e9e30c ("usb: dwc3: Add frame length
>> adjustment quirk").
>> 
>> Signed-off-by: Sean Anderson <sean.anderson@seco.com>
>> ---
>> 
>> Changes in v2:
>> - Also program GFLADJ.240MHZDECR
>> - Don't program GFLADJ if the version is < 2.50a
>> 
>>  drivers/usb/dwc3/core.c | 37 +++++++++++++++++++++++++++++++++++--
>>  drivers/usb/dwc3/core.h |  3 +++
>>  2 files changed, 38 insertions(+), 2 deletions(-)
>> 
>> diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
>> index 5214daceda86..883e119377f0 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 decr, fladj, rate, period;
> 
> Minor style nit: Felipe prefers to keep the declaration on separate
> lines, let's keep it consistent with the rest in this driver.

So 

unsigned int decr;
unsigned int fladj;
unsigned int rate;
unsigned int period;

?

Frankly that seems rather verbose.

>>  
>>  	if (dwc->ref_clk) {
>>  		rate = clk_get_rate(dwc->ref_clk);
>> @@ -357,6 +357,7 @@ 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;
>>  	}
>> @@ -365,9 +366,41 @@ 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.
>> +	 *
>> +	 * 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);
> 
> Can we rearrange the math so we don't have to operate on different data
> type and deal with conversion/truncation?

I don't understand what data types you are referring to.

The truncation above (in the calculaion for rate/period) is intentional,
so we can determine the error introduced by representing period using
only ns.

>> +	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);
> 
> Does this pass checkpatch?

Yes.

--Sean

>> +	dwc3_writel(dwc->regs, DWC3_GFLADJ, reg);
>>  }
>>  
>> -
>>  /**
>>   * dwc3_free_one_event_buffer - Frees one event buffer
>>   * @dwc: Pointer to our controller context structure
>> diff --git a/drivers/usb/dwc3/core.h b/drivers/usb/dwc3/core.h
>> index 45cfa7d9f27a..eb9c1efced05 100644
>> --- a/drivers/usb/dwc3/core.h
>> +++ b/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
> Thanks,
> Thinh
>