linux-kernel - Re: [PATCH 2/2] clk: bcm2835: Minimise clock jitter for PCM clock

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Date:   Wed, 31 May 2017 11:19:11 +0200
From:   Stefan Wahren <stefan.wahren@...e.com>
To:     Phil Elwell <phil@...pberrypi.org>, linux-clk@...r.kernel.org,
        Eric Anholt <eric@...olt.net>, linux-kernel@...r.kernel.org,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        linux-rpi-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2] clk: bcm2835: Minimise clock jitter for PCM clock

Am 31.05.2017 um 10:33 schrieb Phil Elwell:
> Hi Stefan,
>
> On 30/05/2017 20:04, Stefan Wahren wrote:
>> Hi Phil,
>>
>>> Phil Elwell <phil@...pberrypi.org> hat am 30. Mai 2017 um 18:28 geschrieben:
>>>
>>>
>>> Fractional clock dividers generate accurate average frequencies but
>>> with jitter, particularly when the integer divisor is small.
>>>
>>> Introduce a new metric of clock accuracy to penalise clocks with a good
>>> average but worse jitter compared to clocks with an average which is no
>>> better but with lower jitter. The metric is the ideal rate minus the
>>> worse deviation from that ideal using the nearest integer divisors.
>>>
>>> Use this metric for parent selection for clocks requiring low jitter
>>> (currently just PCM).
>>>
>>> Signed-off-by: Phil Elwell <phil@...pberrypi.org>
>>> ---
>>>  drivers/clk/bcm/clk-bcm2835.c | 39 ++++++++++++++++++++++++++++++++++-----
>>>  1 file changed, 34 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c
>>> index facc346..e0ce5e7 100644
>>> --- a/drivers/clk/bcm/clk-bcm2835.c
>>> +++ b/drivers/clk/bcm/clk-bcm2835.c
>>> @@ -530,6 +530,7 @@ struct bcm2835_clock_data {
>>>  
>>>  	bool is_vpu_clock;
>>>  	bool is_mash_clock;
>>> +	bool low_jitter;
>>>  
>>>  	u32 tcnt_mux;
>>>  };
>>> @@ -1124,7 +1125,8 @@ static unsigned long bcm2835_clock_choose_div_and_prate(struct clk_hw *hw,
>>>  							int parent_idx,
>>>  							unsigned long rate,
>>>  							u32 *div,
>>> -							unsigned long *prate)
>>> +							unsigned long *prate,
>>> +							unsigned long *avgrate)
>>>  {
>>>  	struct bcm2835_clock *clock = bcm2835_clock_from_hw(hw);
>>>  	struct bcm2835_cprman *cprman = clock->cprman;
>>> @@ -1136,11 +1138,33 @@ static unsigned long bcm2835_clock_choose_div_and_prate(struct clk_hw *hw,
>>>  	parent = clk_hw_get_parent_by_index(hw, parent_idx);
>>>  
>>>  	if (!(BIT(parent_idx) & data->set_rate_parent)) {
>>> +		unsigned long tmp_rate;
>>> +
>>>  		*prate = clk_hw_get_rate(parent);
>>>  		*div = bcm2835_clock_choose_div(hw, rate, *prate, true);
>>>  
>>> -		return bcm2835_clock_rate_from_divisor(clock, *prate,
>>> -						       *div);
>>> +		tmp_rate = bcm2835_clock_rate_from_divisor(clock, *prate, *div);
>>> +		*avgrate = tmp_rate;
>>> +
>>> +		if (data->low_jitter && (*div & CM_DIV_FRAC_MASK)) {
>>> +			unsigned long high, low;
>>> +			u32 idiv = *div & ~CM_DIV_FRAC_MASK;
>>> +
>>> +			high = bcm2835_clock_rate_from_divisor(clock, *prate,
>>> +							       idiv);
>>> +			idiv += CM_DIV_FRAC_MASK + 1;
>>> +			low = bcm2835_clock_rate_from_divisor(clock, *prate,
>>> +							      idiv);
>> What about int_div or intdiv instead of idiv as variable name?
> OK - int_div matches tmp_rate.
>
>>> +
>>> +			/* Return a value which is the maximum deviation
>>> +			 * below the ideal rate, for use as a metric.
>>> +			 */
>> Please fix coding style of the comment.
> Oops. I'm surprised checkpatch didn't complain about that.

The style above is only used for netdev.

>
> Thanks,
>
> Phil