Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751736AbdFAIqi (ORCPT ); Thu, 1 Jun 2017 04:46:38 -0400 Received: from mx08-00252a01.pphosted.com ([91.207.212.211]:58310 "EHLO mx08-00252a01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751702AbdFAIqg (ORCPT ); Thu, 1 Jun 2017 04:46:36 -0400 Subject: Re: [PATCH v2 2/2] clk: bcm2835: Minimise clock jitter for PCM clock To: Eric Anholt , Michael Turquette , Stephen Boyd , Stefan Wahren , Florian Fainelli , linux-clk@vger.kernel.org, linux-rpi-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <87bmq8btfe.fsf@eliezer.anholt.net> From: Phil Elwell Message-ID: <1c434318-eb0a-b669-9a0a-780993b1c03d@raspberrypi.org> Date: Thu, 1 Jun 2017 09:46:33 +0100 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.1.1 MIME-Version: 1.0 In-Reply-To: <87bmq8btfe.fsf@eliezer.anholt.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-GB Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-06-01_02:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_spam_notspam policy=outbound_spam score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1703280000 definitions=main-1706010162 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3361 Lines: 94 On 31/05/2017 22:36, Eric Anholt wrote: > Phil Elwell writes: > >> 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. > > "worst" the second time According to the rules of English grammar, you should only use the superlative ("worst") when comparing something to a group. In this case we are only comparing two things - the distance to the nearest-neighbour integers - so the comparitive ("worse") is correct. >> Use this metric for parent selection for clocks requiring low jitter >> (currently just PCM). >> >> Signed-off-by: Phil Elwell >> --- >> drivers/clk/bcm/clk-bcm2835.c | 40 +++++++++++++++++++++++++++++++++++----- >> 1 file changed, 35 insertions(+), 5 deletions(-) >> >> diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c >> index 81ecd4c..c7ee951 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 +1138,34 @@ 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 int_div = *div & ~CM_DIV_FRAC_MASK; >> + >> + high = bcm2835_clock_rate_from_divisor(clock, *prate, >> + int_div); >> + int_div += CM_DIV_FRAC_MASK + 1; >> + low = bcm2835_clock_rate_from_divisor(clock, *prate, >> + int_div); >> + >> + /* >> + * Return a value which is the maximum deviation >> + * below the ideal rate, for use as a metric. >> + */ >> + if ((tmp_rate - low) < (high - tmp_rate)) >> + tmp_rate = low; >> + else >> + tmp_rate -= high - tmp_rate; > > Simplification suggestion: Remove tmp_rate variable, just assign to > rate_from_divisor result to *avgrate. At the end of the low_jitter > block, just "return *avgrate - max(*avgrate - low, high - *avgrate)". Yes, I like that. > With that, feel free to add: > > Reviewed-by: Eric Anholt Thanks - I will. Phil