Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752580AbdLLUVW convert rfc822-to-8bit (ORCPT ); Tue, 12 Dec 2017 15:21:22 -0500 Received: from mail.kernel.org ([198.145.29.99]:48316 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752511AbdLLUVT (ORCPT ); Tue, 12 Dec 2017 15:21:19 -0500 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1967820C0C Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=jic23@kernel.org Date: Tue, 12 Dec 2017 20:21:14 +0000 From: Jonathan Cameron To: Stefan =?UTF-8?B?QnLDvG5z?= Cc: , Peter Meerwald-Stadler , Maciej Purski , , "Andrew F . Davis" , Lars-Peter Clausen , Hartmut Knaack , Javier Martinez Canillas Subject: Re: [PATCH v1 5/7] iio: adc: ina2xx: Use a monotonic clock for delay calculation Message-ID: <20171212202114.654fc59b@archlinux> In-Reply-To: <2674819.FhFV8Gkxuq@pebbles> References: <20171208174152.30341-1-stefan.bruens@rwth-aachen.de> <7330c7df-330f-449f-8706-a2407e92ebcf@rwthex-w2-a.rwth-ad.de> <20171210173157.6cc78b4b@archlinux> <2674819.FhFV8Gkxuq@pebbles> X-Mailer: Claws Mail 3.15.1-dirty (GTK+ 2.24.31; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4555 Lines: 137 On Sun, 10 Dec 2017 21:47:37 +0100 Stefan Brüns wrote: > On Sunday, December 10, 2017 6:31:57 PM CET Jonathan Cameron wrote: > > On Fri, 8 Dec 2017 18:41:50 +0100 > > > > Stefan Brüns wrote: > > > The iio timestamp clock is user selectable and may be non-monotonic. Also, > > > only part of the acquisition time is measured, thus the delay was longer > > > than intended. > > > > > > Signed-off-by: Stefan Brüns > > > --- > > > > > > drivers/iio/adc/ina2xx-adc.c | 35 +++++++++++++++++++++-------------- > > > 1 file changed, 21 insertions(+), 14 deletions(-) > > > > > > diff --git a/drivers/iio/adc/ina2xx-adc.c b/drivers/iio/adc/ina2xx-adc.c > > > index 2621a34ee5c6..65bd9e69faf2 100644 > > > --- a/drivers/iio/adc/ina2xx-adc.c > > > +++ b/drivers/iio/adc/ina2xx-adc.c > > > @@ -703,10 +703,10 @@ static int ina2xx_work_buffer(struct iio_dev > > > *indio_dev)> > > > /* data buffer needs space for channel data and timestap */ > > > unsigned short data[4 + sizeof(s64)/sizeof(short)]; > > > int bit, ret, i = 0; > > > > > > - s64 time_a, time_b; > > > + s64 time; > > > > > > unsigned int alert; > > > > > > - time_a = iio_get_time_ns(indio_dev); > > > + time = iio_get_time_ns(indio_dev); > > > > > > /* > > > > > > * Because the timer thread and the chip conversion clock > > > > > > @@ -752,11 +752,9 @@ static int ina2xx_work_buffer(struct iio_dev > > > *indio_dev)> > > > data[i++] = val; > > > > > > } > > > > > > - time_b = iio_get_time_ns(indio_dev); > > > + iio_push_to_buffers_with_timestamp(indio_dev, data, time); > > > > > > - iio_push_to_buffers_with_timestamp(indio_dev, data, time_a); > > > - > > > - return (unsigned long)(time_b - time_a) / 1000; > > > + return 0; > > > > > > }; > > > > > > static int ina2xx_capture_thread(void *data) > > > > > > @@ -764,7 +762,9 @@ static int ina2xx_capture_thread(void *data) > > > > > > struct iio_dev *indio_dev = data; > > > struct ina2xx_chip_info *chip = iio_priv(indio_dev); > > > int sampling_us = SAMPLING_PERIOD(chip); > > > > > > - int buffer_us, delay_us; > > > + int ret; > > > + struct timespec64 next, now, delta; > > > + s64 delay_us; > > > > > > /* > > > > > > * Poll a bit faster than the chip internal Fs, in case > > > > > > @@ -773,15 +773,22 @@ static int ina2xx_capture_thread(void *data) > > > > > > if (!chip->allow_async_readout) > > > > > > sampling_us -= 200; > > > > > > + ktime_get_ts64(&next); > > > + > > > > > > do { > > > > > > - buffer_us = ina2xx_work_buffer(indio_dev); > > > - if (buffer_us < 0) > > > - return buffer_us; > > > + ret = ina2xx_work_buffer(indio_dev); > > > + if (ret < 0) > > > + return ret; > > > > > > - if (sampling_us > buffer_us) { > > > - delay_us = sampling_us - buffer_us; > > > - usleep_range(delay_us, (delay_us * 3) >> 1); > > > - } > > > + ktime_get_ts64(&now); > > > + > > > + do { > > > + timespec64_add_ns(&next, 1000 * sampling_us); > > > + delta = timespec64_sub(next, now); > > > + delay_us = timespec64_to_ns(&delta) / 1000; > > > + } while (delay_us <= 0); > > > > Umm. I'm lost, what is the purpose of the above dance? > > A comment perhaps. > > next is the timestamp for the next read to happen, now is the current time. > Obviously we have to sleep for the remainder. > > Each sampling interval the "next" timestamp is pushed back by sampling_us. > Normally this happens exactly once per read, i.e. we schedule the reads to > happen exactly each sampling interval. > > The sampling inteval is *only* added multiple times if it is faster than the > bus can deliver the data (at 100 kBits/s, each register read takes about 400 > us, so sampling faster than every ~1 ms is not possible. So this is deliberately skipping a sample if this happens? It was this element that I wasn't understanding previously. Add a comment in the code to explain this and I'm happy. It's horrible, but not much we can do if things are simply going too fast. Thanks for the info. Jonathan > > The old code measured the time spent for reading the registers and slept for > the remainder of the interval. This way the sampling drifts, as there is some > time not accounted for - usleep_range, function call overhead, kthread > interrupted. > > Using a timestamp avoids the drift. It also allows simple readjustment of the > "next" sampling time when polling the status register. > > Kind regards, > > Stefan >