[v1,5/7] iio: adc: ina2xx: Use a monotonic clock for delay calculation
diff mbox

Message ID 7330c7df-330f-449f-8706-a2407e92ebcf@rwthex-w2-a.rwth-ad.de
State New
Headers show

Commit Message

Stefan Brüns Dec. 8, 2017, 5:41 p.m. UTC
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 <stefan.bruens@rwth-aachen.de>
---

 drivers/iio/adc/ina2xx-adc.c | 35 +++++++++++++++++++++--------------
 1 file changed, 21 insertions(+), 14 deletions(-)

Comments

Jonathan Cameron Dec. 10, 2017, 5:31 p.m. UTC | #1
On Fri, 8 Dec 2017 18:41:50 +0100
Stefan Brüns <stefan.bruens@rwth-aachen.de> 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 <stefan.bruens@rwth-aachen.de>
> ---
> 
>  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.

> +
> +		usleep_range(delay_us, (delay_us * 3) >> 1);
>  
>  	} while (!kthread_should_stop());
>  

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Stefan Brüns Dec. 10, 2017, 8:47 p.m. UTC | #2
On Sunday, December 10, 2017 6:31:57 PM CET Jonathan Cameron wrote:
> On Fri, 8 Dec 2017 18:41:50 +0100
> 
> Stefan Brüns <stefan.bruens@rwth-aachen.de> 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 <stefan.bruens@rwth-aachen.de>
> > ---
> > 
> >  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.

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
Jonathan Cameron Dec. 12, 2017, 8:21 p.m. UTC | #3
On Sun, 10 Dec 2017 21:47:37 +0100
Stefan Brüns <stefan.bruens@rwth-aachen.de> 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 <stefan.bruens@rwth-aachen.de> 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 <stefan.bruens@rwth-aachen.de>
> > > ---
> > > 
> > >  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
> 

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Jonathan Cameron Dec. 17, 2017, 11:56 a.m. UTC | #4
On Tue, 12 Dec 2017 20:21:14 +0000
Jonathan Cameron <jic23@kernel.org> wrote:

> On Sun, 10 Dec 2017 21:47:37 +0100
> Stefan Brüns <stefan.bruens@rwth-aachen.de> 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 <stefan.bruens@rwth-aachen.de> 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 <stefan.bruens@rwth-aachen.de>
> > > > ---
> > > > 
> > > >  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.
I still want to see a comment in the code making it clear what is
happening in that loop.  So for now I'm going to stop here in applying
this series.

Jonathan

> 
> 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
> >   
> 
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Patch
diff mbox

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);
+
+		usleep_range(delay_us, (delay_us * 3) >> 1);
 
 	} while (!kthread_should_stop());