Re: [PATCH v6 1/3] iio: add watermark logic to iio read and poll

[Jonathan Cameron <jic23@kernel.org>]

On 22/03/15 18:33, Octavian Purdila wrote:
> From: Josselin Costanzi <josselin.costanzi@mobile-devices.fr>
> 
> Currently the IIO buffer blocking read only wait until at least one
> data element is available.
> This patch makes the reader sleep until enough data is collected before
> returning to userspace. This should limit the read() calls count when
> trying to get data in batches.
> 
> Co-author: Yannick Bedhomme <yannick.bedhomme@mobile-devices.fr>
> Signed-off-by: Josselin Costanzi <josselin.costanzi@mobile-devices.fr>
> [rebased and remove buffer timeout]
> Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Great.  Applied to the togreg branch of iio.git - initially pushed out as
testing for the autobuilders to try their best to break things.

Jonathan
> ---
>  Documentation/ABI/testing/sysfs-bus-iio  |  15 ++++
>  drivers/iio/industrialio-buffer.c        | 118 +++++++++++++++++++++++++++----
>  drivers/iio/kfifo_buf.c                  |  11 ++-
>  drivers/staging/iio/accel/sca3000_ring.c |   4 +-
>  include/linux/iio/buffer.h               |   8 ++-
>  5 files changed, 129 insertions(+), 27 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/sysfs-bus-iio b/Documentation/ABI/testing/sysfs-bus-iio
> index 6be17c2..0051641 100644
> --- a/Documentation/ABI/testing/sysfs-bus-iio
> +++ b/Documentation/ABI/testing/sysfs-bus-iio
> @@ -1273,3 +1273,18 @@ Contact:	linux-iio@vger.kernel.org
>  Description:
>  		Specifies number of seconds in which we compute the steps
>  		that occur in order to decide if the consumer is making steps.
> +
> +What:		/sys/bus/iio/devices/iio:deviceX/buffer/watermark
> +KernelVersion:	4.2
> +Contact:	linux-iio@vger.kernel.org
> +Description:
> +		A single positive integer specifying the maximum number of scan
> +		elements to wait for.
> +		Poll will block until the watermark is reached.
> +		Blocking read will wait until the minimum between the requested
> +		read amount or the low water mark is available.
> +		Non-blocking read will retrieve the available samples from the
> +		buffer even if there are less samples then watermark level. This
> +		allows the application to block on poll with a timeout and read
> +		the available samples after the timeout expires and thus have a
> +		maximum delay guarantee.
> diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
> index c2d5440..a4f4f07 100644
> --- a/drivers/iio/industrialio-buffer.c
> +++ b/drivers/iio/industrialio-buffer.c
> @@ -37,11 +37,28 @@ static bool iio_buffer_is_active(struct iio_buffer *buf)
>  	return !list_empty(&buf->buffer_list);
>  }
>  
> -static bool iio_buffer_data_available(struct iio_buffer *buf)
> +static size_t iio_buffer_data_available(struct iio_buffer *buf)
>  {
>  	return buf->access->data_available(buf);
>  }
>  
> +static bool iio_buffer_ready(struct iio_dev *indio_dev, struct iio_buffer *buf,
> +			     size_t to_wait)
> +{
> +	/* wakeup if the device was unregistered */
> +	if (!indio_dev->info)
> +		return true;
> +
> +	/* drain the buffer if it was disabled */
> +	if (!iio_buffer_is_active(buf))
> +		to_wait = min_t(size_t, to_wait, 1);
> +
> +	if (iio_buffer_data_available(buf) >= to_wait)
> +		return true;
> +
> +	return false;
> +}
> +
>  /**
>   * iio_buffer_read_first_n_outer() - chrdev read for buffer access
>   *
> @@ -53,6 +70,8 @@ ssize_t iio_buffer_read_first_n_outer(struct file *filp, char __user *buf,
>  {
>  	struct iio_dev *indio_dev = filp->private_data;
>  	struct iio_buffer *rb = indio_dev->buffer;
> +	size_t datum_size = rb->bytes_per_datum;
> +	size_t to_wait = 0;
>  	int ret;
>  
>  	if (!indio_dev->info)
> @@ -61,19 +80,24 @@ ssize_t iio_buffer_read_first_n_outer(struct file *filp, char __user *buf,
>  	if (!rb || !rb->access->read_first_n)
>  		return -EINVAL;
>  
> +	/*
> +	 * If datum_size is 0 there will never be anything to read from the
> +	 * buffer, so signal end of file now.
> +	 */
> +	if (!datum_size)
> +		return 0;
> +
> +	if (!(filp->f_flags & O_NONBLOCK))
> +		to_wait = min_t(size_t, n / datum_size, rb->watermark);
> +
>  	do {
> -		if (!iio_buffer_data_available(rb)) {
> -			if (filp->f_flags & O_NONBLOCK)
> -				return -EAGAIN;
> +		ret = wait_event_interruptible(rb->pollq,
> +				      iio_buffer_ready(indio_dev, rb, to_wait));
> +		if (ret)
> +			return ret;
>  
> -			ret = wait_event_interruptible(rb->pollq,
> -					iio_buffer_data_available(rb) ||
> -					indio_dev->info == NULL);
> -			if (ret)
> -				return ret;
> -			if (indio_dev->info == NULL)
> -				return -ENODEV;
> -		}
> +		if (!indio_dev->info)
> +			return -ENODEV;
>  
>  		ret = rb->access->read_first_n(rb, n, buf);
>  		if (ret == 0 && (filp->f_flags & O_NONBLOCK))
> @@ -96,9 +120,8 @@ unsigned int iio_buffer_poll(struct file *filp,
>  		return -ENODEV;
>  
>  	poll_wait(filp, &rb->pollq, wait);
> -	if (iio_buffer_data_available(rb))
> +	if (iio_buffer_ready(indio_dev, rb, rb->watermark))
>  		return POLLIN | POLLRDNORM;
> -	/* need a way of knowing if there may be enough data... */
>  	return 0;
>  }
>  
> @@ -123,6 +146,7 @@ void iio_buffer_init(struct iio_buffer *buffer)
>  	INIT_LIST_HEAD(&buffer->buffer_list);
>  	init_waitqueue_head(&buffer->pollq);
>  	kref_init(&buffer->ref);
> +	buffer->watermark = 1;
>  }
>  EXPORT_SYMBOL(iio_buffer_init);
>  
> @@ -416,6 +440,11 @@ static ssize_t iio_buffer_write_length(struct device *dev,
>  		buffer->access->set_length(buffer, val);
>  		ret = 0;
>  	}
> +	if (ret)
> +		goto out;
> +	if (buffer->length && buffer->length < buffer->watermark)
> +		buffer->watermark = buffer->length;
> +out:
>  	mutex_unlock(&indio_dev->mlock);
>  
>  	return ret ? ret : len;
> @@ -472,6 +501,7 @@ static void iio_buffer_activate(struct iio_dev *indio_dev,
>  static void iio_buffer_deactivate(struct iio_buffer *buffer)
>  {
>  	list_del_init(&buffer->buffer_list);
> +	wake_up_interruptible(&buffer->pollq);
>  	iio_buffer_put(buffer);
>  }
>  
> @@ -754,16 +784,64 @@ done:
>  
>  static const char * const iio_scan_elements_group_name = "scan_elements";
>  
> +static ssize_t iio_buffer_show_watermark(struct device *dev,
> +					 struct device_attribute *attr,
> +					 char *buf)
> +{
> +	struct iio_dev *indio_dev = dev_to_iio_dev(dev);
> +	struct iio_buffer *buffer = indio_dev->buffer;
> +
> +	return sprintf(buf, "%u\n", buffer->watermark);
> +}
> +
> +static ssize_t iio_buffer_store_watermark(struct device *dev,
> +					  struct device_attribute *attr,
> +					  const char *buf,
> +					  size_t len)
> +{
> +	struct iio_dev *indio_dev = dev_to_iio_dev(dev);
> +	struct iio_buffer *buffer = indio_dev->buffer;
> +	unsigned int val;
> +	int ret;
> +
> +	ret = kstrtouint(buf, 10, &val);
> +	if (ret)
> +		return ret;
> +	if (!val)
> +		return -EINVAL;
> +
> +	mutex_lock(&indio_dev->mlock);
> +
> +	if (val > buffer->length) {
> +		ret = -EINVAL;
> +		goto out;
> +	}
> +
> +	if (iio_buffer_is_active(indio_dev->buffer)) {
> +		ret = -EBUSY;
> +		goto out;
> +	}
> +
> +	buffer->watermark = val;
> +out:
> +	mutex_unlock(&indio_dev->mlock);
> +
> +	return ret ? ret : len;
> +}
> +
>  static DEVICE_ATTR(length, S_IRUGO | S_IWUSR, iio_buffer_read_length,
>  		   iio_buffer_write_length);
>  static struct device_attribute dev_attr_length_ro = __ATTR(length,
>  	S_IRUGO, iio_buffer_read_length, NULL);
>  static DEVICE_ATTR(enable, S_IRUGO | S_IWUSR,
>  		   iio_buffer_show_enable, iio_buffer_store_enable);
> +static DEVICE_ATTR(watermark, S_IRUGO | S_IWUSR,
> +		   iio_buffer_show_watermark, iio_buffer_store_watermark);
>  
>  static struct attribute *iio_buffer_attrs[] = {
>  	&dev_attr_length.attr,
>  	&dev_attr_enable.attr,
> +	&dev_attr_watermark.attr,
>  };
>  
>  int iio_buffer_alloc_sysfs_and_mask(struct iio_dev *indio_dev)
> @@ -944,8 +1022,18 @@ static const void *iio_demux(struct iio_buffer *buffer,
>  static int iio_push_to_buffer(struct iio_buffer *buffer, const void *data)
>  {
>  	const void *dataout = iio_demux(buffer, data);
> +	int ret;
> +
> +	ret = buffer->access->store_to(buffer, dataout);
> +	if (ret)
> +		return ret;
>  
> -	return buffer->access->store_to(buffer, dataout);
> +	/*
> +	 * We can't just test for watermark to decide if we wake the poll queue
> +	 * because read may request less samples than the watermark.
> +	 */
> +	wake_up_interruptible_poll(&buffer->pollq, POLLIN | POLLRDNORM);
> +	return 0;
>  }
>  
>  static void iio_buffer_demux_free(struct iio_buffer *buffer)
> diff --git a/drivers/iio/kfifo_buf.c b/drivers/iio/kfifo_buf.c
> index b2beea0..847ca56 100644
> --- a/drivers/iio/kfifo_buf.c
> +++ b/drivers/iio/kfifo_buf.c
> @@ -83,9 +83,6 @@ static int iio_store_to_kfifo(struct iio_buffer *r,
>  	ret = kfifo_in(&kf->kf, data, 1);
>  	if (ret != 1)
>  		return -EBUSY;
> -
> -	wake_up_interruptible_poll(&r->pollq, POLLIN | POLLRDNORM);
> -
>  	return 0;
>  }
>  
> @@ -109,16 +106,16 @@ static int iio_read_first_n_kfifo(struct iio_buffer *r,
>  	return copied;
>  }
>  
> -static bool iio_kfifo_buf_data_available(struct iio_buffer *r)
> +static size_t iio_kfifo_buf_data_available(struct iio_buffer *r)
>  {
>  	struct iio_kfifo *kf = iio_to_kfifo(r);
> -	bool empty;
> +	size_t samples;
>  
>  	mutex_lock(&kf->user_lock);
> -	empty = kfifo_is_empty(&kf->kf);
> +	samples = kfifo_len(&kf->kf);
>  	mutex_unlock(&kf->user_lock);
>  
> -	return !empty;
> +	return samples;
>  }
>  
>  static void iio_kfifo_buffer_release(struct iio_buffer *buffer)
> diff --git a/drivers/staging/iio/accel/sca3000_ring.c b/drivers/staging/iio/accel/sca3000_ring.c
> index f76a268..8589ead 100644
> --- a/drivers/staging/iio/accel/sca3000_ring.c
> +++ b/drivers/staging/iio/accel/sca3000_ring.c
> @@ -129,9 +129,9 @@ error_ret:
>  	return ret ? ret : num_read;
>  }
>  
> -static bool sca3000_ring_buf_data_available(struct iio_buffer *r)
> +static size_t sca3000_ring_buf_data_available(struct iio_buffer *r)
>  {
> -	return r->stufftoread;
> +	return r->stufftoread ? r->watermark : 0;
>  }
>  
>  /**
> diff --git a/include/linux/iio/buffer.h b/include/linux/iio/buffer.h
> index b65850a..eb8622b 100644
> --- a/include/linux/iio/buffer.h
> +++ b/include/linux/iio/buffer.h
> @@ -21,8 +21,8 @@ struct iio_buffer;
>   * struct iio_buffer_access_funcs - access functions for buffers.
>   * @store_to:		actually store stuff to the buffer
>   * @read_first_n:	try to get a specified number of bytes (must exist)
> - * @data_available:	indicates whether data for reading from the buffer is
> - *			available.
> + * @data_available:	indicates how much data is available for reading from
> + *			the buffer.
>   * @request_update:	if a parameter change has been marked, update underlying
>   *			storage.
>   * @set_bytes_per_datum:set number of bytes per datum
> @@ -43,7 +43,7 @@ struct iio_buffer_access_funcs {
>  	int (*read_first_n)(struct iio_buffer *buffer,
>  			    size_t n,
>  			    char __user *buf);
> -	bool (*data_available)(struct iio_buffer *buffer);
> +	size_t (*data_available)(struct iio_buffer *buffer);
>  
>  	int (*request_update)(struct iio_buffer *buffer);
>  
> @@ -72,6 +72,7 @@ struct iio_buffer_access_funcs {
>   * @demux_bounce:	[INTERN] buffer for doing gather from incoming scan.
>   * @buffer_list:	[INTERN] entry in the devices list of current buffers.
>   * @ref:		[INTERN] reference count of the buffer.
> + * @watermark:		[INTERN] number of datums to wait for poll/read.
>   */
>  struct iio_buffer {
>  	int					length;
> @@ -90,6 +91,7 @@ struct iio_buffer {
>  	void					*demux_bounce;
>  	struct list_head			buffer_list;
>  	struct kref				ref;
> +	unsigned int				watermark;
>  };
>  
>  /**
>