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Date:   Sun, 14 Feb 2021 16:21:35 +0000
From:   Jonathan Cameron <jic23@kernel.org>
To:     Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc:     <linux-kernel@vger.kernel.org>, <linux-iio@vger.kernel.org>,
        <lars@metafoo.de>, <Michael.Hennerich@analog.com>,
        <nuno.sa@analog.com>, <dragos.bogdan@analog.com>
Subject: Re: [RFC PATCH 1/5] iio: Add output buffer support
Message-ID: <20210214162135.343225e3@archlinux>
In-Reply-To: <20210212102021.47276-2-alexandru.ardelean@analog.com>
References: <20210212102021.47276-1-alexandru.ardelean@analog.com>
        <20210212102021.47276-2-alexandru.ardelean@analog.com>
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On Fri, 12 Feb 2021 12:20:17 +0200
Alexandru Ardelean <alexandru.ardelean@analog.com> wrote:

> From: Lars-Peter Clausen <lars@metafoo.de>
> 
> Currently IIO only supports buffer mode for capture devices like ADCs. Add
> support for buffered mode for output devices like DACs.
> 
> The output buffer implementation is analogous to the input buffer
> implementation. Instead of using read() to get data from the buffer write()
> is used to copy data into the buffer.
> 
> poll() with POLLOUT will wakeup if there is space available for more or
> equal to the configured watermark of samples.
> 
> Drivers can remove data from a buffer using iio_buffer_remove_sample(), the
> function can e.g. called from a trigger handler to write the data to
> hardware.
> 
> A buffer can only be either a output buffer or an input, but not both. So,
> for a device that has an ADC and DAC path, this will mean 2 IIO buffers
> (one for each direction).
> 
> The direction of the buffer is decided by the new direction field of the
> iio_buffer struct and should be set after allocating and before registering
> it.
> 
> Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
> ---
>  drivers/iio/industrialio-buffer.c | 110 ++++++++++++++++++++++++++++--
>  include/linux/iio/buffer.h        |   7 ++
>  include/linux/iio/buffer_impl.h   |  11 +++
>  3 files changed, 124 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
> index a0d1ad86022f..6f4f5f5544f3 100644
> --- a/drivers/iio/industrialio-buffer.c
> +++ b/drivers/iio/industrialio-buffer.c
> @@ -162,6 +162,69 @@ static ssize_t iio_buffer_read(struct file *filp, char __user *buf,
>  	return ret;
>  }
>  
> +static size_t iio_buffer_space_available(struct iio_buffer *buf)
> +{
> +	if (buf->access->space_available)
> +		return buf->access->space_available(buf);
> +
> +	return SIZE_MAX;
> +}
> +
> +static ssize_t iio_buffer_write(struct file *filp, const char __user *buf,
> +				size_t n, loff_t *f_ps)
> +{
> +	struct iio_dev_buffer_pair *ib = filp->private_data;
> +	struct iio_buffer *rb = ib->buffer;
> +	struct iio_dev *indio_dev = ib->indio_dev;
> +	DEFINE_WAIT_FUNC(wait, woken_wake_function);
> +	size_t datum_size;
> +	size_t to_wait;
> +	int ret;
> +
> +	if (!rb || !rb->access->write)
> +		return -EINVAL;
> +
> +	datum_size = rb->bytes_per_datum;
> +
> +	/*
> +	 * 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 = 0;
> +	else
> +		to_wait = min_t(size_t, n / datum_size, rb->watermark);
> +
> +	add_wait_queue(&rb->pollq, &wait);
> +	do {
> +		if (!indio_dev->info) {
> +			ret = -ENODEV;
> +			break;
> +		}
> +
> +		if (iio_buffer_space_available(rb) < to_wait) {

In the non blocking case, we still hit here, but query for less than 0
which seems a bit pointless.  in theory at least
iio_buffer_space_available() might be expensive.  Can we save on that
query?

> +			if (signal_pending(current)) {
> +				ret = -ERESTARTSYS;
> +				break;
> +			}
> +
> +			wait_woken(&wait, TASK_INTERRUPTIBLE,
> +				   MAX_SCHEDULE_TIMEOUT);
> +			continue;
> +		}
> +
> +		ret = rb->access->write(rb, n, buf);
> +		if (ret == 0 && (filp->f_flags & O_NONBLOCK))
> +			ret = -EAGAIN;
> +	} while (ret == 0);
> +	remove_wait_queue(&rb->pollq, &wait);
> +
> +	return ret;
> +}
> +
>  /**
>   * iio_buffer_poll() - poll the buffer to find out if it has data
>   * @filp:	File structure pointer for device access
> @@ -182,8 +245,19 @@ static __poll_t iio_buffer_poll(struct file *filp,
>  		return 0;
>  
>  	poll_wait(filp, &rb->pollq, wait);
> -	if (iio_buffer_ready(indio_dev, rb, rb->watermark, 0))
> -		return EPOLLIN | EPOLLRDNORM;
> +
> +	switch (rb->direction) {
> +	case IIO_BUFFER_DIRECTION_IN:
> +		if (iio_buffer_ready(indio_dev, rb, rb->watermark, 0))
> +			return EPOLLIN | EPOLLRDNORM;
> +		break;
> +	case IIO_BUFFER_DIRECTION_OUT:
> +		if (iio_buffer_space_available(rb) >= rb->watermark)
> +			return EPOLLOUT | EPOLLWRNORM;
> +		break;
> +	}
> +
> +	/* need a way of knowing if there may be enough data... */
>  	return 0;
>  }
>  
> @@ -232,6 +306,16 @@ void iio_buffer_wakeup_poll(struct iio_dev *indio_dev)
>  	}
>  }
>  
> +int iio_buffer_remove_sample(struct iio_buffer *buffer, u8 *data)
> +{
> +	if (!buffer || !buffer->access)
> +		return -EINVAL;
> +	if (!buffer->access->write)
> +		return -ENOSYS;
> +	return buffer->access->remove_from(buffer, data);
> +}
> +EXPORT_SYMBOL_GPL(iio_buffer_remove_sample);
> +
>  void iio_buffer_init(struct iio_buffer *buffer)
>  {
>  	INIT_LIST_HEAD(&buffer->demux_list);
> @@ -803,6 +887,8 @@ static int iio_verify_update(struct iio_dev *indio_dev,
>  	}
>  
>  	if (insert_buffer) {
> +		if (insert_buffer->direction == IIO_BUFFER_DIRECTION_OUT)
> +			strict_scanmask = true;
>  		bitmap_or(compound_mask, compound_mask,
>  			  insert_buffer->scan_mask, indio_dev->masklength);
>  		scan_timestamp |= insert_buffer->scan_timestamp;
> @@ -945,6 +1031,8 @@ static int iio_update_demux(struct iio_dev *indio_dev)
>  	int ret;
>  
>  	list_for_each_entry(buffer, &iio_dev_opaque->buffer_list, buffer_list) {
> +		if (buffer->direction == IIO_BUFFER_DIRECTION_OUT)
> +			continue;
>  		ret = iio_buffer_update_demux(indio_dev, buffer);
>  		if (ret < 0)
>  			goto error_clear_mux_table;
> @@ -1155,6 +1243,11 @@ int iio_update_buffers(struct iio_dev *indio_dev,
>  	mutex_lock(&indio_dev->info_exist_lock);
>  	mutex_lock(&indio_dev->mlock);
>  
> +	if (insert_buffer->direction == IIO_BUFFER_DIRECTION_OUT) {
> +		ret = -EINVAL;
> +		goto out_unlock;
> +	}
> +
>  	if (insert_buffer && iio_buffer_is_active(insert_buffer))
>  		insert_buffer = NULL;
>  
> @@ -1400,6 +1493,7 @@ static const struct file_operations iio_buffer_chrdev_fileops = {
>  	.owner = THIS_MODULE,
>  	.llseek = noop_llseek,
>  	.read = iio_buffer_read,
> +	.write = iio_buffer_write,
>  	.poll = iio_buffer_poll,
>  	.unlocked_ioctl = iio_buffer_ioctl,
>  	.compat_ioctl = compat_ptr_ioctl,
> @@ -1914,8 +2008,16 @@ static int iio_buffer_mmap(struct file *filep, struct vm_area_struct *vma)
>  	if (!(vma->vm_flags & VM_SHARED))
>  		return -EINVAL;
>  
> -	if (!(vma->vm_flags & VM_READ))
> -		return -EINVAL;
> +	switch (buffer->direction) {
> +	case IIO_BUFFER_DIRECTION_IN:
> +		if (!(vma->vm_flags & VM_READ))
> +			return -EINVAL;
> +		break;
> +	case IIO_BUFFER_DIRECTION_OUT:
> +		if (!(vma->vm_flags & VM_WRITE))
> +			return -EINVAL;
> +		break;
> +	}
>  
>  	return buffer->access->mmap(buffer, vma);
>  }
> diff --git a/include/linux/iio/buffer.h b/include/linux/iio/buffer.h
> index b6928ac5c63d..e87b8773253d 100644
> --- a/include/linux/iio/buffer.h
> +++ b/include/linux/iio/buffer.h
> @@ -11,8 +11,15 @@
>  
>  struct iio_buffer;
>  
> +enum iio_buffer_direction {
> +	IIO_BUFFER_DIRECTION_IN,
> +	IIO_BUFFER_DIRECTION_OUT,
> +};
> +
>  int iio_push_to_buffers(struct iio_dev *indio_dev, const void *data);
>  
> +int iio_buffer_remove_sample(struct iio_buffer *buffer, u8 *data);
> +
>  /**
>   * iio_push_to_buffers_with_timestamp() - push data and timestamp to buffers
>   * @indio_dev:		iio_dev structure for device.
> diff --git a/include/linux/iio/buffer_impl.h b/include/linux/iio/buffer_impl.h
> index 1d57dc7ccb4f..47bdbf4a4519 100644
> --- a/include/linux/iio/buffer_impl.h
> +++ b/include/linux/iio/buffer_impl.h
> @@ -7,6 +7,7 @@
>  #ifdef CONFIG_IIO_BUFFER
>  
>  #include <uapi/linux/iio/buffer.h>
> +#include <linux/iio/buffer.h>
>  
>  struct iio_dev;
>  struct iio_buffer;
> @@ -23,6 +24,10 @@ struct iio_buffer;
>   * @read:		try to get a specified number of bytes (must exist)
>   * @data_available:	indicates how much data is available for reading from
>   *			the buffer.
> + * @remove_from:	remove sample from buffer. Drivers should calls this to
> + *			remove a sample from a buffer.
> + * @write:		try to write a number of bytes
> + * @space_available:	returns the amount of bytes available in a buffer
>   * @request_update:	if a parameter change has been marked, update underlying
>   *			storage.
>   * @set_bytes_per_datum:set number of bytes per datum
> @@ -61,6 +66,9 @@ struct iio_buffer_access_funcs {
>  	int (*store_to)(struct iio_buffer *buffer, const void *data);
>  	int (*read)(struct iio_buffer *buffer, size_t n, char __user *buf);
>  	size_t (*data_available)(struct iio_buffer *buffer);
> +	int (*remove_from)(struct iio_buffer *buffer, void *data);
> +	int (*write)(struct iio_buffer *buffer, size_t n, const char __user *buf);
> +	size_t (*space_available)(struct iio_buffer *buffer);
>  
>  	int (*request_update)(struct iio_buffer *buffer);
>  
> @@ -103,6 +111,9 @@ struct iio_buffer {
>  	/**  @bytes_per_datum: Size of individual datum including timestamp. */
>  	size_t bytes_per_datum;
>  
> +	/* @direction: Direction of the data stream (in/out). */
> +	enum iio_buffer_direction direction;
> +
>  	/**
>  	 * @access: Buffer access functions associated with the
>  	 * implementation.