Re: [PATCH 04/15] iio: buffer-dma: Enable buffer write support

[Jonathan Cameron <jic23@kernel.org>]

On Mon, 15 Nov 2021 14:19:14 +0000
Paul Cercueil <paul@crapouillou.net> wrote:

> Adding write support to the buffer-dma code is easy - the write()
> function basically needs to do the exact same thing as the read()
> function: dequeue a block, read or write the data, enqueue the block
> when entirely processed.
> 
> Therefore, the iio_buffer_dma_read() and the new iio_buffer_dma_write()
> now both call a function iio_buffer_dma_io(), which will perform this
> task.
> 
> The .space_available() callback can return the exact same value as the
> .data_available() callback for input buffers, since in both cases we
> count the exact same thing (the number of bytes in each available
> block).
> 
> Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Hi Paul,

There are a few changes in here, such as the bytes_used value being set that
I'm not following the reasoning behind. More info on those?
Also good to provide something about those in this patch description.

Thanks,

Jonathan


> ---
>  drivers/iio/buffer/industrialio-buffer-dma.c | 75 +++++++++++++++-----
>  include/linux/iio/buffer-dma.h               |  7 ++
>  2 files changed, 66 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/iio/buffer/industrialio-buffer-dma.c b/drivers/iio/buffer/industrialio-buffer-dma.c
> index abac88f20104..eeeed6b2e0cf 100644
> --- a/drivers/iio/buffer/industrialio-buffer-dma.c
> +++ b/drivers/iio/buffer/industrialio-buffer-dma.c
> @@ -179,7 +179,8 @@ static struct iio_dma_buffer_block *iio_dma_buffer_alloc_block(
>  	}
>  
>  	block->size = size;
> -	block->state = IIO_BLOCK_STATE_DEQUEUED;
> +	block->bytes_used = size;
> +	block->state = IIO_BLOCK_STATE_DONE;

I don't know why these are here - some more info?

>  	block->queue = queue;
>  	INIT_LIST_HEAD(&block->head);
>  	kref_init(&block->kref);
> @@ -195,6 +196,18 @@ static void _iio_dma_buffer_block_done(struct iio_dma_buffer_block *block)
>  		block->state = IIO_BLOCK_STATE_DONE;
>  }
>  
> +static void iio_dma_buffer_queue_wake(struct iio_dma_buffer_queue *queue)
> +{
> +	__poll_t flags;
> +
> +	if (queue->buffer.direction == IIO_BUFFER_DIRECTION_IN)
> +		flags = EPOLLIN | EPOLLRDNORM;
> +	else
> +		flags = EPOLLOUT | EPOLLWRNORM;
> +
> +	wake_up_interruptible_poll(&queue->buffer.pollq, flags);
> +}
> +
>  /**
>   * iio_dma_buffer_block_done() - Indicate that a block has been completed
>   * @block: The completed block
> @@ -212,7 +225,7 @@ void iio_dma_buffer_block_done(struct iio_dma_buffer_block *block)
>  	spin_unlock_irqrestore(&queue->list_lock, flags);
>  
>  	iio_buffer_block_put_atomic(block);
> -	wake_up_interruptible_poll(&queue->buffer.pollq, EPOLLIN | EPOLLRDNORM);
> +	iio_dma_buffer_queue_wake(queue);
>  }
>  EXPORT_SYMBOL_GPL(iio_dma_buffer_block_done);
>  
> @@ -241,7 +254,7 @@ void iio_dma_buffer_block_list_abort(struct iio_dma_buffer_queue *queue,
>  	}
>  	spin_unlock_irqrestore(&queue->list_lock, flags);
>  
> -	wake_up_interruptible_poll(&queue->buffer.pollq, EPOLLIN | EPOLLRDNORM);
> +	iio_dma_buffer_queue_wake(queue);
>  }
>  EXPORT_SYMBOL_GPL(iio_dma_buffer_block_list_abort);
>  
> @@ -334,7 +347,8 @@ int iio_dma_buffer_request_update(struct iio_buffer *buffer)
>  			queue->fileio.blocks[i] = block;
>  		}
>  
> -		block->state = IIO_BLOCK_STATE_QUEUED;
> +		if (queue->buffer.direction == IIO_BUFFER_DIRECTION_IN)
> +			block->state = IIO_BLOCK_STATE_QUEUED;

Possibly worth a comment on the state being set here.  I figured it out, but might
save some brain cells in future if it's stated in the code.

>  	}
>  
>  out_unlock:
> @@ -467,20 +481,12 @@ static struct iio_dma_buffer_block *iio_dma_buffer_dequeue(
>  	return block;
>  }
>  
> -/**
> - * iio_dma_buffer_read() - DMA buffer read callback
> - * @buffer: Buffer to read form
> - * @n: Number of bytes to read
> - * @user_buffer: Userspace buffer to copy the data to
> - *
> - * Should be used as the read callback for iio_buffer_access_ops
> - * struct for DMA buffers.
> - */
> -int iio_dma_buffer_read(struct iio_buffer *buffer, size_t n,
> -	char __user *user_buffer)
> +static int iio_dma_buffer_io(struct iio_buffer *buffer,
> +			     size_t n, char __user *user_buffer, bool is_write)
>  {
>  	struct iio_dma_buffer_queue *queue = iio_buffer_to_queue(buffer);
>  	struct iio_dma_buffer_block *block;
> +	void *addr;
>  	int ret;
>  
>  	if (n < buffer->bytes_per_datum)
> @@ -503,8 +509,13 @@ int iio_dma_buffer_read(struct iio_buffer *buffer, size_t n,
>  	n = rounddown(n, buffer->bytes_per_datum);
>  	if (n > block->bytes_used - queue->fileio.pos)
>  		n = block->bytes_used - queue->fileio.pos;
> +	addr = block->vaddr + queue->fileio.pos;
>  
> -	if (copy_to_user(user_buffer, block->vaddr + queue->fileio.pos, n)) {
> +	if (is_write)
> +		ret = !!copy_from_user(addr, user_buffer, n);
> +	else
> +		ret = !!copy_to_user(user_buffer, addr, n);

What is the !! gaining us here?  We only care about == 0 vs != 0 so
forcing it to be 0 or 1 isn't useful.

> +	if (ret) {
>  		ret = -EFAULT;
>  		goto out_unlock;
>  	}
> @@ -513,6 +524,7 @@ int iio_dma_buffer_read(struct iio_buffer *buffer, size_t n,
>  
>  	if (queue->fileio.pos == block->bytes_used) {
>  		queue->fileio.active_block = NULL;
> +		block->bytes_used = block->size;

This seems to be a functional change that isn't called out in the patch description.

>  		iio_dma_buffer_enqueue(queue, block);
>  	}
>  
> @@ -523,8 +535,39 @@ int iio_dma_buffer_read(struct iio_buffer *buffer, size_t n,
>  
>  	return ret;
>  }
> +
> +/**
> + * iio_dma_buffer_read() - DMA buffer read callback
> + * @buffer: Buffer to read form
> + * @n: Number of bytes to read
> + * @user_buffer: Userspace buffer to copy the data to
> + *
> + * Should be used as the read callback for iio_buffer_access_ops
> + * struct for DMA buffers.
> + */
> +int iio_dma_buffer_read(struct iio_buffer *buffer, size_t n,
> +	char __user *user_buffer)
> +{
> +	return iio_dma_buffer_io(buffer, n, user_buffer, false);
> +}
>  EXPORT_SYMBOL_GPL(iio_dma_buffer_read);
>  
> +/**
> + * iio_dma_buffer_write() - DMA buffer write callback
> + * @buffer: Buffer to read form
> + * @n: Number of bytes to read
> + * @user_buffer: Userspace buffer to copy the data from
> + *
> + * Should be used as the write callback for iio_buffer_access_ops
> + * struct for DMA buffers.
> + */
> +int iio_dma_buffer_write(struct iio_buffer *buffer, size_t n,
> +			 const char __user *user_buffer)
> +{
> +	return iio_dma_buffer_io(buffer, n, (__force char *)user_buffer, true);
> +}
> +EXPORT_SYMBOL_GPL(iio_dma_buffer_write);
> +
>  /**
>   * iio_dma_buffer_data_available() - DMA buffer data_available callback
>   * @buf: Buffer to check for data availability
> diff --git a/include/linux/iio/buffer-dma.h b/include/linux/iio/buffer-dma.h
> index a65a005c4a19..09c07d5563c0 100644
> --- a/include/linux/iio/buffer-dma.h
> +++ b/include/linux/iio/buffer-dma.h
> @@ -132,6 +132,8 @@ int iio_dma_buffer_disable(struct iio_buffer *buffer,
>  	struct iio_dev *indio_dev);
>  int iio_dma_buffer_read(struct iio_buffer *buffer, size_t n,
>  	char __user *user_buffer);
> +int iio_dma_buffer_write(struct iio_buffer *buffer, size_t n,
> +			 const char __user *user_buffer);
>  size_t iio_dma_buffer_data_available(struct iio_buffer *buffer);
>  int iio_dma_buffer_set_bytes_per_datum(struct iio_buffer *buffer, size_t bpd);
>  int iio_dma_buffer_set_length(struct iio_buffer *buffer, unsigned int length);
> @@ -142,4 +144,9 @@ int iio_dma_buffer_init(struct iio_dma_buffer_queue *queue,
>  void iio_dma_buffer_exit(struct iio_dma_buffer_queue *queue);
>  void iio_dma_buffer_release(struct iio_dma_buffer_queue *queue);
>  
> +static inline size_t iio_dma_buffer_space_available(struct iio_buffer *buffer)
> +{
> +	return iio_dma_buffer_data_available(buffer);
> +}
> +
>  #endif