iio: buffer-dma: Enable buffer write support

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.

Note that we preemptively reset block->bytes_used to the buffer's size
in iio_dma_buffer_request_update(), as in the future the
iio_dma_buffer_enqueue() function won't reset it.

Signed-off-by: Paul Cercueil <[e-mail]>
Reviewed-by: Alexandru Ardelean <[e-mail]>
Signed-off-by: Nuno Sa <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/buffer/industrialio-buffer-dma.c

···
 		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
···
 	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);
 
···
 	}
 	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);
 
···
 			queue->fileio.blocks[i] = block;
 		}
 
-		block->state = IIO_BLOCK_STATE_QUEUED;
-		list_add_tail(&block->head, &queue->incoming);
+		/*
+		 * block->bytes_used may have been modified previously, e.g. by
+		 * iio_dma_buffer_block_list_abort(). Reset it here to the
+		 * block's so that iio_dma_buffer_io() will work.
+		 */
+		block->bytes_used = block->size;
+
+		/*
+		 * If it's an input buffer, mark the block as queued, and
+		 * iio_dma_buffer_enable() will submit it. Otherwise mark it as
+		 * done, which means it's ready to be dequeued.
+		 */
+		if (queue->buffer.direction == IIO_BUFFER_DIRECTION_IN) {
+			block->state = IIO_BLOCK_STATE_QUEUED;
+			list_add_tail(&block->head, &queue->incoming);
+		} else {
+			block->state = IIO_BLOCK_STATE_DONE;
+		}
 	}
 
 out_unlock:
···
 	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_from_user)
 {
 	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)
···
 	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_from_user)
+		ret = copy_from_user(addr, user_buffer, n);
+	else
+		ret = copy_to_user(user_buffer, addr, n);
+	if (ret) {
 		ret = -EFAULT;
 		goto out_unlock;
 	}
···
 
 	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 __user char *)user_buffer, true);
+}
+EXPORT_SYMBOL_GPL(iio_dma_buffer_write);
 
 /**
  * iio_dma_buffer_usage() - DMA buffer data_available and

include/linux/iio/buffer-dma.h

···
 	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_usage(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);