Re: [PATCH 11/15] iio: buffer-dma: Boost performance using write-combine cache setting

[Jonathan Cameron]

On Mon, 15 Nov 2021 14:19:21 +0000
Paul Cercueil <paul@xxxxxxxxxxxxxxx> wrote:

> We can be certain that the input buffers will only be accessed by
> userspace for reading, and output buffers will mostly be accessed by
> userspace for writing.

Mostly?  Perhaps a little more info on why that's not 'only'.

> 
> Therefore, it makes more sense to use only fully cached input buffers,
> and to use the write-combine cache coherency setting for output buffers.
> 
> This boosts performance, as the data written to the output buffers does
> not have to be sync'd for coherency. It will halve performance if the
> userspace application tries to read from the output buffer, but this
> should never happen.
> 
> Since we don't need to sync the cache when disabling CPU access either
> for input buffers or output buffers, the .end_cpu_access() callback can
> be dropped completely.

We have an odd mix of coherent and non coherent DMA in here as you noted,
but are you sure this is safe on all platforms?

> 
> Signed-off-by: Paul Cercueil <paul@xxxxxxxxxxxxxxx>

Any numbers to support this patch?  The mapping types are performance
optimisations so nice to know how much of a difference they make.


> ---
>  drivers/iio/buffer/industrialio-buffer-dma.c | 82 +++++++++++++-------
>  1 file changed, 54 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/iio/buffer/industrialio-buffer-dma.c b/drivers/iio/buffer/industrialio-buffer-dma.c
> index 92356ee02f30..fb39054d8c15 100644
> --- a/drivers/iio/buffer/industrialio-buffer-dma.c
> +++ b/drivers/iio/buffer/industrialio-buffer-dma.c
> @@ -229,8 +229,33 @@ static int iio_buffer_dma_buf_mmap(struct dma_buf *dbuf,
>  	if (vma->vm_ops->open)
>  		vma->vm_ops->open(vma);
>  
> -	return dma_mmap_pages(dev, vma, vma->vm_end - vma->vm_start,
> -			      virt_to_page(block->vaddr));
> +	if (block->queue->buffer.direction == IIO_BUFFER_DIRECTION_IN) {
> +		/*
> +		 * With an input buffer, userspace will only read the data and
> +		 * never write. We can mmap the buffer fully cached.
> +		 */
> +		return dma_mmap_pages(dev, vma, vma->vm_end - vma->vm_start,
> +				      virt_to_page(block->vaddr));
> +	} else {
> +		/*
> +		 * With an output buffer, userspace will only write the data
> +		 * and should rarely (if never) read from it. It is better to
> +		 * use write-combine in this case.
> +		 */
> +		return dma_mmap_wc(dev, vma, block->vaddr, block->phys_addr,
> +				   vma->vm_end - vma->vm_start);
> +	}
> +}
> +
> +static void iio_dma_buffer_free_dmamem(struct iio_dma_buffer_block *block)
> +{
> +	struct device *dev = block->queue->dev;
> +	size_t size = PAGE_ALIGN(block->size);
> +
> +	if (block->queue->buffer.direction == IIO_BUFFER_DIRECTION_IN)
> +		dma_free_coherent(dev, size, block->vaddr, block->phys_addr);
> +	else
> +		dma_free_wc(dev, size, block->vaddr, block->phys_addr);
>  }
>  
>  static void iio_buffer_dma_buf_release(struct dma_buf *dbuf)
> @@ -243,9 +268,7 @@ static void iio_buffer_dma_buf_release(struct dma_buf *dbuf)
>  
>  	mutex_lock(&queue->lock);
>  
> -	dma_free_coherent(queue->dev, PAGE_ALIGN(block->size),
> -			  block->vaddr, block->phys_addr);
> -
> +	iio_dma_buffer_free_dmamem(block);
>  	kfree(block);
>  
>  	queue->num_blocks--;
> @@ -268,19 +291,6 @@ static int iio_buffer_dma_buf_begin_cpu_access(struct dma_buf *dbuf,
>  	return 0;
>  }
>  
> -static int iio_buffer_dma_buf_end_cpu_access(struct dma_buf *dbuf,
> -					     enum dma_data_direction dma_dir)
> -{
> -	struct iio_dma_buffer_block *block = dbuf->priv;
> -	struct device *dev = block->queue->dev;
> -
> -	/* We only need to sync the cache for output buffers */
> -	if (block->queue->buffer.direction == IIO_BUFFER_DIRECTION_OUT)
> -		dma_sync_single_for_device(dev, block->phys_addr, block->size, dma_dir);
> -
> -	return 0;
> -}
> -
>  static const struct dma_buf_ops iio_dma_buffer_dmabuf_ops = {
>  	.attach			= iio_buffer_dma_buf_attach,
>  	.map_dma_buf		= iio_buffer_dma_buf_map,
> @@ -288,9 +298,28 @@ static const struct dma_buf_ops iio_dma_buffer_dmabuf_ops = {
>  	.mmap			= iio_buffer_dma_buf_mmap,
>  	.release		= iio_buffer_dma_buf_release,
>  	.begin_cpu_access	= iio_buffer_dma_buf_begin_cpu_access,
> -	.end_cpu_access		= iio_buffer_dma_buf_end_cpu_access,
>  };
>  
> +static int iio_dma_buffer_alloc_dmamem(struct iio_dma_buffer_block *block)
> +{
> +	struct device *dev = block->queue->dev;
> +	size_t size = PAGE_ALIGN(block->size);
> +
> +	if (block->queue->buffer.direction == IIO_BUFFER_DIRECTION_IN) {
> +		block->vaddr = dma_alloc_coherent(dev, size,
> +						  &block->phys_addr,
> +						  GFP_KERNEL);
> +	} else {
> +		block->vaddr = dma_alloc_wc(dev, size,
> +					    &block->phys_addr,
> +					    GFP_KERNEL);
> +	}
> +	if (!block->vaddr)
> +		return -ENOMEM;
> +
> +	return 0;
> +}
> +
>  static struct iio_dma_buffer_block *iio_dma_buffer_alloc_block(
>  	struct iio_dma_buffer_queue *queue, size_t size, bool fileio)
>  {
> @@ -303,12 +332,12 @@ static struct iio_dma_buffer_block *iio_dma_buffer_alloc_block(
>  	if (!block)
>  		return ERR_PTR(-ENOMEM);
>  
> -	block->vaddr = dma_alloc_coherent(queue->dev, PAGE_ALIGN(size),
> -		&block->phys_addr, GFP_KERNEL);
> -	if (!block->vaddr) {
> -		err = -ENOMEM;
> +	block->size = size;
> +	block->queue = queue;
> +
> +	err = iio_dma_buffer_alloc_dmamem(block);
> +	if (err)
>  		goto err_free_block;
> -	}
>  
>  	einfo.ops = &iio_dma_buffer_dmabuf_ops;
>  	einfo.size = PAGE_ALIGN(size);
> @@ -322,10 +351,8 @@ static struct iio_dma_buffer_block *iio_dma_buffer_alloc_block(
>  	}
>  
>  	block->dmabuf = dmabuf;
> -	block->size = size;
>  	block->bytes_used = size;
>  	block->state = IIO_BLOCK_STATE_DONE;
> -	block->queue = queue;
>  	block->fileio = fileio;
>  	INIT_LIST_HEAD(&block->head);
>  
> @@ -338,8 +365,7 @@ static struct iio_dma_buffer_block *iio_dma_buffer_alloc_block(
>  	return block;
>  
>  err_free_dma:
> -	dma_free_coherent(queue->dev, PAGE_ALIGN(size),
> -			  block->vaddr, block->phys_addr);
> +	iio_dma_buffer_free_dmamem(block);
>  err_free_block:
>  	kfree(block);
>  	return ERR_PTR(err);