Re: [PATCH v9 29/32] rapidio: fix common struct sg_table related issues

[Robin Murphy]

On 2020-08-26 07:33, Marek Szyprowski wrote:
> The Documentation/DMA-API-HOWTO.txt states that the dma_map_sg() function
> returns the number of the created entries in the DMA address space.
> However the subsequent calls to the dma_sync_sg_for_{device,cpu}() and
> dma_unmap_sg must be called with the original number of the entries
> passed to the dma_map_sg().
>
> struct sg_table is a common structure used for describing a non-contiguous
> memory buffer, used commonly in the DRM and graphics subsystems. It
> consists of a scatterlist with memory pages and DMA addresses (sgl entry),
> as well as the number of scatterlist entries: CPU pages (orig_nents entry)
> and DMA mapped pages (nents entry).
>
> It turned out that it was a common mistake to misuse nents and orig_nents
> entries, calling DMA-mapping functions with a wrong number of entries or
> ignoring the number of mapped entries returned by the dma_map_sg()
> function.
>
> To avoid such issues, lets use a common dma-mapping wrappers operating
> directly on the struct sg_table objects and use scatterlist page
> iterators where possible. This, almost always, hides references to the
> nents and orig_nents entries, making the code robust, easier to follow
> and copy/paste safe.

Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>

> Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> ---
>   drivers/rapidio/devices/rio_mport_cdev.c | 11 ++++-------
>   1 file changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/rapidio/devices/rio_mport_cdev.c b/drivers/rapidio/devices/rio_mport_cdev.c
> index a30342942e26..89eb3d212652 100644
> --- a/drivers/rapidio/devices/rio_mport_cdev.c
> +++ b/drivers/rapidio/devices/rio_mport_cdev.c
> @@ -573,8 +573,7 @@ static void dma_req_free(struct kref *ref)
>   			refcount);
>   	struct mport_cdev_priv *priv = req->priv;
>   
> -	dma_unmap_sg(req->dmach->device->dev,
> -		     req->sgt.sgl, req->sgt.nents, req->dir);
> +	dma_unmap_sgtable(req->dmach->device->dev, &req->sgt, req->dir, 0);
>   	sg_free_table(&req->sgt);
>   	if (req->page_list) {
>   		unpin_user_pages(req->page_list, req->nr_pages);
> @@ -814,7 +813,6 @@ rio_dma_transfer(struct file *filp, u32 transfer_mode,
>   	struct mport_dev *md = priv->md;
>   	struct dma_chan *chan;
>   	int ret;
> -	int nents;
>   
>   	if (xfer->length == 0)
>   		return -EINVAL;
> @@ -930,15 +928,14 @@ rio_dma_transfer(struct file *filp, u32 transfer_mode,
>   				xfer->offset, xfer->length);
>   	}
>   
> -	nents = dma_map_sg(chan->device->dev,
> -			   req->sgt.sgl, req->sgt.nents, dir);
> -	if (nents == 0) {
> +	ret = dma_map_sgtable(chan->device->dev, &req->sgt, dir, 0);
> +	if (ret) {
>   		rmcd_error("Failed to map SG list");
>   		ret = -EFAULT;
>   		goto err_pg;
>   	}
>   
> -	ret = do_dma_request(req, xfer, sync, nents);
> +	ret = do_dma_request(req, xfer, sync, req->sgt.nents);
>   
>   	if (ret >= 0) {
>   		if (sync == RIO_TRANSFER_ASYNC)