Re: [PATCH RFC 1/1] uio: Add dma-buf import ioctls

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Sat, Feb 23, 2019 at 12:28:17PM -0800, Hyun Kwon wrote:
> Add the dmabuf map / unmap interfaces. This allows the user driver
> to be able to import the external dmabuf and use it from user space.
> 
> Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
> ---
>  drivers/uio/Makefile         |   2 +-
>  drivers/uio/uio.c            |  43 +++++++++
>  drivers/uio/uio_dmabuf.c     | 210 +++++++++++++++++++++++++++++++++++++++++++
>  drivers/uio/uio_dmabuf.h     |  26 ++++++
>  include/uapi/linux/uio/uio.h |  33 +++++++
>  5 files changed, 313 insertions(+), 1 deletion(-)
>  create mode 100644 drivers/uio/uio_dmabuf.c
>  create mode 100644 drivers/uio/uio_dmabuf.h
>  create mode 100644 include/uapi/linux/uio/uio.h
> 
> diff --git a/drivers/uio/Makefile b/drivers/uio/Makefile
> index c285dd2..5da16c7 100644
> --- a/drivers/uio/Makefile
> +++ b/drivers/uio/Makefile
> @@ -1,5 +1,5 @@
>  # SPDX-License-Identifier: GPL-2.0
> -obj-$(CONFIG_UIO)	+= uio.o
> +obj-$(CONFIG_UIO)	+= uio.o uio_dmabuf.o
>  obj-$(CONFIG_UIO_CIF)	+= uio_cif.o
>  obj-$(CONFIG_UIO_PDRV_GENIRQ)	+= uio_pdrv_genirq.o
>  obj-$(CONFIG_UIO_DMEM_GENIRQ)	+= uio_dmem_genirq.o
> diff --git a/drivers/uio/uio.c b/drivers/uio/uio.c
> index 1313422..6841f98 100644
> --- a/drivers/uio/uio.c
> +++ b/drivers/uio/uio.c
> @@ -24,6 +24,12 @@
>  #include <linux/kobject.h>
>  #include <linux/cdev.h>
>  #include <linux/uio_driver.h>
> +#include <linux/list.h>
> +#include <linux/mutex.h>
> +
> +#include <uapi/linux/uio/uio.h>
> +
> +#include "uio_dmabuf.h"
>  
>  #define UIO_MAX_DEVICES		(1U << MINORBITS)
>  
> @@ -454,6 +460,8 @@ static irqreturn_t uio_interrupt(int irq, void *dev_id)
>  struct uio_listener {
>  	struct uio_device *dev;
>  	s32 event_count;
> +	struct list_head dbufs;
> +	struct mutex dbufs_lock; /* protect @dbufs */
>  };
>  
>  static int uio_open(struct inode *inode, struct file *filep)
> @@ -500,6 +508,9 @@ static int uio_open(struct inode *inode, struct file *filep)
>  	if (ret)
>  		goto err_infoopen;
>  
> +	INIT_LIST_HEAD(&listener->dbufs);
> +	mutex_init(&listener->dbufs_lock);
> +
>  	return 0;
>  
>  err_infoopen:
> @@ -529,6 +540,10 @@ static int uio_release(struct inode *inode, struct file *filep)
>  	struct uio_listener *listener = filep->private_data;
>  	struct uio_device *idev = listener->dev;
>  
> +	ret = uio_dmabuf_cleanup(idev, &listener->dbufs, &listener->dbufs_lock);
> +	if (ret)
> +		dev_err(&idev->dev, "failed to clean up the dma bufs\n");
> +
>  	mutex_lock(&idev->info_lock);
>  	if (idev->info && idev->info->release)
>  		ret = idev->info->release(idev->info, inode);
> @@ -652,6 +667,33 @@ static ssize_t uio_write(struct file *filep, const char __user *buf,
>  	return retval ? retval : sizeof(s32);
>  }
>  
> +static long uio_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)

We have resisted adding a uio ioctl for a long time, can't you do this
through sysfs somehow?

A meta-comment about your ioctl structure:

> +#define UIO_DMABUF_DIR_BIDIR	1
> +#define UIO_DMABUF_DIR_TO_DEV	2
> +#define UIO_DMABUF_DIR_FROM_DEV	3
> +#define UIO_DMABUF_DIR_NONE	4

enumerated type?

> +
> +struct uio_dmabuf_args {
> +	__s32	dbuf_fd;
> +	__u64	dma_addr;
> +	__u64	size;
> +	__u32	dir;

Why the odd alignment?  Are you sure this is the best packing for such a
structure?

Why is dbuf_fd __s32?  dir can be __u8, right?

I don't know that dma layer very well, it would be good to get some
review from others to see if this really is even a viable thing to do.
The fd handling seems a bit "odd" here, but maybe I just do not
understand it.

thanks,

greg k-h