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

[Daniel Vetter]

On Tue, Feb 26, 2019 at 12:53 PM Greg Kroah-Hartman
<gregkh at linuxfoundation.org> wrote:
>
> 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 at 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.

Frankly looks like a ploy to sidestep review by graphics folks. We'd
ask for the userspace first :-)

Also, exporting dma_addr to userspace is considered a very bad idea.
If you want to do this properly, you need a minimal in-kernel memory
manager, and those tend to be based on top of drm_gem.c and merged
through the gpu tree. The last place where we accidentally leaked a
dma addr for gpu buffers was in the fbdev code, and we plugged that
one with

commit 4be9bd10e22dfc7fc101c5cf5969ef2d3a042d8a (tag:
drm-misc-next-fixes-2018-10-03)
Author: Neil Armstrong <narmstrong at baylibre.com>
Date:   Fri Sep 28 14:05:55 2018 +0200

    drm/fb_helper: Allow leaking fbdev smem_start

Together with cuse the above patch should be enough to implement a drm
driver entirely in userspace at least.

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch