usb: gadget: functionfs: Add DMABUF import interface

This patch introduces three new ioctls. They all should be called on a
data endpoint (ie. not ep0). They are:

- FUNCTIONFS_DMABUF_ATTACH, which takes the file descriptor of a DMABUF
  object to attach to the endpoint.

- FUNCTIONFS_DMABUF_DETACH, which takes the file descriptor of the
  DMABUF to detach from the endpoint. Note that closing the endpoint's
  file descriptor will automatically detach all attached DMABUFs.

- FUNCTIONFS_DMABUF_TRANSFER, which requests a data transfer from / to
  the given DMABUF. Its argument is a structure that packs the DMABUF's
  file descriptor, the size in bytes to transfer (which should generally
  be set to the size of the DMABUF), and a 'flags' field which is unused
  for now.
  Before this ioctl can be used, the related DMABUF must be attached
  with FUNCTIONFS_DMABUF_ATTACH.

These three ioctls enable the FunctionFS code to transfer data between
the USB stack and a DMABUF object, which can be provided by a driver
from a completely different subsystem, in a zero-copy fashion.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20240130122340.54813-4-paul@crapouillou.netSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/Kconfig

@@ -190,6 +190,7 @@ config USB_F_MASS_STORAGE
 	tristate
 
 config USB_F_FS
+	select DMA_SHARED_BUFFER
 	tristate
 
 config USB_F_UAC1

include/uapi/linux/usb/functionfs.h

@@ -86,6 +86,22 @@ struct usb_ext_prop_desc {
 	__le16	wPropertyNameLength;
 } __attribute__((packed));
 
+/* Flags for usb_ffs_dmabuf_transfer_req->flags (none for now) */
+#define USB_FFS_DMABUF_TRANSFER_MASK	0x0
+
+/**
+ * struct usb_ffs_dmabuf_transfer_req - Transfer request for a DMABUF object
+ * @fd:		file descriptor of the DMABUF object
+ * @flags:	one or more USB_FFS_DMABUF_TRANSFER_* flags
+ * @length:	number of bytes used in this DMABUF for the data transfer.
+ *		Should generally be set to the DMABUF's size.
+ */
+struct usb_ffs_dmabuf_transfer_req {
+	int fd;
+	__u32 flags;
+	__u64 length;
+} __attribute__((packed));
+
 #ifndef __KERNEL__
 
 /*
@@ -290,6 +306,31 @@ struct usb_functionfs_event {
 #define	FUNCTIONFS_ENDPOINT_DESC	_IOR('g', 130, \
 					     struct usb_endpoint_descriptor)
 
+/*
+ * Attach the DMABUF object, identified by its file descriptor, to the
+ * data endpoint. Returns zero on success, and a negative errno value
+ * on error.
+ */
+#define FUNCTIONFS_DMABUF_ATTACH	_IOW('g', 131, int)
+
 
+/*
+ * Detach the given DMABUF object, identified by its file descriptor,
+ * from the data endpoint. Returns zero on success, and a negative
+ * errno value on error. Note that closing the endpoint's file
+ * descriptor will automatically detach all attached DMABUFs.
+ */
+#define FUNCTIONFS_DMABUF_DETACH	_IOW('g', 132, int)
+
+/*
+ * Enqueue the previously attached DMABUF to the transfer queue.
+ * The argument is a structure that packs the DMABUF's file descriptor,
+ * the size in bytes to transfer (which should generally correspond to
+ * the size of the DMABUF), and a 'flags' field which is unused
+ * for now. Returns zero on success, and a negative errno value on
+ * error.
+ */
+#define FUNCTIONFS_DMABUF_TRANSFER	_IOW('g', 133, \
+					     struct usb_ffs_dmabuf_transfer_req)
 
 #endif /* _UAPI__LINUX_FUNCTIONFS_H__ */