dma-buf: Introduce dma buffer sharing mechanism

This is the first step in defining a dma buffer sharing mechanism.

A new buffer object dma_buf is added, with operations and API to allow easy
sharing of this buffer object across devices.

The framework allows:
- creation of a buffer object, its association with a file pointer, and
   associated allocator-defined operations on that buffer. This operation is
   called the 'export' operation.
- different devices to 'attach' themselves to this exported buffer object, to
  facilitate backing storage negotiation, using dma_buf_attach() API.
- the exported buffer object to be shared with the other entity by asking for
   its 'file-descriptor (fd)', and sharing the fd across.
- a received fd to get the buffer object back, where it can be accessed using
   the associated exporter-defined operations.
- the exporter and user to share the scatterlist associated with this buffer
   object using map_dma_buf and unmap_dma_buf operations.

Atleast one 'attach()' call is required to be made prior to calling the
map_dma_buf() operation.

Couple of building blocks in map_dma_buf() are added to ease introduction
of sync'ing across exporter and users, and late allocation by the exporter.

For this first version, this framework will work with certain conditions:
- *ONLY* exporter will be allowed to mmap to userspace (outside of this
   framework - mmap is not a buffer object operation),
- currently, *ONLY* users that do not need CPU access to the buffer are
   allowed.

More details are there in the documentation patch.

This is based on design suggestions from many people at the mini-summits[1],
most notably from Arnd Bergmann <arnd@arndb.de>, Rob Clark <rob@ti.com> and
Daniel Vetter <daniel@ffwll.ch>.

The implementation is inspired from proof-of-concept patch-set from
Tomasz Stanislawski <t.stanislaws@samsung.com>, who demonstrated buffer sharing
between two v4l2 devices. [2]

[1]: https://wiki.linaro.org/OfficeofCTO/MemoryManagement
[2]: http://lwn.net/Articles/454389

Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Sumit Semwal <sumit.semwal@ti.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-and-Tested-by: Rob Clark <rob.clark@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 176 insertions, 0 deletions

diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
new file mode 100644
index 000000000000..f8ac076afa52
--- /dev/null
+++ b/include/linux/dma-buf.h
@@ -0,0 +1,176 @@
+/*
+ * Header file for dma buffer sharing framework.
+ *
+ * Copyright(C) 2011 Linaro Limited. All rights reserved.
+ * Author: Sumit Semwal <sumit.semwal@ti.com>
+ *
+ * Many thanks to linaro-mm-sig list, and specially
+ * Arnd Bergmann <arnd@arndb.de>, Rob Clark <rob@ti.com> and
+ * Daniel Vetter <daniel@ffwll.ch> for their support in creation and
+ * refining of this idea.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 as published by
+ * the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#ifndef __DMA_BUF_H__
+#define __DMA_BUF_H__
+
+#include <linux/file.h>
+#include <linux/err.h>
+#include <linux/device.h>
+#include <linux/scatterlist.h>
+#include <linux/list.h>
+#include <linux/dma-mapping.h>
+
+struct dma_buf;
+struct dma_buf_attachment;
+
+/**
+ * struct dma_buf_ops - operations possible on struct dma_buf
+ * @attach: [optional] allows different devices to 'attach' themselves to the
+ *          given buffer. It might return -EBUSY to signal that backing storage
+ *          is already allocated and incompatible with the requirements
+ *          of requesting device.
+ * @detach: [optional] detach a given device from this buffer.
+ * @map_dma_buf: returns list of scatter pages allocated, increases usecount
+ *               of the buffer. Requires atleast one attach to be called
+ *               before. Returned sg list should already be mapped into
+ *               _device_ address space. This call may sleep. May also return
+ *               -EINTR. Should return -EINVAL if attach hasn't been called yet.
+ * @unmap_dma_buf: decreases usecount of buffer, might deallocate scatter
+ *                 pages.
+ * @release: release this buffer; to be called after the last dma_buf_put.
+ */
+struct dma_buf_ops {
+        int (*attach)(struct dma_buf *, struct device *,
+                        struct dma_buf_attachment *);
+
+        void (*detach)(struct dma_buf *, struct dma_buf_attachment *);
+
+        /* For {map,unmap}_dma_buf below, any specific buffer attributes
+         * required should get added to device_dma_parameters accessible
+         * via dev->dma_params.
+         */
+        struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *,
+                                                enum dma_data_direction);
+        void (*unmap_dma_buf)(struct dma_buf_attachment *,
+                                                struct sg_table *);
+        /* TODO: Add try_map_dma_buf version, to return immed with -EBUSY
+         * if the call would block.
+         */
+
+        /* after final dma_buf_put() */
+        void (*release)(struct dma_buf *);
+
+};
+
+/**
+ * struct dma_buf - shared buffer object
+ * @size: size of the buffer
+ * @file: file pointer used for sharing buffers across, and for refcounting.
+ * @attachments: list of dma_buf_attachment that denotes all devices attached.
+ * @ops: dma_buf_ops associated with this buffer object.
+ * @priv: exporter specific private data for this buffer object.
+ */
+struct dma_buf {
+        size_t size;
+        struct file *file;
+        struct list_head attachments;
+        const struct dma_buf_ops *ops;
+        /* mutex to serialize list manipulation and other ops */
+        struct mutex lock;
+        void *priv;
+};
+
+/**
+ * struct dma_buf_attachment - holds device-buffer attachment data
+ * @dmabuf: buffer for this attachment.
+ * @dev: device attached to the buffer.
+ * @node: list of dma_buf_attachment.
+ * @priv: exporter specific attachment data.
+ *
+ * This structure holds the attachment information between the dma_buf buffer
+ * and its user device(s). The list contains one attachment struct per device
+ * attached to the buffer.
+ */
+struct dma_buf_attachment {
+        struct dma_buf *dmabuf;
+        struct device *dev;
+        struct list_head node;
+        void *priv;
+};
+
+#ifdef CONFIG_DMA_SHARED_BUFFER
+struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
+                                                        struct device *dev);
+void dma_buf_detach(struct dma_buf *dmabuf,
+                                struct dma_buf_attachment *dmabuf_attach);
+struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops,
+                        size_t size, int flags);
+int dma_buf_fd(struct dma_buf *dmabuf);
+struct dma_buf *dma_buf_get(int fd);
+void dma_buf_put(struct dma_buf *dmabuf);
+
+struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *,
+                                        enum dma_data_direction);
+void dma_buf_unmap_attachment(struct dma_buf_attachment *, struct sg_table *);
+#else
+
+static inline struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
+                                                        struct device *dev)
+{
+        return ERR_PTR(-ENODEV);
+}
+
+static inline void dma_buf_detach(struct dma_buf *dmabuf,
+                                  struct dma_buf_attachment *dmabuf_attach)
+{
+        return;
+}
+
+static inline struct dma_buf *dma_buf_export(void *priv,
+                                                struct dma_buf_ops *ops,
+                                                size_t size, int flags)
+{
+        return ERR_PTR(-ENODEV);
+}
+
+static inline int dma_buf_fd(struct dma_buf *dmabuf)
+{
+        return -ENODEV;
+}
+
+static inline struct dma_buf *dma_buf_get(int fd)
+{
+        return ERR_PTR(-ENODEV);
+}
+
+static inline void dma_buf_put(struct dma_buf *dmabuf)
+{
+        return;
+}
+
+static inline struct sg_table *dma_buf_map_attachment(
+        struct dma_buf_attachment *attach, enum dma_data_direction write)
+{
+        return ERR_PTR(-ENODEV);
+}
+
+static inline void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,
+                                                struct sg_table *sg)
+{
+        return;
+}
+
+#endif /* CONFIG_DMA_SHARED_BUFFER */
+
+#endif /* __DMA_BUF_H__ */