[PATCH 0/3] Introduce DMA buffer sharing mechanism

[PATCH 0/3] Introduce DMA buffer sharing mechanism

[Sumit Semwal]

Hello Everyone,

Post some discussion as an RFC, here is the patch for introducing 
DMA buffer sharing mechanism - change history is in the changelog below.

Various subsystems - V4L2, GPU-accessors, DRI to name a few - have felt the 
need to have a common mechanism to share memory buffers across different
devices - ARM, video hardware, GPU.

This need comes forth from a variety of use cases including cameras, image 
processing, video recorders, sound processing, DMA engines, GPU and display
buffers, amongst others.

This patch attempts to define such a buffer sharing mechanism - it is the
result of discussions from a couple of memory-management mini-summits held by
Linaro to understand and address common needs around memory management. [1]

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

The framework allows:
- a new buffer object to be created with fixed size, associated with a file 
   pointer and allocator-defined operations for this buffer object. This
   operation is called the 'export' operation.
- different devices to 'attach' themselves to this buffer object, to facilitate
   backing storage negotiation, using dma_buf_attach() API.
- this 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 buffer object's scatterlist using
   map_dma_buf and unmap_dma_buf operations.

Documentation present in the patch-set gives more details.

For 1st version, dma-buf is marked as an EXPERIMENTAL driver, which we can
remove for later versions with additional usage and testing.

*IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details]
For this first version, A buffer shared using the dma_buf sharing API:
- *may* be exported to user space using "mmap" *ONLY* by exporter, outside of
   this framework.
- may be used *ONLY* by importers that do not need CPU access to the buffer.

This is based on design suggestions from many people at the mini-summits,
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]

Some sample implementations and WIP for dma-buf users and exporters are
available at [3] and [4]. [These are not being submitted for discussion /
inclusion right now, but are for reference only]

References:
[1]: https://wiki.linaro.org/OfficeofCTO/MemoryManagement
[2]: http://lwn.net/Articles/454389
[3]: Dave Airlie's prime support:
   http://cgit.freedesktop.org/~airlied/linux/log/?h=drm-prime-dmabuf
[4]: Rob Clark's sharing between DRM and V4L2:
   https://github.com/robclark/kernel-omap4/commits/drmplane-dmabuf

Patchset based on top of 3.2-rc7, the current version can be found at

http://git.linaro.org/gitweb?p=people/sumitsemwal/linux-3.x.git
Branch: dmabuf-patch-v1

Earlier versions:
RFC:
v3 at: https://lkml.org/lkml/2011/12/19/50
v2 at: https://lkml.org/lkml/2011/12/2/53
v1 at: https://lkml.org/lkml/2011/10/11/92

Wish you all happy vacations and a very happy, joyous and prosperous new year
2012 :)

Best regards,
~Sumit Semwal

History:
v4:
- Review comments incorporated:
   - from Konrad Rzeszutek Wilk [https://lkml.org/lkml/2011/12/20/209]
     - corrected language in some comments
     - re-ordered struct definitions for readability
     - added might_sleep() call in dma_buf_map_attachment() wrapper
     
   - from Rob Clark [https://lkml.org/lkml/2011/12/23/196]
     - Made dma-buf EXPERIMENTAL for 1st version.

v3:
- Review comments incorporated:
   - from Konrad Rzeszutek Wilk [https://lkml.org/lkml/2011/12/3/45]
     - replaced BUG_ON with WARN_ON - various places
     - added some error-checks
     - replaced EXPORT_SYMBOL with EXPORT_SYMBOL_GPL
     - some cosmetic / documentation comments

   - from Arnd Bergmann, Daniel Vetter, Rob Clark
      [https://lkml.org/lkml/2011/12/5/321]
     - removed mmap() fop and dma_buf_op, also the sg_sync* operations, and
        documented that mmap is not allowed for exported buffer
     - updated documentation to clearly state when migration is allowed
     - changed kconfig
     - some error code checks

   - from Rob Clark [https://lkml.org/lkml/2011/12/5/572]
     - update documentation to allow map_dma_buf to return -EINTR

v2:
- Review comments incorporated:
   - from Tomasz Stanislawski [https://lkml.org/lkml/2011/10/14/136]
     - kzalloc moved out of critical section
     - corrected some in-code comments

   - from Dave Airlie [https://lkml.org/lkml/2011/11/25/123]

   - from Daniel Vetter and Rob Clark [https://lkml.org/lkml/2011/11/26/53]
     - use struct sg_table in place of struct scatterlist
     - rename {get,put}_scatterlist to {map,unmap}_dma_buf
     - add new wrapper APIs dma_buf_{map,unmap}_attachment for ease of users
     
- documentation updates as per review comments from Randy Dunlap
     [https://lkml.org/lkml/2011/10/12/439]

v1: original


Sumit Semwal (3):
  dma-buf: Introduce dma buffer sharing mechanism
  dma-buf: Documentation for buffer sharing framework
  dma-buf: mark EXPERIMENTAL for 1st release.

 Documentation/dma-buf-sharing.txt |  224 ++++++++++++++++++++++++++++
 drivers/base/Kconfig              |   11 ++
 drivers/base/Makefile             |    1 +
 drivers/base/dma-buf.c            |  291 +++++++++++++++++++++++++++++++++++++
 include/linux/dma-buf.h           |  176 ++++++++++++++++++++++
 5 files changed, 703 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/dma-buf-sharing.txt
 create mode 100644 drivers/base/dma-buf.c
 create mode 100644 include/linux/dma-buf.h

-- 
1.7.5.4

[PATCH 1/3] dma-buf: Introduce dma buffer sharing mechanism

[Sumit Semwal]

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>
---
 drivers/base/Kconfig    |   10 ++
 drivers/base/Makefile   |    1 +
 drivers/base/dma-buf.c  |  291 +++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/dma-buf.h |  176 ++++++++++++++++++++++++++++
 4 files changed, 478 insertions(+), 0 deletions(-)
 create mode 100644 drivers/base/dma-buf.c
 create mode 100644 include/linux/dma-buf.h

diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
index 21cf46f..8a0e87f 100644
--- a/drivers/base/Kconfig
+++ b/drivers/base/Kconfig
@@ -174,4 +174,14 @@ config SYS_HYPERVISOR
 
 source "drivers/base/regmap/Kconfig"
 
+config DMA_SHARED_BUFFER
+	bool "Buffer framework to be shared between drivers"
+	default n
+	select ANON_INODES
+	help
+	  This option enables the framework for buffer-sharing between
+	  multiple drivers. A buffer is associated with a file using driver
+	  APIs extension; the file's descriptor can then be passed on to other
+	  driver.
+
 endmenu
diff --git a/drivers/base/Makefile b/drivers/base/Makefile
index 99a375a..d0df046 100644
--- a/drivers/base/Makefile
+++ b/drivers/base/Makefile
@@ -8,6 +8,7 @@ obj-$(CONFIG_DEVTMPFS)	+= devtmpfs.o
 obj-y			+= power/
 obj-$(CONFIG_HAS_DMA)	+= dma-mapping.o
 obj-$(CONFIG_HAVE_GENERIC_DMA_COHERENT) += dma-coherent.o
+obj-$(CONFIG_DMA_SHARED_BUFFER) += dma-buf.o
 obj-$(CONFIG_ISA)	+= isa.o
 obj-$(CONFIG_FW_LOADER)	+= firmware_class.o
 obj-$(CONFIG_NUMA)	+= node.o
diff --git a/drivers/base/dma-buf.c b/drivers/base/dma-buf.c
new file mode 100644
index 0000000..e38ad24
--- /dev/null
+++ b/drivers/base/dma-buf.c
@@ -0,0 +1,291 @@
+/*
+ * Framework for buffer objects that can be shared across devices/subsystems.
+ *
+ * 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/>.
+ */
+
+#include <linux/fs.h>
+#include <linux/slab.h>
+#include <linux/dma-buf.h>
+#include <linux/anon_inodes.h>
+#include <linux/export.h>
+
+static inline int is_dma_buf_file(struct file *);
+
+static int dma_buf_release(struct inode *inode, struct file *file)
+{
+	struct dma_buf *dmabuf;
+
+	if (!is_dma_buf_file(file))
+		return -EINVAL;
+
+	dmabuf = file->private_data;
+
+	dmabuf->ops->release(dmabuf);
+	kfree(dmabuf);
+	return 0;
+}
+
+static const struct file_operations dma_buf_fops = {
+	.release	= dma_buf_release,
+};
+
+/*
+ * is_dma_buf_file - Check if struct file* is associated with dma_buf
+ */
+static inline int is_dma_buf_file(struct file *file)
+{
+	return file->f_op == &dma_buf_fops;
+}
+
+/**
+ * dma_buf_export - Creates a new dma_buf, and associates an anon file
+ * with this buffer, so it can be exported.
+ * Also connect the allocator specific data and ops to the buffer.
+ *
+ * @priv:	[in]	Attach private data of allocator to this buffer
+ * @ops:	[in]	Attach allocator-defined dma buf ops to the new buffer.
+ * @size:	[in]	Size of the buffer
+ * @flags:	[in]	mode flags for the file.
+ *
+ * Returns, on success, a newly created dma_buf object, which wraps the
+ * supplied private data and operations for dma_buf_ops. On either missing
+ * ops, or error in allocating struct dma_buf, will return negative error.
+ *
+ */
+struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops,
+				size_t size, int flags)
+{
+	struct dma_buf *dmabuf;
+	struct file *file;
+
+	if (WARN_ON(!priv || !ops
+			  || !ops->map_dma_buf
+			  || !ops->unmap_dma_buf
+			  || !ops->release)) {
+		return ERR_PTR(-EINVAL);
+	}
+
+	dmabuf = kzalloc(sizeof(struct dma_buf), GFP_KERNEL);
+	if (dmabuf == NULL)
+		return ERR_PTR(-ENOMEM);
+
+	dmabuf->priv = priv;
+	dmabuf->ops = ops;
+	dmabuf->size = size;
+
+	file = anon_inode_getfile("dmabuf", &dma_buf_fops, dmabuf, flags);
+
+	dmabuf->file = file;
+
+	mutex_init(&dmabuf->lock);
+	INIT_LIST_HEAD(&dmabuf->attachments);
+
+	return dmabuf;
+}
+EXPORT_SYMBOL_GPL(dma_buf_export);
+
+
+/**
+ * dma_buf_fd - returns a file descriptor for the given dma_buf
+ * @dmabuf:	[in]	pointer to dma_buf for which fd is required.
+ *
+ * On success, returns an associated 'fd'. Else, returns error.
+ */
+int dma_buf_fd(struct dma_buf *dmabuf)
+{
+	int error, fd;
+
+	if (!dmabuf || !dmabuf->file)
+		return -EINVAL;
+
+	error = get_unused_fd();
+	if (error < 0)
+		return error;
+	fd = error;
+
+	fd_install(fd, dmabuf->file);
+
+	return fd;
+}
+EXPORT_SYMBOL_GPL(dma_buf_fd);
+
+/**
+ * dma_buf_get - returns the dma_buf structure related to an fd
+ * @fd:	[in]	fd associated with the dma_buf to be returned
+ *
+ * On success, returns the dma_buf structure associated with an fd; uses
+ * file's refcounting done by fget to increase refcount. returns ERR_PTR
+ * otherwise.
+ */
+struct dma_buf *dma_buf_get(int fd)
+{
+	struct file *file;
+
+	file = fget(fd);
+
+	if (!file)
+		return ERR_PTR(-EBADF);
+
+	if (!is_dma_buf_file(file)) {
+		fput(file);
+		return ERR_PTR(-EINVAL);
+	}
+
+	return file->private_data;
+}
+EXPORT_SYMBOL_GPL(dma_buf_get);
+
+/**
+ * dma_buf_put - decreases refcount of the buffer
+ * @dmabuf:	[in]	buffer to reduce refcount of
+ *
+ * Uses file's refcounting done implicitly by fput()
+ */
+void dma_buf_put(struct dma_buf *dmabuf)
+{
+	if (WARN_ON(!dmabuf || !dmabuf->file))
+		return;
+
+	fput(dmabuf->file);
+}
+EXPORT_SYMBOL_GPL(dma_buf_put);
+
+/**
+ * dma_buf_attach - Add the device to dma_buf's attachments list; optionally,
+ * calls attach() of dma_buf_ops to allow device-specific attach functionality
+ * @dmabuf:	[in]	buffer to attach device to.
+ * @dev:	[in]	device to be attached.
+ *
+ * Returns struct dma_buf_attachment * for this attachment; may return negative
+ * error codes.
+ *
+ */
+struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
+					  struct device *dev)
+{
+	struct dma_buf_attachment *attach;
+	int ret;
+
+	if (WARN_ON(!dmabuf || !dev || !dmabuf->ops))
+		return ERR_PTR(-EINVAL);
+
+	attach = kzalloc(sizeof(struct dma_buf_attachment), GFP_KERNEL);
+	if (attach == NULL)
+		goto err_alloc;
+
+	mutex_lock(&dmabuf->lock);
+
+	attach->dev = dev;
+	attach->dmabuf = dmabuf;
+	if (dmabuf->ops->attach) {
+		ret = dmabuf->ops->attach(dmabuf, dev, attach);
+		if (ret)
+			goto err_attach;
+	}
+	list_add(&attach->node, &dmabuf->attachments);
+
+	mutex_unlock(&dmabuf->lock);
+	return attach;
+
+err_alloc:
+	return ERR_PTR(-ENOMEM);
+err_attach:
+	kfree(attach);
+	mutex_unlock(&dmabuf->lock);
+	return ERR_PTR(ret);
+}
+EXPORT_SYMBOL_GPL(dma_buf_attach);
+
+/**
+ * dma_buf_detach - Remove the given attachment from dmabuf's attachments list;
+ * optionally calls detach() of dma_buf_ops for device-specific detach
+ * @dmabuf:	[in]	buffer to detach from.
+ * @attach:	[in]	attachment to be detached; is free'd after this call.
+ *
+ */
+void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)
+{
+	if (WARN_ON(!dmabuf || !attach || !dmabuf->ops))
+		return;
+
+	mutex_lock(&dmabuf->lock);
+	list_del(&attach->node);
+	if (dmabuf->ops->detach)
+		dmabuf->ops->detach(dmabuf, attach);
+
+	mutex_unlock(&dmabuf->lock);
+	kfree(attach);
+}
+EXPORT_SYMBOL_GPL(dma_buf_detach);
+
+/**
+ * dma_buf_map_attachment - Returns the scatterlist table of the attachment;
+ * mapped into _device_ address space. Is a wrapper for map_dma_buf() of the
+ * dma_buf_ops.
+ * @attach:	[in]	attachment whose scatterlist is to be returned
+ * @direction:	[in]	direction of DMA transfer
+ *
+ * Returns sg_table containing the scatterlist to be returned; may return NULL
+ * or ERR_PTR.
+ *
+ */
+struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
+					enum dma_data_direction direction)
+{
+	struct sg_table *sg_table = ERR_PTR(-EINVAL);
+
+	might_sleep();
+
+	if (WARN_ON(!attach || !attach->dmabuf || !attach->dmabuf->ops))
+		return ERR_PTR(-EINVAL);
+
+	mutex_lock(&attach->dmabuf->lock);
+	if (attach->dmabuf->ops->map_dma_buf)
+		sg_table = attach->dmabuf->ops->map_dma_buf(attach, direction);
+	mutex_unlock(&attach->dmabuf->lock);
+
+	return sg_table;
+}
+EXPORT_SYMBOL_GPL(dma_buf_map_attachment);
+
+/**
+ * dma_buf_unmap_attachment - unmaps and decreases usecount of the buffer;might
+ * deallocate the scatterlist associated. Is a wrapper for unmap_dma_buf() of
+ * dma_buf_ops.
+ * @attach:	[in]	attachment to unmap buffer from
+ * @sg_table:	[in]	scatterlist info of the buffer to unmap
+ *
+ */
+void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,
+				struct sg_table *sg_table)
+{
+	if (WARN_ON(!attach || !attach->dmabuf || !sg_table
+			    || !attach->dmabuf->ops))
+		return;
+
+	mutex_lock(&attach->dmabuf->lock);
+	if (attach->dmabuf->ops->unmap_dma_buf)
+		attach->dmabuf->ops->unmap_dma_buf(attach, sg_table);
+	mutex_unlock(&attach->dmabuf->lock);
+
+}
+EXPORT_SYMBOL_GPL(dma_buf_unmap_attachment);
diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
new file mode 100644
index 0000000..f8ac076
--- /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__ */
-- 
1.7.5.4

[PATCH 2/3] dma-buf: Documentation for buffer sharing framework

[Sumit Semwal]

Add documentation for dma buffer sharing framework, explaining the
various operations, members and API of the dma buffer sharing
framework.

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>
---
 Documentation/dma-buf-sharing.txt |  224 +++++++++++++++++++++++++++++++++++++
 1 files changed, 224 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/dma-buf-sharing.txt

diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
new file mode 100644
index 0000000..510eab3
--- /dev/null
+++ b/Documentation/dma-buf-sharing.txt
@@ -0,0 +1,224 @@
+                    DMA Buffer Sharing API Guide
+                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+                            Sumit Semwal
+                <sumit dot semwal at linaro dot org>
+                 <sumit dot semwal at ti dot com>
+
+This document serves as a guide to device-driver writers on what is the dma-buf
+buffer sharing API, how to use it for exporting and using shared buffers.
+
+Any device driver which wishes to be a part of DMA buffer sharing, can do so as
+either the 'exporter' of buffers, or the 'user' of buffers.
+
+Say a driver A wants to use buffers created by driver B, then we call B as the
+exporter, and A as buffer-user.
+
+The exporter
+- implements and manages operations[1] for the buffer
+- allows other users to share the buffer by using dma_buf sharing APIs,
+- manages the details of buffer allocation,
+- decides about the actual backing storage where this allocation happens,
+- takes care of any migration of scatterlist - for all (shared) users of this
+   buffer,
+
+The buffer-user
+- is one of (many) sharing users of the buffer.
+- doesn't need to worry about how the buffer is allocated, or where.
+- needs a mechanism to get access to the scatterlist that makes up this buffer
+   in memory, mapped into its own address space, so it can access the same area
+   of memory.
+
+*IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details]
+For this first version, A buffer shared using the dma_buf sharing API:
+- *may* be exported to user space using "mmap" *ONLY* by exporter, outside of
+   this framework.
+- may be used *ONLY* by importers that do not need CPU access to the buffer.
+
+The dma_buf buffer sharing API usage contains the following steps:
+
+1. Exporter announces that it wishes to export a buffer
+2. Userspace gets the file descriptor associated with the exported buffer, and
+   passes it around to potential buffer-users based on use case
+3. Each buffer-user 'connects' itself to the buffer
+4. When needed, buffer-user requests access to the buffer from exporter
+5. When finished with its use, the buffer-user notifies end-of-DMA to exporter
+6. when buffer-user is done using this buffer completely, it 'disconnects'
+   itself from the buffer.
+
+
+1. Exporter's announcement of buffer export
+
+   The buffer exporter announces its wish to export a buffer. In this, it
+   connects its own private buffer data, provides implementation for operations
+   that can be performed on the exported dma_buf, and flags for the file
+   associated with this buffer.
+
+   Interface:
+      struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops,
+				     size_t size, int flags)
+
+   If this succeeds, dma_buf_export allocates a dma_buf structure, and returns a
+   pointer to the same. It also associates an anonymous file with this buffer,
+   so it can be exported. On failure to allocate the dma_buf object, it returns
+   NULL.
+
+2. Userspace gets a handle to pass around to potential buffer-users
+
+   Userspace entity requests for a file-descriptor (fd) which is a handle to the
+   anonymous file associated with the buffer. It can then share the fd with other
+   drivers and/or processes.
+
+   Interface:
+      int dma_buf_fd(struct dma_buf *dmabuf)
+
+   This API installs an fd for the anonymous file associated with this buffer;
+   returns either 'fd', or error.
+
+3. Each buffer-user 'connects' itself to the buffer
+
+   Each buffer-user now gets a reference to the buffer, using the fd passed to
+   it.
+
+   Interface:
+      struct dma_buf *dma_buf_get(int fd)
+
+   This API will return a reference to the dma_buf, and increment refcount for
+   it.
+
+   After this, the buffer-user needs to attach its device with the buffer, which
+   helps the exporter to know of device buffer constraints.
+
+   Interface:
+      struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
+                                                struct device *dev)
+
+   This API returns reference to an attachment structure, which is then used
+   for scatterlist operations. It will optionally call the 'attach' dma_buf
+   operation, if provided by the exporter.
+
+   The dma-buf sharing framework does the bookkeeping bits related to managing
+   the list of all attachments to a buffer.
+
+Until this stage, the buffer-exporter has the option to choose not to actually
+allocate the backing storage for this buffer, but wait for the first buffer-user
+to request use of buffer for allocation.
+
+
+4. When needed, buffer-user requests access to the buffer
+
+   Whenever a buffer-user wants to use the buffer for any DMA, it asks for
+   access to the buffer using dma_buf_map_attachment API. At least one attach to
+   the buffer must have happened before map_dma_buf can be called.
+
+   Interface:
+      struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *,
+                                         enum dma_data_direction);
+
+   This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the
+   "dma_buf->ops->" indirection from the users of this interface.
+
+   In struct dma_buf_ops, map_dma_buf is defined as
+      struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *,
+                                                enum dma_data_direction);
+
+   It is one of the buffer operations that must be implemented by the exporter.
+   It should return the sg_table containing scatterlist for this buffer, mapped
+   into caller's address space.
+
+   If this is being called for the first time, the exporter can now choose to
+   scan through the list of attachments for this buffer, collate the requirements
+   of the attached devices, and choose an appropriate backing storage for the
+   buffer.
+
+   Based on enum dma_data_direction, it might be possible to have multiple users
+   accessing at the same time (for reading, maybe), or any other kind of sharing
+   that the exporter might wish to make available to buffer-users.
+
+   map_dma_buf() operation can return -EINTR if it is interrupted by a signal.
+
+
+5. When finished, the buffer-user notifies end-of-DMA to exporter
+
+   Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to
+   the exporter using the dma_buf_unmap_attachment API.
+
+   Interface:
+      void dma_buf_unmap_attachment(struct dma_buf_attachment *,
+                                    struct sg_table *);
+
+   This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the
+   "dma_buf->ops->" indirection from the users of this interface.
+
+   In struct dma_buf_ops, unmap_dma_buf is defined as
+      void (*unmap_dma_buf)(struct dma_buf_attachment *, struct sg_table *);
+
+   unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like
+   map_dma_buf, this API also must be implemented by the exporter.
+
+
+6. when buffer-user is done using this buffer, it 'disconnects' itself from the
+   buffer.
+
+   After the buffer-user has no more interest in using this buffer, it should
+   disconnect itself from the buffer:
+
+   - it first detaches itself from the buffer.
+
+   Interface:
+      void dma_buf_detach(struct dma_buf *dmabuf,
+                          struct dma_buf_attachment *dmabuf_attach);
+
+   This API removes the attachment from the list in dmabuf, and optionally calls
+   dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits.
+
+   - Then, the buffer-user returns the buffer reference to exporter.
+
+   Interface:
+     void dma_buf_put(struct dma_buf *dmabuf);
+
+   This API then reduces the refcount for this buffer.
+
+   If, as a result of this call, the refcount becomes 0, the 'release' file
+   operation related to this fd is called. It calls the dmabuf->ops->release()
+   operation in turn, and frees the memory allocated for dmabuf when exported.
+
+NOTES:
+- Importance of attach-detach and {map,unmap}_dma_buf operation pairs
+   The attach-detach calls allow the exporter to figure out backing-storage
+   constraints for the currently-interested devices. This allows preferential
+   allocation, and/or migration of pages across different types of storage
+   available, if possible.
+
+   Bracketing of DMA access with {map,unmap}_dma_buf operations is essential
+   to allow just-in-time backing of storage, and migration mid-way through a
+   use-case.
+
+- Migration of backing storage if needed
+   If after
+   - at least one map_dma_buf has happened,
+   - and the backing storage has been allocated for this buffer,
+   another new buffer-user intends to attach itself to this buffer, it might
+   be allowed, if possible for the exporter.
+
+   In case it is allowed by the exporter:
+    if the new buffer-user has stricter 'backing-storage constraints', and the
+    exporter can handle these constraints, the exporter can just stall on the
+    map_dma_buf until all outstanding access is completed (as signalled by
+    unmap_dma_buf).
+    Once all users have finished accessing and have unmapped this buffer, the
+    exporter could potentially move the buffer to the stricter backing-storage,
+    and then allow further {map,unmap}_dma_buf operations from any buffer-user
+    from the migrated backing-storage.
+
+   If the exporter cannot fulfil the backing-storage constraints of the new
+   buffer-user device as requested, dma_buf_attach() would return an error to
+   denote non-compatibility of the new buffer-sharing request with the current
+   buffer.
+
+   If the exporter chooses not to allow an attach() operation once a
+   map_dma_buf() API has been called, it simply returns an error.
+
+References:
+[1] struct dma_buf_ops in include/linux/dma-buf.h
+[2] All interfaces mentioned above defined in include/linux/dma-buf.h
-- 
1.7.5.4

[PATCH 3/3] dma-buf: mark EXPERIMENTAL for 1st release.

[Sumit Semwal]

Mark dma-buf buffer sharing API as EXPERIMENTAL for first release.
We will remove this in later versions, once it gets smoothed out
and has more users.

Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Sumit Semwal <sumit.semwal@ti.com>
---
 drivers/base/Kconfig |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
index 8a0e87f..e95c67e 100644
--- a/drivers/base/Kconfig
+++ b/drivers/base/Kconfig
@@ -178,6 +178,7 @@ config DMA_SHARED_BUFFER
 	bool "Buffer framework to be shared between drivers"
 	default n
 	select ANON_INODES
+	depends on EXPERIMENTAL
 	help
 	  This option enables the framework for buffer-sharing between
 	  multiple drivers. A buffer is associated with a file using driver
-- 
1.7.5.4

Re: [PATCH 1/3] dma-buf: Introduce dma buffer sharing mechanism

[InKi Dae]

2011/12/26 Sumit Semwal <sumit.semwal@ti.com>:
> 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>
> ---
>  drivers/base/Kconfig    |   10 ++
>  drivers/base/Makefile   |    1 +
>  drivers/base/dma-buf.c  |  291 +++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/dma-buf.h |  176 ++++++++++++++++++++++++++++
>  4 files changed, 478 insertions(+), 0 deletions(-)
>  create mode 100644 drivers/base/dma-buf.c
>  create mode 100644 include/linux/dma-buf.h
>
> diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
> index 21cf46f..8a0e87f 100644
> --- a/drivers/base/Kconfig
> +++ b/drivers/base/Kconfig
> @@ -174,4 +174,14 @@ config SYS_HYPERVISOR
>
>  source "drivers/base/regmap/Kconfig"
>
> +config DMA_SHARED_BUFFER
> +       bool "Buffer framework to be shared between drivers"
> +       default n
> +       select ANON_INODES
> +       help
> +         This option enables the framework for buffer-sharing between
> +         multiple drivers. A buffer is associated with a file using driver
> +         APIs extension; the file's descriptor can then be passed on to other
> +         driver.
> +
>  endmenu
> diff --git a/drivers/base/Makefile b/drivers/base/Makefile
> index 99a375a..d0df046 100644
> --- a/drivers/base/Makefile
> +++ b/drivers/base/Makefile
> @@ -8,6 +8,7 @@ obj-$(CONFIG_DEVTMPFS)  += devtmpfs.o
>  obj-y                  += power/
>  obj-$(CONFIG_HAS_DMA)  += dma-mapping.o
>  obj-$(CONFIG_HAVE_GENERIC_DMA_COHERENT) += dma-coherent.o
> +obj-$(CONFIG_DMA_SHARED_BUFFER) += dma-buf.o
>  obj-$(CONFIG_ISA)      += isa.o
>  obj-$(CONFIG_FW_LOADER)        += firmware_class.o
>  obj-$(CONFIG_NUMA)     += node.o
> diff --git a/drivers/base/dma-buf.c b/drivers/base/dma-buf.c
> new file mode 100644
> index 0000000..e38ad24
> --- /dev/null
> +++ b/drivers/base/dma-buf.c
> @@ -0,0 +1,291 @@
> +/*
> + * Framework for buffer objects that can be shared across devices/subsystems.
> + *
> + * 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/>.
> + */
> +
> +#include <linux/fs.h>
> +#include <linux/slab.h>
> +#include <linux/dma-buf.h>
> +#include <linux/anon_inodes.h>
> +#include <linux/export.h>
> +
> +static inline int is_dma_buf_file(struct file *);
> +
> +static int dma_buf_release(struct inode *inode, struct file *file)
> +{
> +       struct dma_buf *dmabuf;
> +
> +       if (!is_dma_buf_file(file))
> +               return -EINVAL;
> +
> +       dmabuf = file->private_data;
> +
> +       dmabuf->ops->release(dmabuf);
> +       kfree(dmabuf);
> +       return 0;
> +}
> +
> +static const struct file_operations dma_buf_fops = {
> +       .release        = dma_buf_release,
> +};
> +
> +/*
> + * is_dma_buf_file - Check if struct file* is associated with dma_buf
> + */
> +static inline int is_dma_buf_file(struct file *file)
> +{
> +       return file->f_op == &dma_buf_fops;
> +}
> +
> +/**
> + * dma_buf_export - Creates a new dma_buf, and associates an anon file
> + * with this buffer, so it can be exported.
> + * Also connect the allocator specific data and ops to the buffer.
> + *
> + * @priv:      [in]    Attach private data of allocator to this buffer
> + * @ops:       [in]    Attach allocator-defined dma buf ops to the new buffer.
> + * @size:      [in]    Size of the buffer
> + * @flags:     [in]    mode flags for the file.
> + *
> + * Returns, on success, a newly created dma_buf object, which wraps the
> + * supplied private data and operations for dma_buf_ops. On either missing
> + * ops, or error in allocating struct dma_buf, will return negative error.
> + *
> + */
> +struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops,
> +                               size_t size, int flags)
> +{
> +       struct dma_buf *dmabuf;
> +       struct file *file;
> +
> +       if (WARN_ON(!priv || !ops
> +                         || !ops->map_dma_buf
> +                         || !ops->unmap_dma_buf
> +                         || !ops->release)) {
> +               return ERR_PTR(-EINVAL);
> +       }
> +
> +       dmabuf = kzalloc(sizeof(struct dma_buf), GFP_KERNEL);
> +       if (dmabuf == NULL)
> +               return ERR_PTR(-ENOMEM);
> +
> +       dmabuf->priv = priv;
> +       dmabuf->ops = ops;
> +       dmabuf->size = size;
> +
> +       file = anon_inode_getfile("dmabuf", &dma_buf_fops, dmabuf, flags);
> +
> +       dmabuf->file = file;
> +
> +       mutex_init(&dmabuf->lock);
> +       INIT_LIST_HEAD(&dmabuf->attachments);
> +
> +       return dmabuf;
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_export);
> +
> +
> +/**
> + * dma_buf_fd - returns a file descriptor for the given dma_buf
> + * @dmabuf:    [in]    pointer to dma_buf for which fd is required.
> + *
> + * On success, returns an associated 'fd'. Else, returns error.
> + */
> +int dma_buf_fd(struct dma_buf *dmabuf)
> +{
> +       int error, fd;
> +
> +       if (!dmabuf || !dmabuf->file)
> +               return -EINVAL;
> +
> +       error = get_unused_fd();
> +       if (error < 0)
> +               return error;
> +       fd = error;
> +
> +       fd_install(fd, dmabuf->file);
> +
> +       return fd;
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_fd);
> +
> +/**
> + * dma_buf_get - returns the dma_buf structure related to an fd
> + * @fd:        [in]    fd associated with the dma_buf to be returned
> + *
> + * On success, returns the dma_buf structure associated with an fd; uses
> + * file's refcounting done by fget to increase refcount. returns ERR_PTR
> + * otherwise.
> + */
> +struct dma_buf *dma_buf_get(int fd)
> +{
> +       struct file *file;
> +
> +       file = fget(fd);
> +
> +       if (!file)
> +               return ERR_PTR(-EBADF);
> +
> +       if (!is_dma_buf_file(file)) {
> +               fput(file);
> +               return ERR_PTR(-EINVAL);
> +       }
> +
> +       return file->private_data;
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_get);
> +
> +/**
> + * dma_buf_put - decreases refcount of the buffer
> + * @dmabuf:    [in]    buffer to reduce refcount of
> + *
> + * Uses file's refcounting done implicitly by fput()
> + */
> +void dma_buf_put(struct dma_buf *dmabuf)
> +{
> +       if (WARN_ON(!dmabuf || !dmabuf->file))
> +               return;
> +
> +       fput(dmabuf->file);
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_put);
> +
> +/**
> + * dma_buf_attach - Add the device to dma_buf's attachments list; optionally,
> + * calls attach() of dma_buf_ops to allow device-specific attach functionality
> + * @dmabuf:    [in]    buffer to attach device to.
> + * @dev:       [in]    device to be attached.
> + *
> + * Returns struct dma_buf_attachment * for this attachment; may return negative
> + * error codes.
> + *
> + */
> +struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
> +                                         struct device *dev)
> +{
> +       struct dma_buf_attachment *attach;
> +       int ret;
> +
> +       if (WARN_ON(!dmabuf || !dev || !dmabuf->ops))
> +               return ERR_PTR(-EINVAL);
> +
> +       attach = kzalloc(sizeof(struct dma_buf_attachment), GFP_KERNEL);
> +       if (attach == NULL)
> +               goto err_alloc;
> +
> +       mutex_lock(&dmabuf->lock);
> +
> +       attach->dev = dev;
> +       attach->dmabuf = dmabuf;
> +       if (dmabuf->ops->attach) {
> +               ret = dmabuf->ops->attach(dmabuf, dev, attach);
> +               if (ret)
> +                       goto err_attach;
> +       }
> +       list_add(&attach->node, &dmabuf->attachments);
> +
> +       mutex_unlock(&dmabuf->lock);
> +       return attach;
> +
> +err_alloc:
> +       return ERR_PTR(-ENOMEM);
> +err_attach:
> +       kfree(attach);
> +       mutex_unlock(&dmabuf->lock);
> +       return ERR_PTR(ret);
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_attach);
> +
> +/**
> + * dma_buf_detach - Remove the given attachment from dmabuf's attachments list;
> + * optionally calls detach() of dma_buf_ops for device-specific detach
> + * @dmabuf:    [in]    buffer to detach from.
> + * @attach:    [in]    attachment to be detached; is free'd after this call.
> + *
> + */
> +void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)
> +{
> +       if (WARN_ON(!dmabuf || !attach || !dmabuf->ops))
> +               return;
> +
> +       mutex_lock(&dmabuf->lock);
> +       list_del(&attach->node);
> +       if (dmabuf->ops->detach)
> +               dmabuf->ops->detach(dmabuf, attach);
> +
> +       mutex_unlock(&dmabuf->lock);
> +       kfree(attach);
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_detach);
> +
> +/**
> + * dma_buf_map_attachment - Returns the scatterlist table of the attachment;
> + * mapped into _device_ address space. Is a wrapper for map_dma_buf() of the
> + * dma_buf_ops.
> + * @attach:    [in]    attachment whose scatterlist is to be returned
> + * @direction: [in]    direction of DMA transfer
> + *
> + * Returns sg_table containing the scatterlist to be returned; may return NULL
> + * or ERR_PTR.
> + *
> + */
> +struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
> +                                       enum dma_data_direction direction)
> +{
> +       struct sg_table *sg_table = ERR_PTR(-EINVAL);
> +
> +       might_sleep();
> +
> +       if (WARN_ON(!attach || !attach->dmabuf || !attach->dmabuf->ops))
> +               return ERR_PTR(-EINVAL);
> +
> +       mutex_lock(&attach->dmabuf->lock);
> +       if (attach->dmabuf->ops->map_dma_buf)
> +               sg_table = attach->dmabuf->ops->map_dma_buf(attach, direction);
> +       mutex_unlock(&attach->dmabuf->lock);
> +
> +       return sg_table;
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_map_attachment);
> +
> +/**
> + * dma_buf_unmap_attachment - unmaps and decreases usecount of the buffer;might
> + * deallocate the scatterlist associated. Is a wrapper for unmap_dma_buf() of
> + * dma_buf_ops.
> + * @attach:    [in]    attachment to unmap buffer from
> + * @sg_table:  [in]    scatterlist info of the buffer to unmap
> + *
> + */
> +void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,
> +                               struct sg_table *sg_table)
> +{
> +       if (WARN_ON(!attach || !attach->dmabuf || !sg_table
> +                           || !attach->dmabuf->ops))
> +               return;
> +
> +       mutex_lock(&attach->dmabuf->lock);
> +       if (attach->dmabuf->ops->unmap_dma_buf)
> +               attach->dmabuf->ops->unmap_dma_buf(attach, sg_table);
> +       mutex_unlock(&attach->dmabuf->lock);
> +
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_unmap_attachment);
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> new file mode 100644
> index 0000000..f8ac076
> --- /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;

how about having sg_table here also? like this, struct sg_table *sgt;
now dmabuf->priv could have duplicated values.
I know that a private buffer object is set to dmabuf->priv when
dma_buf_export function is called. but when  xxx_acquire_dmabuf
callback of vb2 side is called dmabuf->priv would have sg_table object
returned from dmabuf->ops->map_dma_buf() again to release the sg_table
with dmabuf->ops->unmap_dma_buf(). I'm not sure that this situation
could induce some problem but anyway confusing.

> +       /* 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__ */
> --
> 1.7.5.4
>
> --
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> see: http://www.linux-mm.org/ .
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Re: [PATCH 2/3] dma-buf: Documentation for buffer sharing framework

[Sakari Ailus]

Hi Sumit and Arnd,

On Mon, Dec 26, 2011 at 02:53:16PM +0530, Sumit Semwal wrote:
> Add documentation for dma buffer sharing framework, explaining the
> various operations, members and API of the dma buffer sharing
> framework.
> 
> 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>
> ---
>  Documentation/dma-buf-sharing.txt |  224 +++++++++++++++++++++++++++++++++++++
>  1 files changed, 224 insertions(+), 0 deletions(-)
>  create mode 100644 Documentation/dma-buf-sharing.txt
> 
> diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
> new file mode 100644
> index 0000000..510eab3
> --- /dev/null
> +++ b/Documentation/dma-buf-sharing.txt
> @@ -0,0 +1,224 @@
> +                    DMA Buffer Sharing API Guide
> +                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +                            Sumit Semwal
> +                <sumit dot semwal at linaro dot org>
> +                 <sumit dot semwal at ti dot com>
> +
> +This document serves as a guide to device-driver writers on what is the dma-buf
> +buffer sharing API, how to use it for exporting and using shared buffers.
> +
> +Any device driver which wishes to be a part of DMA buffer sharing, can do so as
> +either the 'exporter' of buffers, or the 'user' of buffers.
> +
> +Say a driver A wants to use buffers created by driver B, then we call B as the
> +exporter, and A as buffer-user.
> +
> +The exporter
> +- implements and manages operations[1] for the buffer
> +- allows other users to share the buffer by using dma_buf sharing APIs,
> +- manages the details of buffer allocation,
> +- decides about the actual backing storage where this allocation happens,
> +- takes care of any migration of scatterlist - for all (shared) users of this
> +   buffer,
> +
> +The buffer-user
> +- is one of (many) sharing users of the buffer.
> +- doesn't need to worry about how the buffer is allocated, or where.
> +- needs a mechanism to get access to the scatterlist that makes up this buffer
> +   in memory, mapped into its own address space, so it can access the same area
> +   of memory.
> +
> +*IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details]
> +For this first version, A buffer shared using the dma_buf sharing API:
> +- *may* be exported to user space using "mmap" *ONLY* by exporter, outside of
> +   this framework.
> +- may be used *ONLY* by importers that do not need CPU access to the buffer.
> +
> +The dma_buf buffer sharing API usage contains the following steps:
> +
> +1. Exporter announces that it wishes to export a buffer
> +2. Userspace gets the file descriptor associated with the exported buffer, and
> +   passes it around to potential buffer-users based on use case
> +3. Each buffer-user 'connects' itself to the buffer
> +4. When needed, buffer-user requests access to the buffer from exporter
> +5. When finished with its use, the buffer-user notifies end-of-DMA to exporter
> +6. when buffer-user is done using this buffer completely, it 'disconnects'
> +   itself from the buffer.
> +
> +
> +1. Exporter's announcement of buffer export
> +
> +   The buffer exporter announces its wish to export a buffer. In this, it
> +   connects its own private buffer data, provides implementation for operations
> +   that can be performed on the exported dma_buf, and flags for the file
> +   associated with this buffer.
> +
> +   Interface:
> +      struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops,
> +				     size_t size, int flags)
> +
> +   If this succeeds, dma_buf_export allocates a dma_buf structure, and returns a
> +   pointer to the same. It also associates an anonymous file with this buffer,
> +   so it can be exported. On failure to allocate the dma_buf object, it returns
> +   NULL.
> +
> +2. Userspace gets a handle to pass around to potential buffer-users
> +
> +   Userspace entity requests for a file-descriptor (fd) which is a handle to the
> +   anonymous file associated with the buffer. It can then share the fd with other
> +   drivers and/or processes.
> +
> +   Interface:
> +      int dma_buf_fd(struct dma_buf *dmabuf)
> +
> +   This API installs an fd for the anonymous file associated with this buffer;
> +   returns either 'fd', or error.
> +
> +3. Each buffer-user 'connects' itself to the buffer
> +
> +   Each buffer-user now gets a reference to the buffer, using the fd passed to
> +   it.
> +
> +   Interface:
> +      struct dma_buf *dma_buf_get(int fd)
> +
> +   This API will return a reference to the dma_buf, and increment refcount for
> +   it.
> +
> +   After this, the buffer-user needs to attach its device with the buffer, which
> +   helps the exporter to know of device buffer constraints.
> +
> +   Interface:
> +      struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
> +                                                struct device *dev)
> +
> +   This API returns reference to an attachment structure, which is then used
> +   for scatterlist operations. It will optionally call the 'attach' dma_buf
> +   operation, if provided by the exporter.
> +
> +   The dma-buf sharing framework does the bookkeeping bits related to managing
> +   the list of all attachments to a buffer.
> +
> +Until this stage, the buffer-exporter has the option to choose not to actually
> +allocate the backing storage for this buffer, but wait for the first buffer-user
> +to request use of buffer for allocation.
> +
> +
> +4. When needed, buffer-user requests access to the buffer
> +
> +   Whenever a buffer-user wants to use the buffer for any DMA, it asks for
> +   access to the buffer using dma_buf_map_attachment API. At least one attach to
> +   the buffer must have happened before map_dma_buf can be called.
> +
> +   Interface:
> +      struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *,
> +                                         enum dma_data_direction);
> +
> +   This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the
> +   "dma_buf->ops->" indirection from the users of this interface.
> +
> +   In struct dma_buf_ops, map_dma_buf is defined as
> +      struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *,
> +                                                enum dma_data_direction);
> +
> +   It is one of the buffer operations that must be implemented by the exporter.
> +   It should return the sg_table containing scatterlist for this buffer, mapped
> +   into caller's address space.
> +
> +   If this is being called for the first time, the exporter can now choose to
> +   scan through the list of attachments for this buffer, collate the requirements
> +   of the attached devices, and choose an appropriate backing storage for the
> +   buffer.
> +
> +   Based on enum dma_data_direction, it might be possible to have multiple users
> +   accessing at the same time (for reading, maybe), or any other kind of sharing
> +   that the exporter might wish to make available to buffer-users.
> +
> +   map_dma_buf() operation can return -EINTR if it is interrupted by a signal.
> +
> +
> +5. When finished, the buffer-user notifies end-of-DMA to exporter
> +
> +   Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to
> +   the exporter using the dma_buf_unmap_attachment API.
> +
> +   Interface:
> +      void dma_buf_unmap_attachment(struct dma_buf_attachment *,
> +                                    struct sg_table *);
> +
> +   This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the
> +   "dma_buf->ops->" indirection from the users of this interface.
> +
> +   In struct dma_buf_ops, unmap_dma_buf is defined as
> +      void (*unmap_dma_buf)(struct dma_buf_attachment *, struct sg_table *);
> +
> +   unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like
> +   map_dma_buf, this API also must be implemented by the exporter.

How is this API expected to be used with user space APIs which use
V4L2-style queueing of the buffers, i.e. several hardware devices may have a
single buffer mapped at any given point of time and the user is responsible
for passing the buffer for processing between hardware devices?

In that case also cache handling would need to be performed explicitly by
drivers --- the V4L2 API already provides a way to tell drivers to skip
cache cleaning or invalidation if the user does not intend to touch the
buffer between passing it between two separate devices.

> +
> +6. when buffer-user is done using this buffer, it 'disconnects' itself from the
> +   buffer.
> +
> +   After the buffer-user has no more interest in using this buffer, it should
> +   disconnect itself from the buffer:
> +
> +   - it first detaches itself from the buffer.
> +
> +   Interface:
> +      void dma_buf_detach(struct dma_buf *dmabuf,
> +                          struct dma_buf_attachment *dmabuf_attach);
> +
> +   This API removes the attachment from the list in dmabuf, and optionally calls
> +   dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits.
> +
> +   - Then, the buffer-user returns the buffer reference to exporter.
> +
> +   Interface:
> +     void dma_buf_put(struct dma_buf *dmabuf);
> +
> +   This API then reduces the refcount for this buffer.
> +
> +   If, as a result of this call, the refcount becomes 0, the 'release' file
> +   operation related to this fd is called. It calls the dmabuf->ops->release()
> +   operation in turn, and frees the memory allocated for dmabuf when exported.
> +
> +NOTES:
> +- Importance of attach-detach and {map,unmap}_dma_buf operation pairs
> +   The attach-detach calls allow the exporter to figure out backing-storage
> +   constraints for the currently-interested devices. This allows preferential
> +   allocation, and/or migration of pages across different types of storage
> +   available, if possible.
> +
> +   Bracketing of DMA access with {map,unmap}_dma_buf operations is essential
> +   to allow just-in-time backing of storage, and migration mid-way through a
> +   use-case.
> +
> +- Migration of backing storage if needed
> +   If after
> +   - at least one map_dma_buf has happened,
> +   - and the backing storage has been allocated for this buffer,
> +   another new buffer-user intends to attach itself to this buffer, it might
> +   be allowed, if possible for the exporter.
> +
> +   In case it is allowed by the exporter:
> +    if the new buffer-user has stricter 'backing-storage constraints', and the
> +    exporter can handle these constraints, the exporter can just stall on the
> +    map_dma_buf until all outstanding access is completed (as signalled by
> +    unmap_dma_buf).

I would expect this to take place in V4L2 context when streaming is
disabled; it would make sense to return EBUSY instead since it's not known
when the unmapping will be done.

> +    Once all users have finished accessing and have unmapped this buffer, the
> +    exporter could potentially move the buffer to the stricter backing-storage,
> +    and then allow further {map,unmap}_dma_buf operations from any buffer-user
> +    from the migrated backing-storage.
> +
> +   If the exporter cannot fulfil the backing-storage constraints of the new
> +   buffer-user device as requested, dma_buf_attach() would return an error to
> +   denote non-compatibility of the new buffer-sharing request with the current
> +   buffer.
> +
> +   If the exporter chooses not to allow an attach() operation once a
> +   map_dma_buf() API has been called, it simply returns an error.
> +
> +References:
> +[1] struct dma_buf_ops in include/linux/dma-buf.h
> +[2] All interfaces mentioned above defined in include/linux/dma-buf.h

Kind regards,

-- 
Sakari Ailus
e-mail: sakari.ailus@iki.fi	jabber/XMPP/Gmail: sailus@retiisi.org.uk

Re: [PATCH 2/3] dma-buf: Documentation for buffer sharing framework

[Rob Clark]

On Sun, Jan 1, 2012 at 2:09 PM, Sakari Ailus <sakari.ailus@iki.fi> wrote:
> Hi Sumit and Arnd,
>
> On Mon, Dec 26, 2011 at 02:53:16PM +0530, Sumit Semwal wrote:
>> Add documentation for dma buffer sharing framework, explaining the
>> various operations, members and API of the dma buffer sharing
>> framework.
>>
>> 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>
>> ---
>>  Documentation/dma-buf-sharing.txt |  224 +++++++++++++++++++++++++++++++++++++
>>  1 files changed, 224 insertions(+), 0 deletions(-)
>>  create mode 100644 Documentation/dma-buf-sharing.txt
>>
>> diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
>> new file mode 100644
>> index 0000000..510eab3
>> --- /dev/null
>> +++ b/Documentation/dma-buf-sharing.txt
>> @@ -0,0 +1,224 @@
>> +                    DMA Buffer Sharing API Guide
>> +                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> +
>> +                            Sumit Semwal
>> +                <sumit dot semwal at linaro dot org>
>> +                 <sumit dot semwal at ti dot com>
>> +
>> +This document serves as a guide to device-driver writers on what is the dma-buf
>> +buffer sharing API, how to use it for exporting and using shared buffers.
>> +
>> +Any device driver which wishes to be a part of DMA buffer sharing, can do so as
>> +either the 'exporter' of buffers, or the 'user' of buffers.
>> +
>> +Say a driver A wants to use buffers created by driver B, then we call B as the
>> +exporter, and A as buffer-user.
>> +
>> +The exporter
>> +- implements and manages operations[1] for the buffer
>> +- allows other users to share the buffer by using dma_buf sharing APIs,
>> +- manages the details of buffer allocation,
>> +- decides about the actual backing storage where this allocation happens,
>> +- takes care of any migration of scatterlist - for all (shared) users of this
>> +   buffer,
>> +
>> +The buffer-user
>> +- is one of (many) sharing users of the buffer.
>> +- doesn't need to worry about how the buffer is allocated, or where.
>> +- needs a mechanism to get access to the scatterlist that makes up this buffer
>> +   in memory, mapped into its own address space, so it can access the same area
>> +   of memory.
>> +
>> +*IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details]
>> +For this first version, A buffer shared using the dma_buf sharing API:
>> +- *may* be exported to user space using "mmap" *ONLY* by exporter, outside of
>> +   this framework.
>> +- may be used *ONLY* by importers that do not need CPU access to the buffer.
>> +
>> +The dma_buf buffer sharing API usage contains the following steps:
>> +
>> +1. Exporter announces that it wishes to export a buffer
>> +2. Userspace gets the file descriptor associated with the exported buffer, and
>> +   passes it around to potential buffer-users based on use case
>> +3. Each buffer-user 'connects' itself to the buffer
>> +4. When needed, buffer-user requests access to the buffer from exporter
>> +5. When finished with its use, the buffer-user notifies end-of-DMA to exporter
>> +6. when buffer-user is done using this buffer completely, it 'disconnects'
>> +   itself from the buffer.
>> +
>> +
>> +1. Exporter's announcement of buffer export
>> +
>> +   The buffer exporter announces its wish to export a buffer. In this, it
>> +   connects its own private buffer data, provides implementation for operations
>> +   that can be performed on the exported dma_buf, and flags for the file
>> +   associated with this buffer.
>> +
>> +   Interface:
>> +      struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops,
>> +                                  size_t size, int flags)
>> +
>> +   If this succeeds, dma_buf_export allocates a dma_buf structure, and returns a
>> +   pointer to the same. It also associates an anonymous file with this buffer,
>> +   so it can be exported. On failure to allocate the dma_buf object, it returns
>> +   NULL.
>> +
>> +2. Userspace gets a handle to pass around to potential buffer-users
>> +
>> +   Userspace entity requests for a file-descriptor (fd) which is a handle to the
>> +   anonymous file associated with the buffer. It can then share the fd with other
>> +   drivers and/or processes.
>> +
>> +   Interface:
>> +      int dma_buf_fd(struct dma_buf *dmabuf)
>> +
>> +   This API installs an fd for the anonymous file associated with this buffer;
>> +   returns either 'fd', or error.
>> +
>> +3. Each buffer-user 'connects' itself to the buffer
>> +
>> +   Each buffer-user now gets a reference to the buffer, using the fd passed to
>> +   it.
>> +
>> +   Interface:
>> +      struct dma_buf *dma_buf_get(int fd)
>> +
>> +   This API will return a reference to the dma_buf, and increment refcount for
>> +   it.
>> +
>> +   After this, the buffer-user needs to attach its device with the buffer, which
>> +   helps the exporter to know of device buffer constraints.
>> +
>> +   Interface:
>> +      struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
>> +                                                struct device *dev)
>> +
>> +   This API returns reference to an attachment structure, which is then used
>> +   for scatterlist operations. It will optionally call the 'attach' dma_buf
>> +   operation, if provided by the exporter.
>> +
>> +   The dma-buf sharing framework does the bookkeeping bits related to managing
>> +   the list of all attachments to a buffer.
>> +
>> +Until this stage, the buffer-exporter has the option to choose not to actually
>> +allocate the backing storage for this buffer, but wait for the first buffer-user
>> +to request use of buffer for allocation.
>> +
>> +
>> +4. When needed, buffer-user requests access to the buffer
>> +
>> +   Whenever a buffer-user wants to use the buffer for any DMA, it asks for
>> +   access to the buffer using dma_buf_map_attachment API. At least one attach to
>> +   the buffer must have happened before map_dma_buf can be called.
>> +
>> +   Interface:
>> +      struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *,
>> +                                         enum dma_data_direction);
>> +
>> +   This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the
>> +   "dma_buf->ops->" indirection from the users of this interface.
>> +
>> +   In struct dma_buf_ops, map_dma_buf is defined as
>> +      struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *,
>> +                                                enum dma_data_direction);
>> +
>> +   It is one of the buffer operations that must be implemented by the exporter.
>> +   It should return the sg_table containing scatterlist for this buffer, mapped
>> +   into caller's address space.
>> +
>> +   If this is being called for the first time, the exporter can now choose to
>> +   scan through the list of attachments for this buffer, collate the requirements
>> +   of the attached devices, and choose an appropriate backing storage for the
>> +   buffer.
>> +
>> +   Based on enum dma_data_direction, it might be possible to have multiple users
>> +   accessing at the same time (for reading, maybe), or any other kind of sharing
>> +   that the exporter might wish to make available to buffer-users.
>> +
>> +   map_dma_buf() operation can return -EINTR if it is interrupted by a signal.
>> +
>> +
>> +5. When finished, the buffer-user notifies end-of-DMA to exporter
>> +
>> +   Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to
>> +   the exporter using the dma_buf_unmap_attachment API.
>> +
>> +   Interface:
>> +      void dma_buf_unmap_attachment(struct dma_buf_attachment *,
>> +                                    struct sg_table *);
>> +
>> +   This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the
>> +   "dma_buf->ops->" indirection from the users of this interface.
>> +
>> +   In struct dma_buf_ops, unmap_dma_buf is defined as
>> +      void (*unmap_dma_buf)(struct dma_buf_attachment *, struct sg_table *);
>> +
>> +   unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like
>> +   map_dma_buf, this API also must be implemented by the exporter.
>
> How is this API expected to be used with user space APIs which use
> V4L2-style queueing of the buffers, i.e. several hardware devices may have a
> single buffer mapped at any given point of time and the user is responsible
> for passing the buffer for processing between hardware devices?

The intention is that the v4l2 device would attach in when the dmabuf
descriptor is first seen, and then on subsequent QBUF/DQBUF (or maybe
just before/after DMA to/from buffer) would map/unmap.  It would be
the responsibility of the exporter to cache the mapping if appropriate
between map/unmap calls.  The importer should not care about this.

Have a look at https://github.com/robclark/kernel-omap4/commits/drmplane-dmabuf
(or I think sumit has some updated patches for vb2) for an example.

BR,
-R

> In that case also cache handling would need to be performed explicitly by
> drivers --- the V4L2 API already provides a way to tell drivers to skip
> cache cleaning or invalidation if the user does not intend to touch the
> buffer between passing it between two separate devices.
>
>> +
>> +6. when buffer-user is done using this buffer, it 'disconnects' itself from the
>> +   buffer.
>> +
>> +   After the buffer-user has no more interest in using this buffer, it should
>> +   disconnect itself from the buffer:
>> +
>> +   - it first detaches itself from the buffer.
>> +
>> +   Interface:
>> +      void dma_buf_detach(struct dma_buf *dmabuf,
>> +                          struct dma_buf_attachment *dmabuf_attach);
>> +
>> +   This API removes the attachment from the list in dmabuf, and optionally calls
>> +   dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits.
>> +
>> +   - Then, the buffer-user returns the buffer reference to exporter.
>> +
>> +   Interface:
>> +     void dma_buf_put(struct dma_buf *dmabuf);
>> +
>> +   This API then reduces the refcount for this buffer.
>> +
>> +   If, as a result of this call, the refcount becomes 0, the 'release' file
>> +   operation related to this fd is called. It calls the dmabuf->ops->release()
>> +   operation in turn, and frees the memory allocated for dmabuf when exported.
>> +
>> +NOTES:
>> +- Importance of attach-detach and {map,unmap}_dma_buf operation pairs
>> +   The attach-detach calls allow the exporter to figure out backing-storage
>> +   constraints for the currently-interested devices. This allows preferential
>> +   allocation, and/or migration of pages across different types of storage
>> +   available, if possible.
>> +
>> +   Bracketing of DMA access with {map,unmap}_dma_buf operations is essential
>> +   to allow just-in-time backing of storage, and migration mid-way through a
>> +   use-case.
>> +
>> +- Migration of backing storage if needed
>> +   If after
>> +   - at least one map_dma_buf has happened,
>> +   - and the backing storage has been allocated for this buffer,
>> +   another new buffer-user intends to attach itself to this buffer, it might
>> +   be allowed, if possible for the exporter.
>> +
>> +   In case it is allowed by the exporter:
>> +    if the new buffer-user has stricter 'backing-storage constraints', and the
>> +    exporter can handle these constraints, the exporter can just stall on the
>> +    map_dma_buf until all outstanding access is completed (as signalled by
>> +    unmap_dma_buf).
>
> I would expect this to take place in V4L2 context when streaming is
> disabled; it would make sense to return EBUSY instead since it's not known
> when the unmapping will be done.
>
>> +    Once all users have finished accessing and have unmapped this buffer, the
>> +    exporter could potentially move the buffer to the stricter backing-storage,
>> +    and then allow further {map,unmap}_dma_buf operations from any buffer-user
>> +    from the migrated backing-storage.
>> +
>> +   If the exporter cannot fulfil the backing-storage constraints of the new
>> +   buffer-user device as requested, dma_buf_attach() would return an error to
>> +   denote non-compatibility of the new buffer-sharing request with the current
>> +   buffer.
>> +
>> +   If the exporter chooses not to allow an attach() operation once a
>> +   map_dma_buf() API has been called, it simply returns an error.
>> +
>> +References:
>> +[1] struct dma_buf_ops in include/linux/dma-buf.h
>> +[2] All interfaces mentioned above defined in include/linux/dma-buf.h
>
> Kind regards,
>
> --
> Sakari Ailus
> e-mail: sakari.ailus@iki.fi     jabber/XMPP/Gmail: sailus@retiisi.org.uk
> --
> To unsubscribe from this list: send the line "unsubscribe linux-media" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: [Linaro-mm-sig] [PATCH 1/3] dma-buf: Introduce dma buffer sharing mechanism

[Laurent Pinchart]

Hi Summit,

Sorry for the late review. I know that this code is now in mainline, but I 
still have a couple of comments. I'll send patches if you agree with them.

On Monday 26 December 2011 10:23:15 Sumit Semwal wrote:
> 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>
> ---
>  drivers/base/Kconfig    |   10 ++
>  drivers/base/Makefile   |    1 +
>  drivers/base/dma-buf.c  |  291
> +++++++++++++++++++++++++++++++++++++++++++++++ include/linux/dma-buf.h | 
> 176 ++++++++++++++++++++++++++++
>  4 files changed, 478 insertions(+), 0 deletions(-)
>  create mode 100644 drivers/base/dma-buf.c
>  create mode 100644 include/linux/dma-buf.h
> 

[snip]

> diff --git a/drivers/base/dma-buf.c b/drivers/base/dma-buf.c
> new file mode 100644
> index 0000000..e38ad24
> --- /dev/null
> +++ b/drivers/base/dma-buf.c
> @@ -0,0 +1,291 @@

[snip]

> +/**
> + * dma_buf_export - Creates a new dma_buf, and associates an anon file
> + * with this buffer, so it can be exported.
> + * Also connect the allocator specific data and ops to the buffer.
> + *
> + * @priv:	[in]	Attach private data of allocator to this buffer
> + * @ops:	[in]	Attach allocator-defined dma buf ops to the new buffer.
> + * @size:	[in]	Size of the buffer
> + * @flags:	[in]	mode flags for the file.
> + *
> + * Returns, on success, a newly created dma_buf object, which wraps the
> + * supplied private data and operations for dma_buf_ops. On either missing
> + * ops, or error in allocating struct dma_buf, will return negative error.
> + *
> + */
> +struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops,
> +				size_t size, int flags)
> +{
> +	struct dma_buf *dmabuf;
> +	struct file *file;
> +
> +	if (WARN_ON(!priv || !ops
> +			  || !ops->map_dma_buf
> +			  || !ops->unmap_dma_buf
> +			  || !ops->release)) {
> +		return ERR_PTR(-EINVAL);
> +	}
> +
> +	dmabuf = kzalloc(sizeof(struct dma_buf), GFP_KERNEL);
> +	if (dmabuf == NULL)
> +		return ERR_PTR(-ENOMEM);
> +
> +	dmabuf->priv = priv;
> +	dmabuf->ops = ops;

dmabuf->ops will never but NULL, but (see below)

> +	dmabuf->size = size;
> +
> +	file = anon_inode_getfile("dmabuf", &dma_buf_fops, dmabuf, flags);
> +
> +	dmabuf->file = file;
> +
> +	mutex_init(&dmabuf->lock);
> +	INIT_LIST_HEAD(&dmabuf->attachments);
> +
> +	return dmabuf;
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_export);

[snip]

> +/**
> + * dma_buf_attach - Add the device to dma_buf's attachments list;
> optionally, + * calls attach() of dma_buf_ops to allow device-specific
> attach functionality + * @dmabuf:	[in]	buffer to attach device to.
> + * @dev:	[in]	device to be attached.
> + *
> + * Returns struct dma_buf_attachment * for this attachment; may return
> negative + * error codes.
> + *
> + */
> +struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
> +					  struct device *dev)
> +{
> +	struct dma_buf_attachment *attach;
> +	int ret;
> +
> +	if (WARN_ON(!dmabuf || !dev || !dmabuf->ops))

you still check dmabuf->ops here, as well as in several places below. 
Shouldn't these checks be removed ?

> +		return ERR_PTR(-EINVAL);
> +
> +	attach = kzalloc(sizeof(struct dma_buf_attachment), GFP_KERNEL);
> +	if (attach == NULL)
> +		goto err_alloc;

What about returning ERR_PTR(-ENOMEM) directly here ?

> +
> +	mutex_lock(&dmabuf->lock);
> +
> +	attach->dev = dev;
> +	attach->dmabuf = dmabuf;

These two lines can be moved before mutex_lock().

> +	if (dmabuf->ops->attach) {
> +		ret = dmabuf->ops->attach(dmabuf, dev, attach);
> +		if (ret)
> +			goto err_attach;
> +	}
> +	list_add(&attach->node, &dmabuf->attachments);
> +
> +	mutex_unlock(&dmabuf->lock);
> +	return attach;
> +
> +err_alloc:
> +	return ERR_PTR(-ENOMEM);
> +err_attach:
> +	kfree(attach);
> +	mutex_unlock(&dmabuf->lock);
> +	return ERR_PTR(ret);
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_attach);

-- 
Regards,

Laurent Pinchart

Re: [Linaro-mm-sig] [PATCH 1/3] dma-buf: Introduce dma buffer sharing mechanism

[Semwal, Sumit]

On Fri, Jan 20, 2012 at 6:53 PM, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
> Hi Summit,
>
> Sorry for the late review. I know that this code is now in mainline, but I
> still have a couple of comments. I'll send patches if you agree with them.
Hi Laurent,

Thanks for your review; apologies for being late in replying - I was
OoO for last couple of days.
>
> On Monday 26 December 2011 10:23:15 Sumit Semwal wrote:
<snip>
>>
>
> [snip]
>
>> diff --git a/drivers/base/dma-buf.c b/drivers/base/dma-buf.c
>> new file mode 100644
>> index 0000000..e38ad24
>> --- /dev/null
>> +++ b/drivers/base/dma-buf.c
>> @@ -0,0 +1,291 @@
>
> [snip]
>
>> +/**
>> + * dma_buf_export - Creates a new dma_buf, and associates an anon file
>> + * with this buffer, so it can be exported.
>> + * Also connect the allocator specific data and ops to the buffer.
>> + *
>> + * @priv:    [in]    Attach private data of allocator to this buffer
>> + * @ops:     [in]    Attach allocator-defined dma buf ops to the new buffer.
>> + * @size:    [in]    Size of the buffer
>> + * @flags:   [in]    mode flags for the file.
>> + *
>> + * Returns, on success, a newly created dma_buf object, which wraps the
>> + * supplied private data and operations for dma_buf_ops. On either missing
>> + * ops, or error in allocating struct dma_buf, will return negative error.
>> + *
>> + */
>> +struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops,
>> +                             size_t size, int flags)
>> +{
>> +     struct dma_buf *dmabuf;
>> +     struct file *file;
>> +
>> +     if (WARN_ON(!priv || !ops
>> +                       || !ops->map_dma_buf
>> +                       || !ops->unmap_dma_buf
>> +                       || !ops->release)) {
>> +             return ERR_PTR(-EINVAL);
>> +     }
>> +
>> +     dmabuf = kzalloc(sizeof(struct dma_buf), GFP_KERNEL);
>> +     if (dmabuf == NULL)
>> +             return ERR_PTR(-ENOMEM);
>> +
>> +     dmabuf->priv = priv;
>> +     dmabuf->ops = ops;
>
> dmabuf->ops will never but NULL, but (see below)
>
<snip>
>> +struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
>> +                                       struct device *dev)
>> +{
>> +     struct dma_buf_attachment *attach;
>> +     int ret;
>> +
>> +     if (WARN_ON(!dmabuf || !dev || !dmabuf->ops))
>
> you still check dmabuf->ops here, as well as in several places below.
> Shouldn't these checks be removed ?
You're right - these can be removed.
>
>> +             return ERR_PTR(-EINVAL);
>> +
>> +     attach = kzalloc(sizeof(struct dma_buf_attachment), GFP_KERNEL);
>> +     if (attach == NULL)
>> +             goto err_alloc;
>
> What about returning ERR_PTR(-ENOMEM) directly here ?
>
Right; we can do that.
>> +
>> +     mutex_lock(&dmabuf->lock);
>> +
>> +     attach->dev = dev;
>> +     attach->dmabuf = dmabuf;
>
> These two lines can be moved before mutex_lock().
>
:) Yes - thanks for catching this.
<snip>
> --
> Regards,
>
> Laurent Pinchart

Let me know if you'd send patches for these, or should I just go ahead
and correct.

Best regards,
~Sumit.

Re: [PATCH 1/3] dma-buf: Introduce dma buffer sharing mechanism

[Tomasz Stanislawski]

Hi Sumit,

On 12/26/2011 10:23 AM, Sumit Semwal wrote:
> 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.
>

[snip]

> +/**
> + * 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 *);

I think that you should add enum dma_data_direction as an argument
unmap function. It was mentioned that the dma_buf_attachment should keep
cached and mapped sg_table for performance reasons. The field
dma_buf_attachment::priv seams to be a natural place to keep this sg_table.
To map a buffer the exporter calls dma_map_sg. It needs dma direction
as an argument. The problem is that dma_unmap_sg also needs this
argument but dma direction is not available neither in
dma_buf_unmap_attachment nor in unmap callback. Therefore the exporter
is forced to embed returned sg_table into a bigger structure where dma 
direction is remembered. Refer to function vb2_dc_dmabuf_ops_map at
link below as an example:

http://thread.gmane.org/gmane.linux.drivers.video-input-infrastructure/43793/focus=43797

Regards,
Tomasz Stanislawski

Re: [PATCH 1/3] dma-buf: Introduce dma buffer sharing mechanism

[Daniel Vetter]

On Wed, Jan 25, 2012 at 06:02:41PM +0100, Tomasz Stanislawski wrote:
> Hi Sumit,
> 
> On 12/26/2011 10:23 AM, Sumit Semwal wrote:
> >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.
> >
> 
> [snip]
> 
> >+/**
> >+ * 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 *);
> 
> I think that you should add enum dma_data_direction as an argument
> unmap function. It was mentioned that the dma_buf_attachment should keep
> cached and mapped sg_table for performance reasons. The field
> dma_buf_attachment::priv seams to be a natural place to keep this sg_table.
> To map a buffer the exporter calls dma_map_sg. It needs dma direction
> as an argument. The problem is that dma_unmap_sg also needs this
> argument but dma direction is not available neither in
> dma_buf_unmap_attachment nor in unmap callback. Therefore the exporter
> is forced to embed returned sg_table into a bigger structure where
> dma direction is remembered. Refer to function vb2_dc_dmabuf_ops_map
> at

Oops, makes sense. I've totally overlooked that we need to pass in the dma
direction also for the unmap call to the dma subsystem. Sumit, can you
stitch together that small patch?
-Daniel
-- 
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48

Re: [PATCH 1/3] dma-buf: Introduce dma buffer sharing mechanism

[Semwal, Sumit]

On Thu, Jan 26, 2012 at 1:37 AM, Daniel Vetter <daniel@ffwll.ch> wrote:
> On Wed, Jan 25, 2012 at 06:02:41PM +0100, Tomasz Stanislawski wrote:
>> Hi Sumit,
>>
>> On 12/26/2011 10:23 AM, Sumit Semwal wrote:
>> >This is the first step in defining a dma buffer sharing mechanism.
>> >
>> [snip]
>>
>> >+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 *);
>>
>> I think that you should add enum dma_data_direction as an argument
>> unmap function. It was mentioned that the dma_buf_attachment should keep
>> cached and mapped sg_table for performance reasons. The field
>> dma_buf_attachment::priv seams to be a natural place to keep this sg_table.
>> To map a buffer the exporter calls dma_map_sg. It needs dma direction
>> as an argument. The problem is that dma_unmap_sg also needs this
>> argument but dma direction is not available neither in
>> dma_buf_unmap_attachment nor in unmap callback. Therefore the exporter
>> is forced to embed returned sg_table into a bigger structure where
>> dma direction is remembered. Refer to function vb2_dc_dmabuf_ops_map
>> at
>
> Oops, makes sense. I've totally overlooked that we need to pass in the dma
> direction also for the unmap call to the dma subsystem. Sumit, can you
> stitch together that small patch?

Right, of course. I will do that by tomorrow; it is a bank holiday
today here in India, so.

regards,
~Sumit.
> -Daniel
> --
> Daniel Vetter
> Mail: daniel@ffwll.ch
> Mobile: +41 (0)79 365 57 48

Re: [Linaro-mm-sig] [PATCH 1/3] dma-buf: Introduce dma buffer sharing mechanism

[Laurent Pinchart]

Hi Sumit,

On Wednesday 25 January 2012 14:56:52 Semwal, Sumit wrote:
> On Fri, Jan 20, 2012 at 6:53 PM, Laurent Pinchart wrote:
> > Hi Summit,
> > 
> > Sorry for the late review. I know that this code is now in mainline, but
> > I still have a couple of comments. I'll send patches if you agree with
> > them.
> 
> Hi Laurent,
> 
> Thanks for your review; apologies for being late in replying - I was
> OoO for last couple of days.

No worries.

[snip]

> Let me know if you'd send patches for these, or should I just go ahead and
> correct.

I'll send patches.

Another small comment. The map_dma_buf operation is defined as

        struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *,
                                                enum dma_data_direction);

If we want to let exporters cache the sg_table we should return a const struct 
sg_table *. unmap_dma_buf will then take a const pointer as well, which would 
need to be cast to a non-const pointer internally. What's your opinion on that 
?

-- 
Regards,

Laurent Pinchart