Subject: [RFC v3 1/2] dma-buf: Introduce dma buffer sharing mechanism

From: [email-masked] (Konrad Rzeszutek Wilk)
Date: Tue, 20 Dec 2011 11:36:50 -0500

On Mon, Dec 19, 2011 at 02:03:30PM +0530, 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:
> - different devices to 'attach' themselves to this buffer, to facilitate
>   backing storage negotiation, using dma_buf_attach() API.

Any thoughts of adding facility to track them? So you can see who is using what?

> - association of a file pointer with each user-buffer and associated
>    allocator-defined operations on that buffer. This operation is called the
>    'export' operation.

 'create'? or 'alloc' ?

export implies an import somwhere and I don't think that is the case here.

> - 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 scatterlist 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.

for the whole memory region or just for the device itself?

> 
> 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.
> 
> 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 <[email-masked]>, Rob Clark <[email-masked]> and
> Daniel Vetter <[email-masked]>.
> 
> The implementation is inspired from proof-of-concept patch-set from
> Tomasz Stanislawski <[email-masked]>, 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 <[email-masked]>
> Signed-off-by: Sumit Semwal <[email-masked]>
> ---
>  drivers/base/Kconfig    |   10 ++
>  drivers/base/Makefile   |    1 +
>  drivers/base/dma-buf.c  |  289 +++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/dma-buf.h |  172 ++++++++++++++++++++++++++++
>  4 files changed, 472 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..e920709
> --- /dev/null
> +++ b/drivers/base/dma-buf.c
> @@ -0,0 +1,289 @@
> +/*
> + * Framework for buffer objects that can be shared across devices/subsystems.
> + *
> + * Copyright(C) 2011 Linaro Limited. All rights reserved.
> + * Author: Sumit Semwal <[email-masked]>
> + *
> + * Many thanks to linaro-mm-sig list, and specially
> + * Arnd Bergmann <[email-masked]>, Rob Clark <[email-masked]> and
> + * Daniel Vetter <[email-masked]> 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;
> +}
> +
> +/**

Wrong kerneldoc.

> + * 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);
> +
> +	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..3e3ecf3
> --- /dev/null
> +++ b/include/linux/dma-buf.h
> @@ -0,0 +1,172 @@
> +/*
> + * Header file for dma buffer sharing framework.
> + *
> + * Copyright(C) 2011 Linaro Limited. All rights reserved.
> + * Author: Sumit Semwal <[email-masked]>
> + *
> + * Many thanks to linaro-mm-sig list, and specially
> + * Arnd Bergmann <[email-masked]>, Rob Clark <[email-masked]> and
> + * Daniel Vetter <[email-masked]> 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 - holds device-buffer attachment data

OK, but what is the purpose of it?

> + * @dmabuf: buffer for this attachment.
> + * @dev: device attached to the buffer.
                                ^^^ this
> + * @node: list_head to allow manipulation of list of dma_buf_attachment.

Just say: "list of dma_buf_attachment"'

> + * @priv: exporter-specific attachment data.

That "exporter-specific.." brings to my mind custom decleration forms. But maybe that is me.

> + */
> +struct dma_buf_attachment {
> +	struct dma_buf *dmabuf;
> +	struct device *dev;
> +	struct list_head node;
> +	void *priv;
> +};

Why don't you move the decleration of this below 'struct dma_buf'?
It would easier than to read this structure..

> +
> +/**
> + * struct dma_buf_ops - operations possible on struct dma_buf
> + * @attach: allows different devices to 'attach' themselves to the given

register?
> + *	    buffer. It might return -EBUSY to signal that backing storage
> + *	    is already allocated and incompatible with the requirements

Wait.. allocated or attached?

> + *	    of requesting device. [optional]

What is optional? The return value? Or the 'attach' call? If the later , say
that in the first paragraph.

> + * @detach: detach a given device from this buffer. [optional]
> + * @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

Ok, there is some __might_sleep macro you should put on the function.

> + *		 -EINTR.

Ok. What is the return code if attach has _not_ been called?

> + * @unmap_dma_buf: decreases usecount of buffer, might deallocate scatter
> + *		   pages.
> + * @release: release this buffer; to be called after the last dma_buf_put.
> + * @sync_sg_for_cpu: sync the sg list for cpu.
> + * @sync_sg_for_device: synch the sg list for device.

Not seeing those two.
> + */
> +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

Ewww. Why? Why not just just the 'map_dma_buf' and return that?

> +	 * if the call would block.
> +	 */
> +
> +	/* after final dma_buf_put() */
> +	void (*release)(struct dma_buf *);
> +
> +};
> +
> +/**
> + * struct dma_buf - shared buffer object

Missing the 'size'.

> + * @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: user specific private data

Can you elaborate on this? Is this the "exporter" using this? Or is
it for the "user" using it? If so, why provide it? Wouldn't the 
user of this have something like this:

struct my_dma_bufs {
	struct dma_buf[20];
	void *priv;
}

Anyhow?

> + */
> +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;
> +};
> +
> +#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__ */
> -- 
> 1.7.4.1
> 
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