text: /*
   * videobuf2-dma-contig.c - DMA contig memory allocator for videobuf2
   *
   * Copyright (C) 2010 Samsung Electronics
   *
   * Author: Pawel Osciak <pawel@osciak.com>
   *
   * This program is free software; you can redistribute it and/or modify
   * it under the terms of the GNU General Public License as published by
   * the Free Software Foundation.
   */
  
  #include <linux/dma-buf.h>
  #include <linux/module.h>
  #include <linux/refcount.h>
  #include <linux/scatterlist.h>
  #include <linux/sched.h>
  #include <linux/slab.h>
  #include <linux/dma-mapping.h>
  
  #include <media/videobuf2-v4l2.h>
  #include <media/videobuf2-dma-contig.h>
  #include <media/videobuf2-memops.h>
  
  struct vb2_dc_buf {
  	struct device			*dev;
  	void				*vaddr;
  	unsigned long			size;
  	void				*cookie;
  	dma_addr_t			dma_addr;
  	unsigned long			attrs;
  	enum dma_data_direction		dma_dir;
  	struct sg_table			*dma_sgt;
  	struct frame_vector		*vec;
  
  	/* MMAP related */
  	struct vb2_vmarea_handler	handler;
  	refcount_t			refcount;
  	struct sg_table			*sgt_base;
  
  	/* DMABUF related */
  	struct dma_buf_attachment	*db_attach;
  };
  
  /*********************************************/
  /*        scatterlist table functions        */
  /*********************************************/
  
  static unsigned long vb2_dc_get_contiguous_size(struct sg_table *sgt)
  {
  	struct scatterlist *s;
  	dma_addr_t expected = sg_dma_address(sgt->sgl);
  	unsigned int i;
  	unsigned long size = 0;
  
  	for_each_sg(sgt->sgl, s, sgt->nents, i) {
  		if (sg_dma_address(s) != expected)
  			break;
  		expected = sg_dma_address(s) + sg_dma_len(s);
  		size += sg_dma_len(s);
  	}
  	return size;
  }
  
  /*********************************************/
  /*         callbacks for all buffers         */
  /*********************************************/
  
  static void *vb2_dc_cookie(void *buf_priv)
  {
  	struct vb2_dc_buf *buf = buf_priv;
  
  	return &buf->dma_addr;
  }
  
  static void *vb2_dc_vaddr(void *buf_priv)
  {
  	struct vb2_dc_buf *buf = buf_priv;
  
  	if (!buf->vaddr && buf->db_attach)
  		buf->vaddr = dma_buf_vmap(buf->db_attach->dmabuf);
  
  	return buf->vaddr;
  }
  
  static unsigned int vb2_dc_num_users(void *buf_priv)
  {
  	struct vb2_dc_buf *buf = buf_priv;
  
  	return refcount_read(&buf->refcount);
  }
  
  static void vb2_dc_prepare(void *buf_priv)
  {
  	struct vb2_dc_buf *buf = buf_priv;
  	struct sg_table *sgt = buf->dma_sgt;
  
  	/* DMABUF exporter will flush the cache for us */
  	if (!sgt || buf->db_attach)
  		return;
  
  	dma_sync_sg_for_device(buf->dev, sgt->sgl, sgt->orig_nents,
  			       buf->dma_dir);
  }
  
  static void vb2_dc_finish(void *buf_priv)
  {
  	struct vb2_dc_buf *buf = buf_priv;
  	struct sg_table *sgt = buf->dma_sgt;
  
  	/* DMABUF exporter will flush the cache for us */
  	if (!sgt || buf->db_attach)
  		return;
  
  	dma_sync_sg_for_cpu(buf->dev, sgt->sgl, sgt->orig_nents, buf->dma_dir);
  }
  
  /*********************************************/
  /*        callbacks for MMAP buffers         */
  /*********************************************/
  
  static void vb2_dc_put(void *buf_priv)
  {
  	struct vb2_dc_buf *buf = buf_priv;
  
  	if (!refcount_dec_and_test(&buf->refcount))
  		return;
  
  	if (buf->sgt_base) {
  		sg_free_table(buf->sgt_base);
  		kfree(buf->sgt_base);
  	}
  	dma_free_attrs(buf->dev, buf->size, buf->cookie, buf->dma_addr,
  		       buf->attrs);
  	put_device(buf->dev);
  	kfree(buf);
  }
  
  static void *vb2_dc_alloc(struct device *dev, unsigned long attrs,
  			  unsigned long size, enum dma_data_direction dma_dir,
  			  gfp_t gfp_flags)
  {
  	struct vb2_dc_buf *buf;
  
  	if (WARN_ON(!dev))
  		return ERR_PTR(-EINVAL);
  
  	buf = kzalloc(sizeof *buf, GFP_KERNEL);
  	if (!buf)
  		return ERR_PTR(-ENOMEM);
  
  	if (attrs)
  		buf->attrs = attrs;
  	buf->cookie = dma_alloc_attrs(dev, size, &buf->dma_addr,
  					GFP_KERNEL | gfp_flags, buf->attrs);
  	if (!buf->cookie) {
  		dev_err(dev, "dma_alloc_coherent of size %ld failed\n", size);
  		kfree(buf);
  		return ERR_PTR(-ENOMEM);
  	}
  
  	if ((buf->attrs & DMA_ATTR_NO_KERNEL_MAPPING) == 0)
  		buf->vaddr = buf->cookie;
  
  	/* Prevent the device from being released while the buffer is used */
  	buf->dev = get_device(dev);
  	buf->size = size;
  	buf->dma_dir = dma_dir;
  
  	buf->handler.refcount = &buf->refcount;
  	buf->handler.put = vb2_dc_put;
  	buf->handler.arg = buf;
  
  	refcount_set(&buf->refcount, 1);
  
  	return buf;
  }
  
  static int vb2_dc_mmap(void *buf_priv, struct vm_area_struct *vma)
  {
  	struct vb2_dc_buf *buf = buf_priv;
  	int ret;
  
  	if (!buf) {
  		printk(KERN_ERR "No buffer to map\n");
  		return -EINVAL;
  	}
  
  	ret = dma_mmap_attrs(buf->dev, vma, buf->cookie,
  		buf->dma_addr, buf->size, buf->attrs);
  
  	if (ret) {
  		pr_err("Remapping memory failed, error: %d\n", ret);
  		return ret;
  	}
  
  	vma->vm_flags		|= VM_DONTEXPAND | VM_DONTDUMP;
  	vma->vm_private_data	= &buf->handler;
  	vma->vm_ops		= &vb2_common_vm_ops;
  
  	vma->vm_ops->open(vma);
  
  	pr_debug("%s: mapped dma addr 0x%08lx at 0x%08lx, size %ld\n",
  		__func__, (unsigned long)buf->dma_addr, vma->vm_start,
  		buf->size);
  
  	return 0;
  }
  
  /*********************************************/
  /*         DMABUF ops for exporters          */
  /*********************************************/
  
  struct vb2_dc_attachment {
  	struct sg_table sgt;
  	enum dma_data_direction dma_dir;
  };
  
  static int vb2_dc_dmabuf_ops_attach(struct dma_buf *dbuf,
  	struct dma_buf_attachment *dbuf_attach)
  {
  	struct vb2_dc_attachment *attach;
  	unsigned int i;
  	struct scatterlist *rd, *wr;
  	struct sg_table *sgt;
  	struct vb2_dc_buf *buf = dbuf->priv;
  	int ret;
  
  	attach = kzalloc(sizeof(*attach), GFP_KERNEL);
  	if (!attach)
  		return -ENOMEM;
  
  	sgt = &attach->sgt;
  	/* Copy the buf->base_sgt scatter list to the attachment, as we can't
  	 * map the same scatter list to multiple attachments at the same time.
  	 */
  	ret = sg_alloc_table(sgt, buf->sgt_base->orig_nents, GFP_KERNEL);
  	if (ret) {
  		kfree(attach);
  		return -ENOMEM;
  	}
  
  	rd = buf->sgt_base->sgl;
  	wr = sgt->sgl;
  	for (i = 0; i < sgt->orig_nents; ++i) {
  		sg_set_page(wr, sg_page(rd), rd->length, rd->offset);
  		rd = sg_next(rd);
  		wr = sg_next(wr);
  	}
  
  	attach->dma_dir = DMA_NONE;
  	dbuf_attach->priv = attach;
  
  	return 0;
  }
  
  static void vb2_dc_dmabuf_ops_detach(struct dma_buf *dbuf,
  	struct dma_buf_attachment *db_attach)
  {
  	struct vb2_dc_attachment *attach = db_attach->priv;
  	struct sg_table *sgt;
  
  	if (!attach)
  		return;
  
  	sgt = &attach->sgt;
  
  	/* release the scatterlist cache */
  	if (attach->dma_dir != DMA_NONE)
  		/*
  		 * Cache sync can be skipped here, as the vb2_dc memory is
  		 * allocated from device coherent memory, which means the
  		 * memory locations do not require any explicit cache
  		 * maintenance prior or after being used by the device.
  		 */
  		dma_unmap_sg_attrs(db_attach->dev, sgt->sgl, sgt->orig_nents,
  				   attach->dma_dir, DMA_ATTR_SKIP_CPU_SYNC);
  	sg_free_table(sgt);
  	kfree(attach);
  	db_attach->priv = NULL;
  }
  
  static struct sg_table *vb2_dc_dmabuf_ops_map(
  	struct dma_buf_attachment *db_attach, enum dma_data_direction dma_dir)
  {
  	struct vb2_dc_attachment *attach = db_attach->priv;
  	/* stealing dmabuf mutex to serialize map/unmap operations */
  	struct mutex *lock = &db_attach->dmabuf->lock;
  	struct sg_table *sgt;
  
  	mutex_lock(lock);
  
  	sgt = &attach->sgt;
  	/* return previously mapped sg table */
  	if (attach->dma_dir == dma_dir) {
  		mutex_unlock(lock);
  		return sgt;
  	}
  
  	/* release any previous cache */
  	if (attach->dma_dir != DMA_NONE) {
  		dma_unmap_sg_attrs(db_attach->dev, sgt->sgl, sgt->orig_nents,
  				   attach->dma_dir, DMA_ATTR_SKIP_CPU_SYNC);
  		attach->dma_dir = DMA_NONE;
  	}
  
  	/*
  	 * mapping to the client with new direction, no cache sync
  	 * required see comment in vb2_dc_dmabuf_ops_detach()
  	 */
  	sgt->nents = dma_map_sg_attrs(db_attach->dev, sgt->sgl, sgt->orig_nents,
  				      dma_dir, DMA_ATTR_SKIP_CPU_SYNC);
  	if (!sgt->nents) {
  		pr_err("failed to map scatterlist\n");
  		mutex_unlock(lock);
  		return ERR_PTR(-EIO);
  	}
  
  	attach->dma_dir = dma_dir;
  
  	mutex_unlock(lock);
  
  	return sgt;
  }
  
  static void vb2_dc_dmabuf_ops_unmap(struct dma_buf_attachment *db_attach,
  	struct sg_table *sgt, enum dma_data_direction dma_dir)
  {
  	/* nothing to be done here */
  }
  
  static void vb2_dc_dmabuf_ops_release(struct dma_buf *dbuf)
  {
  	/* drop reference obtained in vb2_dc_get_dmabuf */
  	vb2_dc_put(dbuf->priv);
  }
  
  static void *vb2_dc_dmabuf_ops_kmap(struct dma_buf *dbuf, unsigned long pgnum)
  {
  	struct vb2_dc_buf *buf = dbuf->priv;
  
  	return buf->vaddr ? buf->vaddr + pgnum * PAGE_SIZE : NULL;
  }
  
  static void *vb2_dc_dmabuf_ops_vmap(struct dma_buf *dbuf)
  {
  	struct vb2_dc_buf *buf = dbuf->priv;
  
  	return buf->vaddr;
  }
  
  static int vb2_dc_dmabuf_ops_mmap(struct dma_buf *dbuf,
  	struct vm_area_struct *vma)
  {
  	return vb2_dc_mmap(dbuf->priv, vma);
  }
  
  static const struct dma_buf_ops vb2_dc_dmabuf_ops = {
  	.attach = vb2_dc_dmabuf_ops_attach,
  	.detach = vb2_dc_dmabuf_ops_detach,
  	.map_dma_buf = vb2_dc_dmabuf_ops_map,
  	.unmap_dma_buf = vb2_dc_dmabuf_ops_unmap,
  	.map = vb2_dc_dmabuf_ops_kmap,
  	.vmap = vb2_dc_dmabuf_ops_vmap,
  	.mmap = vb2_dc_dmabuf_ops_mmap,
  	.release = vb2_dc_dmabuf_ops_release,
  };
  
  static struct sg_table *vb2_dc_get_base_sgt(struct vb2_dc_buf *buf)
  {
  	int ret;
  	struct sg_table *sgt;
  
  	sgt = kmalloc(sizeof(*sgt), GFP_KERNEL);
  	if (!sgt) {
  		dev_err(buf->dev, "failed to alloc sg table\n");
  		return NULL;
  	}
  
  	ret = dma_get_sgtable_attrs(buf->dev, sgt, buf->cookie, buf->dma_addr,
  		buf->size, buf->attrs);
  	if (ret < 0) {
  		dev_err(buf->dev, "failed to get scatterlist from DMA API\n");
  		kfree(sgt);
  		return NULL;
  	}
  
  	return sgt;
  }
  
  static struct dma_buf *vb2_dc_get_dmabuf(void *buf_priv, unsigned long flags)
  {
  	struct vb2_dc_buf *buf = buf_priv;
  	struct dma_buf *dbuf;
  	DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
  
  	exp_info.ops = &vb2_dc_dmabuf_ops;
  	exp_info.size = buf->size;
  	exp_info.flags = flags;
  	exp_info.priv = buf;
  
  	if (!buf->sgt_base)
  		buf->sgt_base = vb2_dc_get_base_sgt(buf);
  
  	if (WARN_ON(!buf->sgt_base))
  		return NULL;
  
  	dbuf = dma_buf_export(&exp_info);
  	if (IS_ERR(dbuf))
  		return NULL;
  
  	/* dmabuf keeps reference to vb2 buffer */
  	refcount_inc(&buf->refcount);
  
  	return dbuf;
  }
  
  /*********************************************/
  /*       callbacks for USERPTR buffers       */
  /*********************************************/
  
  static void vb2_dc_put_userptr(void *buf_priv)
  {
  	struct vb2_dc_buf *buf = buf_priv;
  	struct sg_table *sgt = buf->dma_sgt;
  	int i;
  	struct page **pages;
  
  	if (sgt) {
  		/*
  		 * No need to sync to CPU, it's already synced to the CPU
  		 * since the finish() memop will have been called before this.
  		 */
  		dma_unmap_sg_attrs(buf->dev, sgt->sgl, sgt->orig_nents,
  				   buf->dma_dir, DMA_ATTR_SKIP_CPU_SYNC);
  		pages = frame_vector_pages(buf->vec);
  		/* sgt should exist only if vector contains pages... */
  		BUG_ON(IS_ERR(pages));
  		if (buf->dma_dir == DMA_FROM_DEVICE ||
  		    buf->dma_dir == DMA_BIDIRECTIONAL)
  			for (i = 0; i < frame_vector_count(buf->vec); i++)
  				set_page_dirty_lock(pages[i]);
  		sg_free_table(sgt);
  		kfree(sgt);
  	} else {
  		dma_unmap_resource(buf->dev, buf->dma_addr, buf->size,
  				   buf->dma_dir, 0);
  	}
  	vb2_destroy_framevec(buf->vec);
  	kfree(buf);
  }
  
  static void *vb2_dc_get_userptr(struct device *dev, unsigned long vaddr,
  	unsigned long size, enum dma_data_direction dma_dir)
  {
  	struct vb2_dc_buf *buf;
  	struct frame_vector *vec;
  	unsigned int offset;
  	int n_pages, i;
  	int ret = 0;
  	struct sg_table *sgt;
  	unsigned long contig_size;
  	unsigned long dma_align = dma_get_cache_alignment();
  
  	/* Only cache aligned DMA transfers are reliable */
  	if (!IS_ALIGNED(vaddr | size, dma_align)) {
  		pr_debug("user data must be aligned to %lu bytes\n", dma_align);
  		return ERR_PTR(-EINVAL);
  	}
  
  	if (!size) {
  		pr_debug("size is zero\n");
  		return ERR_PTR(-EINVAL);
  	}
  
  	if (WARN_ON(!dev))
  		return ERR_PTR(-EINVAL);
  
  	buf = kzalloc(sizeof *buf, GFP_KERNEL);
  	if (!buf)
  		return ERR_PTR(-ENOMEM);
  
  	buf->dev = dev;
  	buf->dma_dir = dma_dir;
  
  	offset = lower_32_bits(offset_in_page(vaddr));
  	vec = vb2_create_framevec(vaddr, size);
  	if (IS_ERR(vec)) {
  		ret = PTR_ERR(vec);
  		goto fail_buf;
  	}
  	buf->vec = vec;
  	n_pages = frame_vector_count(vec);
  	ret = frame_vector_to_pages(vec);
  	if (ret < 0) {
  		unsigned long *nums = frame_vector_pfns(vec);
  
  		/*
  		 * Failed to convert to pages... Check the memory is physically
  		 * contiguous and use direct mapping
  		 */
  		for (i = 1; i < n_pages; i++)
  			if (nums[i-1] + 1 != nums[i])
  				goto fail_pfnvec;
  		buf->dma_addr = dma_map_resource(buf->dev,
  				__pfn_to_phys(nums[0]), size, buf->dma_dir, 0);
  		if (dma_mapping_error(buf->dev, buf->dma_addr)) {
  			ret = -ENOMEM;
  			goto fail_pfnvec;
  		}
  		goto out;
  	}
  
  	sgt = kzalloc(sizeof(*sgt), GFP_KERNEL);
  	if (!sgt) {
  		pr_err("failed to allocate sg table\n");
  		ret = -ENOMEM;
  		goto fail_pfnvec;
  	}
  
  	ret = sg_alloc_table_from_pages(sgt, frame_vector_pages(vec), n_pages,
  		offset, size, GFP_KERNEL);
  	if (ret) {
  		pr_err("failed to initialize sg table\n");
  		goto fail_sgt;
  	}
  
  	/*
  	 * No need to sync to the device, this will happen later when the
  	 * prepare() memop is called.
  	 */
  	sgt->nents = dma_map_sg_attrs(buf->dev, sgt->sgl, sgt->orig_nents,
  				      buf->dma_dir, DMA_ATTR_SKIP_CPU_SYNC);
  	if (sgt->nents <= 0) {
  		pr_err("failed to map scatterlist\n");
  		ret = -EIO;
  		goto fail_sgt_init;
  	}
  
  	contig_size = vb2_dc_get_contiguous_size(sgt);
  	if (contig_size < size) {
  		pr_err("contiguous mapping is too small %lu/%lu\n",
  			contig_size, size);
  		ret = -EFAULT;
  		goto fail_map_sg;
  	}
  
  	buf->dma_addr = sg_dma_address(sgt->sgl);
  	buf->dma_sgt = sgt;
  out:
  	buf->size = size;
  
  	return buf;
  
  fail_map_sg:
  	dma_unmap_sg_attrs(buf->dev, sgt->sgl, sgt->orig_nents,
  			   buf->dma_dir, DMA_ATTR_SKIP_CPU_SYNC);
  
  fail_sgt_init:
  	sg_free_table(sgt);
  
  fail_sgt:
  	kfree(sgt);
  
  fail_pfnvec:
  	vb2_destroy_framevec(vec);
  
  fail_buf:
  	kfree(buf);
  
  	return ERR_PTR(ret);
  }
  
  /*********************************************/
  /*       callbacks for DMABUF buffers        */
  /*********************************************/
  
  static int vb2_dc_map_dmabuf(void *mem_priv)
  {
  	struct vb2_dc_buf *buf = mem_priv;
  	struct sg_table *sgt;
  	unsigned long contig_size;
  
  	if (WARN_ON(!buf->db_attach)) {
  		pr_err("trying to pin a non attached buffer\n");
  		return -EINVAL;
  	}
  
  	if (WARN_ON(buf->dma_sgt)) {
  		pr_err("dmabuf buffer is already pinned\n");
  		return 0;
  	}
  
  	/* get the associated scatterlist for this buffer */
  	sgt = dma_buf_map_attachment(buf->db_attach, buf->dma_dir);
  	if (IS_ERR(sgt)) {
  		pr_err("Error getting dmabuf scatterlist\n");
  		return -EINVAL;
  	}
  
  	/* checking if dmabuf is big enough to store contiguous chunk */
  	contig_size = vb2_dc_get_contiguous_size(sgt);
  	if (contig_size < buf->size) {
  		pr_err("contiguous chunk is too small %lu/%lu b\n",
  			contig_size, buf->size);
  		dma_buf_unmap_attachment(buf->db_attach, sgt, buf->dma_dir);
  		return -EFAULT;
  	}
  
  	buf->dma_addr = sg_dma_address(sgt->sgl);
  	buf->dma_sgt = sgt;
  	buf->vaddr = NULL;
  
  	return 0;
  }
  
  static void vb2_dc_unmap_dmabuf(void *mem_priv)
  {
  	struct vb2_dc_buf *buf = mem_priv;
  	struct sg_table *sgt = buf->dma_sgt;
  
  	if (WARN_ON(!buf->db_attach)) {
  		pr_err("trying to unpin a not attached buffer\n");
  		return;
  	}
  
  	if (WARN_ON(!sgt)) {
  		pr_err("dmabuf buffer is already unpinned\n");
  		return;
  	}
  
  	if (buf->vaddr) {
  		dma_buf_vunmap(buf->db_attach->dmabuf, buf->vaddr);
  		buf->vaddr = NULL;
  	}
  	dma_buf_unmap_attachment(buf->db_attach, sgt, buf->dma_dir);
  
  	buf->dma_addr = 0;
  	buf->dma_sgt = NULL;
  }
  
  static void vb2_dc_detach_dmabuf(void *mem_priv)
  {
  	struct vb2_dc_buf *buf = mem_priv;
  
  	/* if vb2 works correctly you should never detach mapped buffer */
  	if (WARN_ON(buf->dma_addr))
  		vb2_dc_unmap_dmabuf(buf);
  
  	/* detach this attachment */
  	dma_buf_detach(buf->db_attach->dmabuf, buf->db_attach);
  	kfree(buf);
  }
  
  static void *vb2_dc_attach_dmabuf(struct device *dev, struct dma_buf *dbuf,
  	unsigned long size, enum dma_data_direction dma_dir)
  {
  	struct vb2_dc_buf *buf;
  	struct dma_buf_attachment *dba;
  
  	if (dbuf->size < size)
  		return ERR_PTR(-EFAULT);
  
  	if (WARN_ON(!dev))
  		return ERR_PTR(-EINVAL);
  
  	buf = kzalloc(sizeof(*buf), GFP_KERNEL);
  	if (!buf)
  		return ERR_PTR(-ENOMEM);
  
  	buf->dev = dev;
  	/* create attachment for the dmabuf with the user device */
  	dba = dma_buf_attach(dbuf, buf->dev);
  	if (IS_ERR(dba)) {
  		pr_err("failed to attach dmabuf\n");
  		kfree(buf);
  		return dba;
  	}
  
  	buf->dma_dir = dma_dir;
  	buf->size = size;
  	buf->db_attach = dba;
  
  	return buf;
  }
  
  /*********************************************/
  /*       DMA CONTIG exported functions       */
  /*********************************************/
  
  const struct vb2_mem_ops vb2_dma_contig_memops = {
  	.alloc		= vb2_dc_alloc,
  	.put		= vb2_dc_put,
  	.get_dmabuf	= vb2_dc_get_dmabuf,
  	.cookie		= vb2_dc_cookie,
  	.vaddr		= vb2_dc_vaddr,
  	.mmap		= vb2_dc_mmap,
  	.get_userptr	= vb2_dc_get_userptr,
  	.put_userptr	= vb2_dc_put_userptr,
  	.prepare	= vb2_dc_prepare,
  	.finish		= vb2_dc_finish,
  	.map_dmabuf	= vb2_dc_map_dmabuf,
  	.unmap_dmabuf	= vb2_dc_unmap_dmabuf,
  	.attach_dmabuf	= vb2_dc_attach_dmabuf,
  	.detach_dmabuf	= vb2_dc_detach_dmabuf,
  	.num_users	= vb2_dc_num_users,
  };
  EXPORT_SYMBOL_GPL(vb2_dma_contig_memops);
  
  /**
   * vb2_dma_contig_set_max_seg_size() - configure DMA max segment size
   * @dev:	device for configuring DMA parameters
   * @size:	size of DMA max segment size to set
   *
   * To allow mapping the scatter-list into a single chunk in the DMA
   * address space, the device is required to have the DMA max segment
   * size parameter set to a value larger than the buffer size. Otherwise,
   * the DMA-mapping subsystem will split the mapping into max segment
   * size chunks. This function sets the DMA max segment size
   * parameter to let DMA-mapping map a buffer as a single chunk in DMA
   * address space.
   * This code assumes that the DMA-mapping subsystem will merge all
   * scatterlist segments if this is really possible (for example when
   * an IOMMU is available and enabled).
   * Ideally, this parameter should be set by the generic bus code, but it
   * is left with the default 64KiB value due to historical litmiations in
   * other subsystems (like limited USB host drivers) and there no good
   * place to set it to the proper value.
   * This function should be called from the drivers, which are known to
   * operate on platforms with IOMMU and provide access to shared buffers
   * (either USERPTR or DMABUF). This should be done before initializing
   * videobuf2 queue.
   */
  int vb2_dma_contig_set_max_seg_size(struct device *dev, unsigned int size)
  {
  	if (!dev->dma_parms) {
  		dev->dma_parms = kzalloc(sizeof(*dev->dma_parms), GFP_KERNEL);
  		if (!dev->dma_parms)
  			return -ENOMEM;
  	}
  	if (dma_get_max_seg_size(dev) < size)
  		return dma_set_max_seg_size(dev, size);
  
  	return 0;
  }
  EXPORT_SYMBOL_GPL(vb2_dma_contig_set_max_seg_size);
  
  /*
   * vb2_dma_contig_clear_max_seg_size() - release resources for DMA parameters
   * @dev:	device for configuring DMA parameters
   *
   * This function releases resources allocated to configure DMA parameters
   * (see vb2_dma_contig_set_max_seg_size() function). It should be called from
   * device drivers on driver remove.
   */
  void vb2_dma_contig_clear_max_seg_size(struct device *dev)
  {
  	kfree(dev->dma_parms);
  	dev->dma_parms = NULL;
  }
  EXPORT_SYMBOL_GPL(vb2_dma_contig_clear_max_seg_size);
  
  MODULE_DESCRIPTION("DMA-contig memory handling routines for videobuf2");
  MODULE_AUTHOR("Pawel Osciak <pawel@osciak.com>");
  MODULE_LICENSE("GPL");
  