Re: [RFC] Experimental DMA-BUF Device Heaps

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On Sun, Aug 16, 2020 at 02:22:46PM -0300, Ezequiel Garcia wrote:
> This heap is basically a wrapper around DMA-API dma_alloc_attrs,
> which will allocate memory suitable for the given device.
> 
> The implementation is mostly a port of the Contiguous Videobuf2
> memory allocator (see videobuf2/videobuf2-dma-contig.c)
> over to the DMA-BUF Heap interface.
> 
> The intention of this allocator is to provide applications
> with a more system-agnostic API: the only thing the application
> needs to know is which device to get the buffer for.
> 
> Whether the buffer is backed by CMA, IOMMU or a DMA Pool
> is unknown to the application.
> 
> I'm not really expecting this patch to be correct or even
> a good idea, but just submitting it to start a discussion on DMA-BUF
> heap discovery and negotiation.
> 
> Given Plumbers is just a couple weeks from now, I've submitted
> a BoF proposal to discuss this, as perhaps it would make
> sense to discuss this live?
> 
> Not-signed-off-by: Ezequiel Garcia <ezequiel@xxxxxxxxxxxxx>

I think on the uapi/constraint solving side there's been already tons of
discussions while I enjoyed vacations, so different concern from me
entirely on the implementation side:

In the past the only thing we had in upstream was subsystem/driver
specific allocators, and dma-buf for sharing. With dma-buf heaps we kinda
get a central allocator, which duplicates large chunks of of all these
allocators. And since it's a central allocator by design, the reason for
having per-subsystem allocators is kinda gone.

I think there's two approaches here:
- we convert e.g. drm allocator helpers to internally use the right heap
  implementation. That would also give us some fairly direct way to expose
  these constraints in sysfs so a userspace allocator knows which dma-buf
  heap to pick for shared stuff.

- we require that any heap is just a different uapi for an existing driver
  allocator, e.g. by having a dma-buf wrapper for all gem drivers.

Otherwise I think what we end up with is a pile of dma-buf heaps for
android's blob gpu driver world, and not used anywhere else. Not something
even remotely interesting for upstream :-)

tldr; I'd like to see how dma-buf heaps closely integrate with all the
existing buffer management code we have. Both kernel (and throuhg some
allocator library effort) in userspace.

Cheers, Daniel

> ---
>  drivers/dma-buf/heaps/Kconfig       |   9 +
>  drivers/dma-buf/heaps/Makefile      |   1 +
>  drivers/dma-buf/heaps/device_heap.c | 268 ++++++++++++++++++++++++++++
>  include/linux/device.h              |   5 +
>  include/linux/dma-heap.h            |   6 +
>  5 files changed, 289 insertions(+)
>  create mode 100644 drivers/dma-buf/heaps/device_heap.c
> 
> diff --git a/drivers/dma-buf/heaps/Kconfig b/drivers/dma-buf/heaps/Kconfig
> index a5eef06c4226..2bb3604184bd 100644
> --- a/drivers/dma-buf/heaps/Kconfig
> +++ b/drivers/dma-buf/heaps/Kconfig
> @@ -12,3 +12,12 @@ config DMABUF_HEAPS_CMA
>  	  Choose this option to enable dma-buf CMA heap. This heap is backed
>  	  by the Contiguous Memory Allocator (CMA). If your system has these
>  	  regions, you should say Y here.
> +
> +config DMABUF_HEAPS_DEVICES
> +	bool "DMA-BUF Device DMA Heap (Experimental)"
> +	depends on DMABUF_HEAPS
> +	help
> +	  Choose this option to enable dma-buf per-device heap. This heap is backed
> +	  by the DMA-API and it's an Experimental feature, meant mostly for testing
> +	  and experimentation.
> +	  Just say N here.
> diff --git a/drivers/dma-buf/heaps/Makefile b/drivers/dma-buf/heaps/Makefile
> index 6e54cdec3da0..c691d85b3044 100644
> --- a/drivers/dma-buf/heaps/Makefile
> +++ b/drivers/dma-buf/heaps/Makefile
> @@ -2,3 +2,4 @@
>  obj-y					+= heap-helpers.o
>  obj-$(CONFIG_DMABUF_HEAPS_SYSTEM)	+= system_heap.o
>  obj-$(CONFIG_DMABUF_HEAPS_CMA)		+= cma_heap.o
> +obj-$(CONFIG_DMABUF_HEAPS_DEVICES)	+= device_heap.o
> diff --git a/drivers/dma-buf/heaps/device_heap.c b/drivers/dma-buf/heaps/device_heap.c
> new file mode 100644
> index 000000000000..1803dc622dd8
> --- /dev/null
> +++ b/drivers/dma-buf/heaps/device_heap.c
> @@ -0,0 +1,268 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * DMABUF Device DMA heap exporter
> + *
> + * Copyright (C) 2020, Collabora Ltd.
> + *
> + * Based on:
> + *   videobuf2-dma-contig.c - DMA contig memory allocator for videobuf2
> + *   Copyright (C) 2010 Samsung Electronics
> + */
> +
> +#include <linux/device.h>
> +#include <linux/dma-buf.h>
> +#include <linux/dma-heap.h>
> +#include <linux/dma-mapping.h>
> +#include <linux/scatterlist.h>
> +#include <linux/slab.h>
> +#include <linux/module.h>
> +
> +struct dev_dmabuf_attachment {
> +	struct sg_table sgt;
> +	enum dma_data_direction dma_dir;
> +};
> +
> +struct dev_dmabuf {
> +	struct dma_heap *heap;
> +	struct dma_buf *dmabuf;
> +	struct device *dev;
> +	size_t size;
> +	void *vaddr;
> +	dma_addr_t dma_addr;
> +	unsigned long attrs;
> +
> +	struct sg_table sgt;
> +};
> +
> +static struct sg_table *dev_dmabuf_ops_map(struct dma_buf_attachment *db_attach,
> +					   enum dma_data_direction dma_dir)
> +{
> +	struct dev_dmabuf_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 .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) {
> +		dev_err(db_attach->dev, "failed to map scatterlist\n");
> +		mutex_unlock(lock);
> +		return ERR_PTR(-EIO);
> +	}
> +
> +	attach->dma_dir = dma_dir;
> +
> +	mutex_unlock(lock);
> +
> +	return sgt;
> +}
> +
> +static void dev_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 int dev_dmabuf_ops_attach(struct dma_buf *dmabuf,
> +				 struct dma_buf_attachment *dbuf_attach)
> +{
> +	struct dev_dmabuf_attachment *attach;
> +	unsigned int i;
> +	struct scatterlist *rd, *wr;
> +	struct sg_table *sgt;
> +	struct dev_dmabuf *buf = dmabuf->priv;
> +	int ret;
> +
> +	attach = kzalloc(sizeof(*attach), GFP_KERNEL);
> +	if (!attach)
> +		return -ENOMEM;
> +	sgt = &attach->sgt;
> +
> +	/*
> +	 * Copy the buf->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.orig_nents, GFP_KERNEL);
> +	if (ret) {
> +		kfree(attach);
> +		return -ENOMEM;
> +	}
> +
> +	rd = buf->sgt.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 dev_dmabuf_ops_detach(struct dma_buf *dmabuf,
> +				  struct dma_buf_attachment *db_attach)
> +{
> +	struct dev_dmabuf_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 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.
> +		 *
> +		 * XXX: This needs a revisit.
> +		 */
> +		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 void *dev_dmabuf_ops_vmap(struct dma_buf *dmabuf)
> +{
> +	struct dev_dmabuf *buf = dmabuf->priv;
> +
> +	return buf->vaddr;
> +}
> +
> +static void dev_dmabuf_ops_release(struct dma_buf *dmabuf)
> +{
> +	struct dev_dmabuf *buf = dmabuf->priv;
> +
> +	sg_free_table(&buf->sgt);
> +	dma_free_attrs(buf->dev, buf->size, buf->vaddr,
> +		       buf->dma_addr, buf->attrs);
> +	put_device(buf->dev);
> +	kfree(buf);
> +}
> +
> +static int dev_dmabuf_ops_mmap(struct dma_buf *dmabuf,
> +			       struct vm_area_struct *vma)
> +{
> +	struct dev_dmabuf *buf = dmabuf->priv;
> +	int ret;
> +
> +	ret = dma_mmap_attrs(buf->dev, vma, buf->vaddr,
> +			     buf->dma_addr, buf->size,
> +			     buf->attrs);
> +	if (ret) {
> +		dev_err(buf->dev, "remapping memory failed, error: %d\n", ret);
> +		return ret;
> +	}
> +	vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;
> +
> +	return 0;
> +}
> +
> +static const struct dma_buf_ops dev_dmabuf_ops = {
> +	.attach = dev_dmabuf_ops_attach,
> +	.detach = dev_dmabuf_ops_detach,
> +	.map_dma_buf = dev_dmabuf_ops_map,
> +	.unmap_dma_buf = dev_dmabuf_ops_unmap,
> +	.vmap = dev_dmabuf_ops_vmap,
> +	.mmap = dev_dmabuf_ops_mmap,
> +	.release = dev_dmabuf_ops_release,
> +};
> +
> +static int dev_heap_allocate(struct dma_heap *heap,
> +			unsigned long size,
> +			unsigned long fd_flags,
> +			unsigned long heap_flags)
> +{
> +	struct device *dev = dma_heap_get_drvdata(heap);
> +	struct dev_dmabuf *buf;
> +	struct dma_buf_export_info exp_info = {};
> +	unsigned long attrs = 0;
> +	int ret = -ENOMEM;
> +
> +	buf = kzalloc(sizeof(*buf), GFP_KERNEL);
> +	if (!buf)
> +		return -ENOMEM;
> +
> +	buf->vaddr = dma_alloc_attrs(dev, size, &buf->dma_addr,
> +				     GFP_KERNEL, attrs);
> +	/* Prevent the device from being released while the buffer is used */
> +	buf->dev = get_device(dev);
> +	buf->heap = heap;
> +	buf->size = size;
> +	buf->attrs = attrs;
> +
> +	/* XXX: This call is documented as unsafe. See dma_get_sgtable_attrs(). */
> +	ret = dma_get_sgtable_attrs(buf->dev, &buf->sgt,
> +				    buf->vaddr, buf->dma_addr,
> +				    buf->size, buf->attrs);
> +	if (ret < 0) {
> +		dev_err(buf->dev, "failed to get scatterlist from DMA API\n");
> +		return ret;
> +	}
> +
> +	exp_info.exp_name = dev_name(dev);
> +	exp_info.owner = THIS_MODULE;
> +	exp_info.ops = &dev_dmabuf_ops;
> +	exp_info.size = size;
> +	exp_info.flags = fd_flags;
> +	exp_info.priv = buf;
> +
> +	buf->dmabuf = dma_buf_export(&exp_info);
> +	if (IS_ERR(buf->dmabuf)) {
> +		dev_err(buf->dev, "failed to export dmabuf\n");
> +		return PTR_ERR(buf->dmabuf);
> +	}
> +
> +	ret = dma_buf_fd(buf->dmabuf, fd_flags);
> +	if (ret < 0) {
> +		dev_err(buf->dev, "failed to get dmabuf fd: %d\n", ret);
> +		return ret;
> +	}
> +
> +	return ret;
> +}
> +
> +static const struct dma_heap_ops dev_heap_ops = {
> +	.allocate = dev_heap_allocate,
> +};
> +
> +void dev_dma_heap_add(struct device *dev)
> +{
> +	struct dma_heap_export_info exp_info;
> +
> +	exp_info.name = dev_name(dev);
> +	exp_info.ops = &dev_heap_ops;
> +	exp_info.priv = dev;
> +
> +	dev->heap = dma_heap_add(&exp_info);
> +}
> +EXPORT_SYMBOL(dev_dma_heap_add);
> diff --git a/include/linux/device.h b/include/linux/device.h
> index ca18da4768e3..1fae95d55ea1 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -45,6 +45,7 @@ struct iommu_ops;
>  struct iommu_group;
>  struct dev_pin_info;
>  struct dev_iommu;
> +struct dma_heap;
>  
>  /**
>   * struct subsys_interface - interfaces to device functions
> @@ -597,6 +598,10 @@ struct device {
>  	struct iommu_group	*iommu_group;
>  	struct dev_iommu	*iommu;
>  
> +#ifdef CONFIG_DMABUF_HEAPS_DEVICES
> +	struct dma_heap		*heap;
> +#endif
> +
>  	bool			offline_disabled:1;
>  	bool			offline:1;
>  	bool			of_node_reused:1;
> diff --git a/include/linux/dma-heap.h b/include/linux/dma-heap.h
> index 454e354d1ffb..dcf7cca2f487 100644
> --- a/include/linux/dma-heap.h
> +++ b/include/linux/dma-heap.h
> @@ -56,4 +56,10 @@ void *dma_heap_get_drvdata(struct dma_heap *heap);
>   */
>  struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info);
>  
> +#ifdef CONFIG_DMABUF_HEAPS_DEVICES
> +void dev_dma_heap_add(struct device *dev);
> +#else
> +static inline void dev_dma_heap_add(struct device *dev) {}
> +#endif
> +
>  #endif /* _DMA_HEAPS_H */
> -- 
> 2.27.0
> 
> _______________________________________________
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> dri-devel@xxxxxxxxxxxxxxxxxxxxx
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch



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