Re: [RFC] Experimental DMA-BUF Device Heaps

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On Tue, Sep 08, 2020 at 08:43:59AM +0300, Laurent Pinchart wrote:
> Hi Daniel,
> 
> On Tue, Sep 01, 2020 at 09:32:22AM +0200, Daniel Vetter wrote:
> > 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.
> 
> We however don't always have a need for a dmabuf, as not all buffers are
> meant to be shared (and we often can't tell beforehand, at allocation
> time, if a given buffer will be shared or not). While the overhead of
> allocating a dmabuf should be file, assigning a file descriptor to each
> buffer would be prohibitely expensive.
> 
> We would need to decouple the dma heaps from file descriptors. I think
> that's doable, but it changes the philosophy of dma heaps and needs
> careful consideration. In particular, one may wonder what that would
> bring us, compared to the DMA mapping API for instance.

Oh I don't mean that we use the dma-buf heaps interfaces, but the backend.
Underneath it's all a struct dma_buf, that's where I figured we should
intercept the code sharing. Definitely not file descriptor handles.

> > 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.
> 
> sysfs won't work I'm afraid, as constraints may depend on the format for
> instace. We need subsystem-specific APIs to expose constraints.

Yeah sysfs would be the default, and maybe a list of useable heaps that
the subsystem api then tells you which to use when.
-Daniel

> 
> > - 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.
> > 
> > > ---
> > >  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 */
> 
> -- 
> Regards,
> 
> Laurent Pinchart

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