[PATCH] staging: comedi: use dma_mmap_coherent for DMA-able buffer mmap

Ian Abbott abbotti at mev.co.uk
Tue Jun 25 11:26:59 UTC 2019


Comedi's acquisition buffer allocation code can allocate the buffer from
normal kernel memory or from DMA coherent memory depending on the
`dma_async_dir` value in the comedi subdevice.  (A value of `DMA_NONE`
causes the buffer to be allocated from normal kernel memory.  Other
values cause the buffer to be allocated from DMA coherent memory.)   The
buffer currently consists of a bunch of page-sized blocks that are
vmap'ed into a contiguous range of virtual addresses. The pages are also
mmap'able into user address space.  For DMA'able buffers, these
page-sized blocks are allocated by `dma_alloc_coherent()`.

For DMA-able buffers, the DMA API is currently abused in various ways,
the most serious abuse being the calling of `virt_to_page()` on the
blocks allocated by `dma_alloc_coherent()` and passing those pages to
`vmap()` (for mapping to the kernels vmalloc address space) and via
`page_to_pfn()` to `remap_pfn_range()` (for mmap'ing to user space).  it
also uses the `__GFP_COMP` flag when allocating the blocks, and marks
the allocated pages as reserved (which is unnecessary for DMA coherent
allocations).

The code can be changed to get rid of the vmap'ed address altogether if
necessary, since there are only a few places in the comedi code that use
the vmap'ed address directly and we also keep a list of the kernel
addresses for the individual pages prior to the vmap operation. This
would add some run-time overhead to buffer accesses.  The real killer is
the mmap operation.

For mmap, the address range specified in the VMA needs to be mmap'ed to
the individually allocated page-sized blocks.  That is not a problem
when the pages are allocated from normal kernel memory as the individual
pages can be remapped by `remap_pfn_range()`, but it is a problem when
the page-sized blocks are allocated by `dma_alloc_coherent()` because
the DMA API currently has no support for splitting a VMA across multiple
blocks of DMA coherent memory (or rather, no support for mapping part of
a VMA range to a single block of DMA coherent memory).

In order to comply with the DMA API and allow the buffer to be mmap'ed,
the buffer needs to be allocated as a single block by a single call to
`dma_alloc_coherent()`, freed by a single call to `dma_free_coherent()`,
and mmap'ed to user space by a single call to `dma_mmap_coherent()`.
This patch changes the buffer allocation, freeing, and mmap'ing code to
do that, with the unfortunate consequence that buffer allocation is more
likely to fail.  It also no longer uses the `__GFP_COMP` flag when
allocating DMA coherent memory, no longer marks the
allocated pages of DMA coherent memory as reserved, and no longer vmap's
the DMA coherent memory pages (since they are contiguous anyway).

Cc: Christoph Hellwig <hch at lst.de>
Signed-off-by: Ian Abbott <abbotti at mev.co.uk>
---
 drivers/staging/comedi/comedi_buf.c  | 150 ++++++++++++++++++---------
 drivers/staging/comedi/comedi_fops.c |  39 ++++---
 2 files changed, 125 insertions(+), 64 deletions(-)

diff --git a/drivers/staging/comedi/comedi_buf.c b/drivers/staging/comedi/comedi_buf.c
index d2c8cc72a99d..3ef3ddabf139 100644
--- a/drivers/staging/comedi/comedi_buf.c
+++ b/drivers/staging/comedi/comedi_buf.c
@@ -27,18 +27,19 @@ static void comedi_buf_map_kref_release(struct kref *kref)
 	unsigned int i;
 
 	if (bm->page_list) {
-		for (i = 0; i < bm->n_pages; i++) {
-			buf = &bm->page_list[i];
-			clear_bit(PG_reserved,
-				  &(virt_to_page(buf->virt_addr)->flags));
-			if (bm->dma_dir != DMA_NONE) {
-#ifdef CONFIG_HAS_DMA
-				dma_free_coherent(bm->dma_hw_dev,
-						  PAGE_SIZE,
-						  buf->virt_addr,
-						  buf->dma_addr);
-#endif
-			} else {
+		if (bm->dma_dir != DMA_NONE) {
+			/*
+			 * DMA buffer was allocated as a single block.
+			 * Address is in page_list[0].
+			 */
+			buf = &bm->page_list[0];
+			dma_free_coherent(bm->dma_hw_dev,
+					  PAGE_SIZE * bm->n_pages,
+					  buf->virt_addr, buf->dma_addr);
+		} else {
+			for (i = 0; i < bm->n_pages; i++) {
+				buf = &bm->page_list[i];
+				ClearPageReserved(virt_to_page(buf->virt_addr));
 				free_page((unsigned long)buf->virt_addr);
 			}
 		}
@@ -57,7 +58,8 @@ static void __comedi_buf_free(struct comedi_device *dev,
 	unsigned long flags;
 
 	if (async->prealloc_buf) {
-		vunmap(async->prealloc_buf);
+		if (s->async_dma_dir == DMA_NONE)
+			vunmap(async->prealloc_buf);
 		async->prealloc_buf = NULL;
 		async->prealloc_bufsz = 0;
 	}
@@ -69,6 +71,72 @@ static void __comedi_buf_free(struct comedi_device *dev,
 	comedi_buf_map_put(bm);
 }
 
+static struct comedi_buf_map *
+comedi_buf_map_alloc(struct comedi_device *dev, enum dma_data_direction dma_dir,
+		     unsigned int n_pages)
+{
+	struct comedi_buf_map *bm;
+	struct comedi_buf_page *buf;
+	unsigned int i;
+
+	bm = kzalloc(sizeof(*bm), GFP_KERNEL);
+	if (!bm)
+		return NULL;
+
+	kref_init(&bm->refcount);
+	bm->dma_dir = dma_dir;
+	if (bm->dma_dir != DMA_NONE) {
+		/* Need ref to hardware device to free buffer later. */
+		bm->dma_hw_dev = get_device(dev->hw_dev);
+	}
+
+	bm->page_list = vzalloc(sizeof(*buf) * n_pages);
+	if (!bm->page_list)
+		goto err;
+
+	if (bm->dma_dir != DMA_NONE) {
+		void *virt_addr;
+		dma_addr_t dma_addr;
+
+		/*
+		 * Currently, the DMA buffer needs to be allocated as a
+		 * single block so that it can be mmap()'ed.
+		 */
+		virt_addr = dma_alloc_coherent(bm->dma_hw_dev,
+					       PAGE_SIZE * n_pages, &dma_addr,
+					       GFP_KERNEL);
+		if (!virt_addr)
+			goto err;
+
+		for (i = 0; i < n_pages; i++) {
+			buf = &bm->page_list[i];
+			buf->virt_addr = virt_addr + (i << PAGE_SHIFT);
+			buf->dma_addr = dma_addr + (i << PAGE_SHIFT);
+		}
+
+		bm->n_pages = i;
+	} else {
+		for (i = 0; i < n_pages; i++) {
+			buf = &bm->page_list[i];
+			buf->virt_addr = (void *)get_zeroed_page(GFP_KERNEL);
+			if (!buf->virt_addr)
+				break;
+
+			SetPageReserved(virt_to_page(buf->virt_addr));
+		}
+
+		bm->n_pages = i;
+		if (i < n_pages)
+			goto err;
+	}
+
+	return bm;
+
+err:
+	comedi_buf_map_put(bm);
+	return NULL;
+}
+
 static void __comedi_buf_alloc(struct comedi_device *dev,
 			       struct comedi_subdevice *s,
 			       unsigned int n_pages)
@@ -86,57 +154,37 @@ static void __comedi_buf_alloc(struct comedi_device *dev,
 		return;
 	}
 
-	bm = kzalloc(sizeof(*async->buf_map), GFP_KERNEL);
+	bm = comedi_buf_map_alloc(dev, s->async_dma_dir, n_pages);
 	if (!bm)
 		return;
 
-	kref_init(&bm->refcount);
 	spin_lock_irqsave(&s->spin_lock, flags);
 	async->buf_map = bm;
 	spin_unlock_irqrestore(&s->spin_lock, flags);
-	bm->dma_dir = s->async_dma_dir;
-	if (bm->dma_dir != DMA_NONE)
-		/* Need ref to hardware device to free buffer later. */
-		bm->dma_hw_dev = get_device(dev->hw_dev);
 
-	bm->page_list = vzalloc(sizeof(*buf) * n_pages);
-	if (bm->page_list)
+	if (bm->dma_dir != DMA_NONE) {
+		/*
+		 * DMA buffer was allocated as a single block.
+		 * Address is in page_list[0].
+		 */
+		buf = &bm->page_list[0];
+		async->prealloc_buf = buf->virt_addr;
+	} else {
 		pages = vmalloc(sizeof(struct page *) * n_pages);
+		if (!pages)
+			return;
 
-	if (!pages)
-		return;
-
-	for (i = 0; i < n_pages; i++) {
-		buf = &bm->page_list[i];
-		if (bm->dma_dir != DMA_NONE)
-#ifdef CONFIG_HAS_DMA
-			buf->virt_addr = dma_alloc_coherent(bm->dma_hw_dev,
-							    PAGE_SIZE,
-							    &buf->dma_addr,
-							    GFP_KERNEL |
-							    __GFP_COMP);
-#else
-			break;
-#endif
-		else
-			buf->virt_addr = (void *)get_zeroed_page(GFP_KERNEL);
-		if (!buf->virt_addr)
-			break;
-
-		set_bit(PG_reserved, &(virt_to_page(buf->virt_addr)->flags));
-
-		pages[i] = virt_to_page(buf->virt_addr);
-	}
-	spin_lock_irqsave(&s->spin_lock, flags);
-	bm->n_pages = i;
-	spin_unlock_irqrestore(&s->spin_lock, flags);
+		for (i = 0; i < n_pages; i++) {
+			buf = &bm->page_list[i];
+			pages[i] = virt_to_page(buf->virt_addr);
+		}
 
-	/* vmap the prealloc_buf if all the pages were allocated */
-	if (i == n_pages)
+		/* vmap the pages to prealloc_buf */
 		async->prealloc_buf = vmap(pages, n_pages, VM_MAP,
 					   COMEDI_PAGE_PROTECTION);
 
-	vfree(pages);
+		vfree(pages);
+	}
 }
 
 void comedi_buf_map_get(struct comedi_buf_map *bm)
diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
index f6d1287c7b83..08d1bbbebf2d 100644
--- a/drivers/staging/comedi/comedi_fops.c
+++ b/drivers/staging/comedi/comedi_fops.c
@@ -2301,11 +2301,12 @@ static int comedi_mmap(struct file *file, struct vm_area_struct *vma)
 	struct comedi_subdevice *s;
 	struct comedi_async *async;
 	struct comedi_buf_map *bm = NULL;
+	struct comedi_buf_page *buf;
 	unsigned long start = vma->vm_start;
 	unsigned long size;
 	int n_pages;
 	int i;
-	int retval;
+	int retval = 0;
 
 	/*
 	 * 'trylock' avoids circular dependency with current->mm->mmap_sem
@@ -2361,24 +2362,36 @@ static int comedi_mmap(struct file *file, struct vm_area_struct *vma)
 		retval = -EINVAL;
 		goto done;
 	}
-	for (i = 0; i < n_pages; ++i) {
-		struct comedi_buf_page *buf = &bm->page_list[i];
+	if (bm->dma_dir != DMA_NONE) {
+		/*
+		 * DMA buffer was allocated as a single block.
+		 * Address is in page_list[0].
+		 */
+		buf = &bm->page_list[0];
+		retval = dma_mmap_coherent(bm->dma_hw_dev, vma, buf->virt_addr,
+					   buf->dma_addr, n_pages * PAGE_SIZE);
+	} else {
+		for (i = 0; i < n_pages; ++i) {
+			unsigned long pfn;
+
+			buf = &bm->page_list[i];
+			pfn = page_to_pfn(virt_to_page(buf->virt_addr));
+			retval = remap_pfn_range(vma, start, pfn, PAGE_SIZE,
+						 PAGE_SHARED);
+			if (retval)
+				break;
 
-		if (remap_pfn_range(vma, start,
-				    page_to_pfn(virt_to_page(buf->virt_addr)),
-				    PAGE_SIZE, PAGE_SHARED)) {
-			retval = -EAGAIN;
-			goto done;
+			start += PAGE_SIZE;
 		}
-		start += PAGE_SIZE;
 	}
 
-	vma->vm_ops = &comedi_vm_ops;
-	vma->vm_private_data = bm;
+	if (retval == 0) {
+		vma->vm_ops = &comedi_vm_ops;
+		vma->vm_private_data = bm;
 
-	vma->vm_ops->open(vma);
+		vma->vm_ops->open(vma);
+	}
 
-	retval = 0;
 done:
 	up_read(&dev->attach_lock);
 	comedi_buf_map_put(bm);	/* put reference to buf map - okay if NULL */
-- 
2.20.1



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