Support for zerocopy to DSP on OMAP3

Hoi,

[Please CC me, I'm not subscribed.]

I'm implementing zerocopy transfer from a v4l2 camera to the DSP on an
OMAP3 (based on earlier work by Stefan Kost [1][2]). Therefore I'm using V4L2_MEMORY_USERPTR to pass in the memory area allocated by TI's DMAI driver. However, this has flags VM_IO | VM_PFNMAP. This means that it is not possible to do get_user_pages() on it - it's an area that is not pageable and possibly even doesn't pass the MMU.

In order to support this kind of zerocopy construct, I propose to add
checks for VM_IO | VM_PFNMAP and only get pages from areas that don't have these flags set.

If I get positive feedback on this, I'll supply a patch.

[1] https://bugzilla.gnome.org/show_bug.cgi?id=583890
[2] http://thread.gmane.org/gmane.linux.drivers.video-input-
infrastructure/6209

Arnout Vandecappelle                               arnout at mind be
Senior Embedded Software Architect                 +32-16-286540
Essensium/Mind                                     http://www.mind.be
G.Geenslaan 9, 3001 Leuven, Belgium                BE 872 984 063 RPR Leuven
LinkedIn profile: http://www.linkedin.com/in/arnoutvandecappelle
GPG fingerprint:  31BB CF53 8660 6F88 345D  54CC A836 5879 20D7 CF43

Here's a first attempt at allowing IO memory for USERPTR buffers.

It also fixes another issue: it was assumed that
dma->sglen == dma->nr_pages. I'll split that up in a separate patch in the final version.

Regards,
Arnout

Arnout Vandecappelle                               arnout at mind be
Senior Embedded Software Architect                 +32-16-286540
Essensium/Mind                                     http://www.mind.be
G.Geenslaan 9, 3001 Leuven, Belgium                BE 872 984 063 RPR Leuven
LinkedIn profile: http://www.linkedin.com/in/arnoutvandecappelle
GPG fingerprint:  31BB CF53 8660 6F88 345D  54CC A836 5879 20D7 CF43

Hi Arnout,

-EMISSINGPATCH :-)

Please split the patch and CC Sakari Ailus when you submit them.

Regards,

Laurent Pinchart

Here is an updated patch, properly split up and with commit messages.

I've tested it on my OMAP3 board an it works. I don't have anything else
to test it with.

Regards,
Arnout

[PATCH 1/2] V4L/DVB: buf-dma-sg.c: don't assume nr_pages == sglen [PATCH 2/2] V4L/DVB: buf-dma-sg.c: support non-pageable user-allocated memory

videobuf_pages_to_sg() and videobuf_vmalloc_to_sg() happen to create a scatterlist element for every page. However, this is not true for bus addresses, so other functions shouldn't rely on the length of the scatter list being equal to nr_pages.

---
 drivers/media/video/videobuf-dma-sg.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

index da1790e..3b6f1b8 100644
@@ -244,7 +244,7 @@ int videobuf_dma_map(struct videobuf_queue* q, struct videobuf_dmabuf *dma)
 	}
 	if (!dma->bus_addr) {
 		dma->sglen = dma_map_sg(q->dev, dma->sglist,
-					dma->nr_pages, dma->direction);
+					dma->sglen, dma->direction);
 		if (0 == dma->sglen) {
 			printk(KERN_WARNING
 			       "%s: videobuf_map_sg failed\n",__func__);

@@ -262,7 +262,7 @@ int videobuf_dma_sync(struct videobuf_queue *q, struct videobuf_dmabuf *dma)
 	MAGIC_CHECK(dma->magic, MAGIC_DMABUF);
 	BUG_ON(!dma->sglen);
 
-	dma_sync_sg_for_cpu(q->dev, dma->sglist, dma->nr_pages, dma->direction);
+	dma_sync_sg_for_cpu(q->dev, dma->sglist, dma->sglen, dma->direction);
 	return 0;
 }
 

@@ -272,7 +272,7 @@ int videobuf_dma_unmap(struct videobuf_queue* q,struct videobuf_dmabuf *dma)
 	if (!dma->sglen)
 		return 0;
 
-	dma_unmap_sg(q->dev, dma->sglist, dma->nr_pages, dma->direction);
+	dma_unmap_sg(q->dev, dma->sglist, dma->sglen, dma->direction);
 
 	kfree(dma->sglist);
 	dma->sglist = NULL;

1.6.3.3

Hi Arnout,

Sakari Ailus
[email protected]

It's assigned there exactly - nr_pages shouldn't have been replaced there.

Updated patches follow.

Regards,
Arnout

Arnout Vandecappelle                               arnout at mind be
Senior Embedded Software Architect                 +32-16-286540
Essensium/Mind                                     http://www.mind.be
G.Geenslaan 9, 3001 Leuven, Belgium                BE 872 984 063 RPR Leuven
LinkedIn profile: http://www.linkedin.com/in/arnoutvandecappelle
GPG fingerprint:  31BB CF53 8660 6F88 345D  54CC A836 5879 20D7 CF43

videobuf_dma_init_user_locked() uses get_user_pages() to get the virtual-to-physical address mapping for user-allocated memory. However, the user-allocated memory may be non-pageable because it is an I/O range or similar. get_user_pages() fails with -EFAULT in that case.

If the user-allocated memory is physically contiguous, the approach of V4L2_MEMORY_OVERLAY can be used. If it is not, -EFAULT is still returned.

---
 drivers/media/video/videobuf-dma-sg.c |   18 ++++++++++++++++++
 1 files changed, 18 insertions(+), 0 deletions(-)

index 3b6f1b8..7884207 100644
@@ -136,6 +136,7 @@ static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
 {
 	unsigned long first,last;
 	int err, rw = 0;
+	struct vm_area_struct *vma;
 
 	dma->direction = direction;
 	switch (dma->direction) {

@@ -153,6 +154,23 @@ static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
 	last  = ((data+size-1) & PAGE_MASK) >> PAGE_SHIFT;
 	dma->offset   = data & ~PAGE_MASK;
 	dma->nr_pages = last-first+1;
+
+	/* In case the buffer is user-allocated and is actually an IO buffer for
+	   some other hardware, we cannot map pages for it.  It in fact behaves
+	   the same as an overlay. */
+	vma = find_vma (current->mm, data);
+	if (vma && (vma->vm_flags & VM_IO)) {
+		/* Only a single contiguous buffer is supported. */
+		if (vma->vm_end < data + size) {
+			dprintk(1, "init user: non-contiguous IO buffer.\n");
+			return -EFAULT; /* same error that get_user_pages() would give */
+		}
+		dma->bus_addr = (vma->vm_pgoff << PAGE_SHIFT) +	(data - vma->vm_start);
+		dprintk(1,"init user IO [0x%lx+0x%lx => %d pages at 0x%x]\n",
+			data, size, dma->nr_pages, dma->bus_addr);
+		return 0;
+	}
+
 	dma->pages = kmalloc(dma->nr_pages * sizeof(struct page*),
 			     GFP_KERNEL);
 	if (NULL == dma->pages)

1.6.3.3