[v4] media: vb2-dma-contig: configure DMA max segment size properly

Message ID 1462352403-27418-1-git-send-email-m.szyprowski@samsung.com
State New
Headers show

Commit Message

Marek Szyprowski May 4, 2016, 9 a.m.
This patch lets vb2-dma-contig memory allocator to configure DMA max
segment size properly for the client device. Setting it is needed to let
DMA-mapping subsystem to create a single, contiguous mapping in DMA
address space. This is essential for all devices, which use dma-contig
videobuf2 memory allocator and shared buffers (in USERPTR or DMAbuf modes
of operations).

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>


This patch is a follow-up of my previous attempts to let Exynos
multimedia devices to work properly with shared buffers when IOMMU is
1. https://www.mail-archive.com/linux-media@vger.kernel.org/msg96946.html
2. http://thread.gmane.org/gmane.linux.drivers.video-input-infrastructure/97316
3. https://patchwork.linuxtv.org/patch/30870/

As sugested by Hans, configuring DMA max segment size should be done by
videobuf2-dma-contig module instead of requiring all device drivers to
do it on their own.

Here is some backgroud why this is done in videobuf2-dc not in the
respective generic bus code:

Best regards,
Marek Szyprowski

- rebased onto media master tree
- call vb2_dc_set_max_seg_size after allocating vb2 buf object

- added FIXME note about possible memory leak

- fixes typos and other language issues in the comments

v1: http://article.gmane.org/gmane.linux.kernel.samsung-soc/53690
 drivers/media/v4l2-core/videobuf2-dma-contig.c | 53 +++++++++++++++++++++++++-
 1 file changed, 51 insertions(+), 2 deletions(-)


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diff --git a/drivers/media/v4l2-core/videobuf2-dma-contig.c b/drivers/media/v4l2-core/videobuf2-dma-contig.c
index 5361197f3e57..6291842a889f 100644
--- a/drivers/media/v4l2-core/videobuf2-dma-contig.c
+++ b/drivers/media/v4l2-core/videobuf2-dma-contig.c
@@ -448,6 +448,42 @@  static void vb2_dc_put_userptr(void *buf_priv)
+ * 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 increases 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. It is done here to avoid fixing
+ * all the vb2-dc client drivers.
+ *
+ * FIXME: the allocated dma_params structure is leaked because there
+ * is completely no way to determine when to free it (dma_params might have
+ * been also already allocated by the bus code). However in typical
+ * use cases this function will be called for platform devices, which are
+ * not hot-plugged and exist all the time in the target system.
+ */
+static int vb2_dc_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;
  * For some kind of reserved memory there might be no struct page available,
  * so all that can be done to support such 'pages' is to try to convert
  * pfn to dma address or at the last resort just assume that
@@ -509,6 +545,10 @@  static void *vb2_dc_get_userptr(void *alloc_ctx, unsigned long vaddr,
 	if (!buf)
 		return ERR_PTR(-ENOMEM);
+	ret = vb2_dc_set_max_seg_size(conf->dev, PAGE_ALIGN(size + PAGE_SIZE));
+	if (!ret)
+		goto fail_buf;
 	buf->dev = conf->dev;
 	buf->dma_dir = dma_dir;
@@ -682,6 +722,7 @@  static void *vb2_dc_attach_dmabuf(void *alloc_ctx, struct dma_buf *dbuf,
 	struct vb2_dc_conf *conf = alloc_ctx;
 	struct vb2_dc_buf *buf;
 	struct dma_buf_attachment *dba;
+	int ret;
 	if (dbuf->size < size)
 		return ERR_PTR(-EFAULT);
@@ -690,13 +731,17 @@  static void *vb2_dc_attach_dmabuf(void *alloc_ctx, struct dma_buf *dbuf,
 	if (!buf)
 		return ERR_PTR(-ENOMEM);
+	ret = vb2_dc_set_max_seg_size(conf->dev, PAGE_ALIGN(size));
+	if (!ret)
+		goto fail_buf;
 	buf->dev = conf->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;
+		ret = PTR_ERR(dba);
+		goto fail_buf;
 	buf->dma_dir = dma_dir;
@@ -704,6 +749,10 @@  static void *vb2_dc_attach_dmabuf(void *alloc_ctx, struct dma_buf *dbuf,
 	buf->db_attach = dba;
 	return buf;
+	kfree(buf);
+	return ERR_PTR(ret);