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authorMarek Szyprowski <m.szyprowski@samsung.com>2016-05-24 09:16:06 +0200
committerSylwester Nawrocki <s.nawrocki@samsung.com>2016-06-03 11:12:50 +0200
commit3f03396918962b2f8b888d02b23cd1e0c88bf5e5 (patch)
parentc974c436eaf406225b7f4c5d27ae033a12667a38 (diff)
media: vb2-dma-contig: add helper for setting dma max seg size
Add a helper function for device drivers to set DMA's max_seg_size. Setting it to largest possible value lets DMA-mapping API always create contiguous mappings in DMA address space. This is essential for all devices, which use dma-contig videobuf2 memory allocator and shared buffers. Till now, the only case when vb2-dma-contig really 'worked' was a case where userspace provided USERPTR buffer, which was in fact mmaped contiguous buffer from the other v4l2/drm device. Also DMABUF made of contiguous buffer worked only when its exporter did not split it into several chunks in the scatter-list. Any other buffer failed, regardless of the arch/platform used and the presence of the IOMMU of the device bus. This patch provides interface to fix this issue. Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com> Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
2 files changed, 55 insertions, 0 deletions
diff --git a/drivers/media/v4l2-core/videobuf2-dma-contig.c b/drivers/media/v4l2-core/videobuf2-dma-contig.c
index 5361197f3e57..e3e47ace7daf 100644
--- a/drivers/media/v4l2-core/videobuf2-dma-contig.c
+++ b/drivers/media/v4l2-core/videobuf2-dma-contig.c
@@ -753,6 +753,59 @@ void vb2_dma_contig_cleanup_ctx(void *alloc_ctx)
+ * vb2_dma_contig_set_max_seg_size() - configure DMA max segment size
+ * @dev: device for configuring DMA parameters
+ * @size: size of DMA max segment size to set
+ *
+ * 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 sets 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.
+ * This function should be called from the drivers, which are known to
+ * operate on platforms with IOMMU and provide access to shared buffers
+ * (either USERPTR or DMABUF). This should be done before initializing
+ * videobuf2 queue.
+ */
+int vb2_dma_contig_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;
+ * vb2_dma_contig_clear_max_seg_size() - release resources for DMA parameters
+ * @dev: device for configuring DMA parameters
+ *
+ * This function releases resources allocated to configure DMA parameters
+ * (see vb2_dma_contig_set_max_seg_size() function). It should be called from
+ * device drivers on driver remove.
+ */
+void vb2_dma_contig_clear_max_seg_size(struct device *dev)
+ kfree(dev->dma_parms);
+ dev->dma_parms = NULL;
MODULE_DESCRIPTION("DMA-contig memory handling routines for videobuf2");
MODULE_AUTHOR("Pawel Osciak <pawel@osciak.com>");
diff --git a/include/media/videobuf2-dma-contig.h b/include/media/videobuf2-dma-contig.h
index 2087c9a68be3..f7dc8401817e 100644
--- a/include/media/videobuf2-dma-contig.h
+++ b/include/media/videobuf2-dma-contig.h
@@ -35,6 +35,8 @@ static inline void *vb2_dma_contig_init_ctx(struct device *dev)
void vb2_dma_contig_cleanup_ctx(void *alloc_ctx);
+int vb2_dma_contig_set_max_seg_size(struct device *dev, unsigned int size);
+void vb2_dma_contig_clear_max_seg_size(struct device *dev);
extern const struct vb2_mem_ops vb2_dma_contig_memops;

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