[PATCH v2] media: vb2-dma-contig: configure DMA max segment size properly

[Marek Szyprowski <m.szyprowski@samsung.com>]

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>
---
Hello,

This patch is a follow-up of my previous attempts to let Exynos
multimedia devices to work properly with shared buffers when IOMMU is
enabled:
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:
http://lists.infradead.org/pipermail/linux-arm-kernel/2014-November/305913.html

Best regards,
Marek Szyprowski

changelog:
v2:
- 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 | 39 ++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/drivers/media/v4l2-core/videobuf2-dma-contig.c b/drivers/media/v4l2-core/videobuf2-dma-contig.c
index 461ae55eaa98..d0382d62954d 100644
--- a/drivers/media/v4l2-core/videobuf2-dma-contig.c
+++ b/drivers/media/v4l2-core/videobuf2-dma-contig.c
@@ -443,6 +443,36 @@ 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
+ * scatter-list 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.
+ */
+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
@@ -499,6 +529,10 @@ static void *vb2_dc_get_userptr(struct device *dev, unsigned long vaddr,
 		return ERR_PTR(-EINVAL);
 	}
 
+	ret = vb2_dc_set_max_seg_size(dev, PAGE_ALIGN(size + PAGE_SIZE));
+	if (!ret)
+		return ERR_PTR(ret);
+
 	buf = kzalloc(sizeof *buf, GFP_KERNEL);
 	if (!buf)
 		return ERR_PTR(-ENOMEM);
@@ -675,10 +709,15 @@ static void *vb2_dc_attach_dmabuf(struct device *dev, struct dma_buf *dbuf,
 {
 	struct vb2_dc_buf *buf;
 	struct dma_buf_attachment *dba;
+	int ret;
 
 	if (dbuf->size < size)
 		return ERR_PTR(-EFAULT);
 
+	ret = vb2_dc_set_max_seg_size(dev, PAGE_ALIGN(size));
+	if (!ret)
+		return ERR_PTR(ret);
+
 	buf = kzalloc(sizeof(*buf), GFP_KERNEL);
 	if (!buf)
 		return ERR_PTR(-ENOMEM);
-- 
1.9.2