videobuf-dma-contig: zero copy USERPTR support

Since videobuf-dma-contig is designed to handle physically contiguous
memory, this patch modifies the videobuf-dma-contig code to only accept a
user space pointer to physically contiguous memory.  For now only
VM_PFNMAP vmas are supported, so forget hotplug.

On SuperH Mobile we use this with our sh_mobile_ceu_camera driver together
with various multimedia accelerator blocks that are exported to user space
using UIO.  The UIO kernel code exports physically contiguous memory to
user space and lets the user space application mmap() this memory and pass
a pointer using the USERPTR interface for V4L2 zero copy operation.

With this approach we support zero copy capture, hardware scaling and
various forms of hardware encoding and decoding.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Magnus Damm <damm@igel.co.jp>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Paul Mundt <lethal@linux-sh.org>
Acked-by: Mauro Carvalho Chehab <mchehab@infradead.org>
Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/media/video/videobuf-dma-contig.c

@@ -17,6 +17,7 @@
 #include <linux/init.h>
 #include <linux/module.h>
 #include <linux/mm.h>
+#include <linux/pagemap.h>
 #include <linux/dma-mapping.h>
 #include <media/videobuf-dma-contig.h>
 
@@ -25,6 +26,7 @@ struct videobuf_dma_contig_memory {
 	void *vaddr;
 	dma_addr_t dma_handle;
 	unsigned long size;
+	int is_userptr;
 };
 
 #define MAGIC_DC_MEM 0x0733ac61
@@ -108,6 +110,82 @@ static struct vm_operations_struct videobuf_vm_ops = {
 	.close    = videobuf_vm_close,
 };
 
+/**
+ * videobuf_dma_contig_user_put() - reset pointer to user space buffer
+ * @mem: per-buffer private videobuf-dma-contig data
+ *
+ * This function resets the user space pointer
+ */
+static void videobuf_dma_contig_user_put(struct videobuf_dma_contig_memory *mem)
+{
+	mem->is_userptr = 0;
+	mem->dma_handle = 0;
+	mem->size = 0;
+}
+
+/**
+ * videobuf_dma_contig_user_get() - setup user space memory pointer
+ * @mem: per-buffer private videobuf-dma-contig data
+ * @vb: video buffer to map
+ *
+ * This function validates and sets up a pointer to user space memory.
+ * Only physically contiguous pfn-mapped memory is accepted.
+ *
+ * Returns 0 if successful.
+ */
+static int videobuf_dma_contig_user_get(struct videobuf_dma_contig_memory *mem,
+					struct videobuf_buffer *vb)
+{
+	struct mm_struct *mm = current->mm;
+	struct vm_area_struct *vma;
+	unsigned long prev_pfn, this_pfn;
+	unsigned long pages_done, user_address;
+	int ret;
+
+	mem->size = PAGE_ALIGN(vb->size);
+	mem->is_userptr = 0;
+	ret = -EINVAL;
+
+	down_read(&mm->mmap_sem);
+
+	vma = find_vma(mm, vb->baddr);
+	if (!vma)
+		goto out_up;
+
+	if ((vb->baddr + mem->size) > vma->vm_end)
+		goto out_up;
+
+	pages_done = 0;
+	prev_pfn = 0; /* kill warning */
+	user_address = vb->baddr;
+
+	while (pages_done < (mem->size >> PAGE_SHIFT)) {
+		ret = follow_pfn(vma, user_address, &this_pfn);
+		if (ret)
+			break;
+
+		if (pages_done == 0)
+			mem->dma_handle = this_pfn << PAGE_SHIFT;
+		else if (this_pfn != (prev_pfn + 1))
+			ret = -EFAULT;
+
+		if (ret)
+			break;
+
+		prev_pfn = this_pfn;
+		user_address += PAGE_SIZE;
+		pages_done++;
+	}
+
+	if (!ret)
+		mem->is_userptr = 1;
+
+ out_up:
+	up_read(&current->mm->mmap_sem);
+
+	return ret;
+}
+
 static void *__videobuf_alloc(size_t size)
 {
 	struct videobuf_dma_contig_memory *mem;
@@ -154,12 +232,11 @@ static int __videobuf_iolock(struct videobuf_queue *q,
 	case V4L2_MEMORY_USERPTR:
 		dev_dbg(q->dev, "%s memory method USERPTR\n", __func__);
 
-		/* The only USERPTR currently supported is the one needed for
-		   read() method.
-		 */
+		/* handle pointer from user space */
 		if (vb->baddr)
-			return -EINVAL;
+			return videobuf_dma_contig_user_get(mem, vb);
 
+		/* allocate memory for the read() method */
 		mem->size = PAGE_ALIGN(vb->size);
 		mem->vaddr = dma_alloc_coherent(q->dev, mem->size,
 						&mem->dma_handle, GFP_KERNEL);
@@ -400,14 +477,21 @@ void videobuf_dma_contig_free(struct videobuf_queue *q,
 	   So, it should free memory only if the memory were allocated for
 	   read() operation.
 	 */
-	if ((buf->memory != V4L2_MEMORY_USERPTR) || buf->baddr)
+	if (buf->memory != V4L2_MEMORY_USERPTR)
 		return;
 
 	if (!mem)
 		return;
 
 	MAGIC_CHECK(mem->magic, MAGIC_DC_MEM);
 
+	/* handle user space pointer case */
+	if (buf->baddr) {
+		videobuf_dma_contig_user_put(mem);
+		return;
+	}
+
+	/* read() method */
 	dma_free_coherent(q->dev, mem->size, mem->vaddr, mem->dma_handle);
 	mem->vaddr = NULL;
 }