Re: [PATCH v14 02/56] media: videobuf2: Rework offset 'cookie' encoding pattern

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Le 08/11/2023 à 07:23, Tomasz Figa a écrit :
Hi Benjamin,

On Tue, Oct 31, 2023 at 05:30:10PM +0100, Benjamin Gaignard wrote:
Change how offset 'cookie' field value is computed to make possible
to use more buffers.
The maximum number of buffers depends of PAGE_SHIFT value and can
go up to 0x7fff when PAGE_SHIFT = 12.
With this encoding pattern we know the maximum number that a queue
could store so we can check it at  queue init time.
It also make easier and faster to find buffer and plane from using
the offset field.
Change __find_plane_by_offset() prototype to return the video buffer
itself rather than it index.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@xxxxxxxxxxxxx>
---
  .../media/common/videobuf2/videobuf2-core.c   | 73 +++++++++----------
  1 file changed, 35 insertions(+), 38 deletions(-)

Thanks a lot for working on this series. Just one comment inline.

diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
index a5e57affeb30..09be8e026044 100644
--- a/drivers/media/common/videobuf2/videobuf2-core.c
+++ b/drivers/media/common/videobuf2/videobuf2-core.c
@@ -31,6 +31,11 @@
#include <trace/events/vb2.h> +#define PLANE_INDEX_SHIFT (PAGE_SHIFT + 3)
+#define PLANE_INDEX_MASK	0x7
Could we somehow ensure that it indeed matches VIDEO_MAX_PLANES?

One thing that comes to my mind would be

#define PLANE_INDEX_BITS	3
#define PLANE_INDEX_SHIFT	(PAGE_SHIFT + PLANE_INDEX_BITS)
#define PLANE_INDEX_MASK	BIT_MASK(PLANE_INDEX_BITS)

BUILD_BUG_ON(BIT(PLANE_INDEX_BITS) != VIDEO_MAX_PLANES);

Alternatively we could change the videodev2.h, so that we have

#define VIDEO_MAX_PLANES_BITS	3
#define VIDEO_MAX_PLANES	(1U < VIDEO_MAX_PLANES_BITS)

and then we can just VIDEO_MAX_PLANES_BITS in our definitions in this file
and no checks would be necessary.

I would prefer this solution.
Hans should I do that in an other patch ?

Regards,
Benjamin


Best regards,
Tomasz

+#define MAX_BUFFER_INDEX	BIT_MASK(30 - PLANE_INDEX_SHIFT)
+#define BUFFER_INDEX_MASK	(MAX_BUFFER_INDEX - 1)
+
  static int debug;
  module_param(debug, int, 0644);
@@ -358,21 +363,27 @@ static void __setup_offsets(struct vb2_buffer *vb)
  	unsigned int plane;
  	unsigned long offset = 0;
- if (vb->index) {
-		struct vb2_buffer *prev = q->bufs[vb->index - 1];
-		struct vb2_plane *p = &prev->planes[prev->num_planes - 1];
-
-		offset = PAGE_ALIGN(p->m.offset + p->length);
-	}
+	/*
+	 * The offset "cookie" value has the following constraints:
+	 * - a buffer can have up to 8 planes.
+	 * - v4l2 mem2mem uses bit 30 to distinguish between
+	 *   OUTPUT (aka "source", bit 30 is 0) and
+	 *   CAPTURE (aka "destination", bit 30 is 1) buffers.
+	 * - must be page aligned
+	 * That led to this bit mapping when PAGE_SHIFT = 12:
+	 * |30                |29        15|14       12|11 0|
+	 * |DST_QUEUE_OFF_BASE|buffer index|plane index| 0  |
+	 * where there are 15 bits to store the buffer index.
+	 * Depending on PAGE_SHIFT value we can have fewer bits
+	 * to store the buffer index.
+	 */
+	offset = vb->index << PLANE_INDEX_SHIFT;
for (plane = 0; plane < vb->num_planes; ++plane) {
-		vb->planes[plane].m.offset = offset;
+		vb->planes[plane].m.offset = offset + (plane << PAGE_SHIFT);
dprintk(q, 3, "buffer %d, plane %d offset 0x%08lx\n",
  				vb->index, plane, offset);
-
-		offset += vb->planes[plane].length;
-		offset = PAGE_ALIGN(offset);
  	}
  }
@@ -2188,10 +2199,9 @@ EXPORT_SYMBOL_GPL(vb2_core_streamoff);
   * __find_plane_by_offset() - find plane associated with the given offset
   */
  static int __find_plane_by_offset(struct vb2_queue *q, unsigned long offset,
-			unsigned int *_buffer, unsigned int *_plane)
+			struct vb2_buffer **vb, unsigned int *plane)
  {
-	struct vb2_buffer *vb;
-	unsigned int buffer, plane;
+	unsigned int buffer;
/*
  	 * Sanity checks to ensure the lock is held, MEMORY_MMAP is
@@ -2209,24 +2219,15 @@ static int __find_plane_by_offset(struct vb2_queue *q, unsigned long offset,
  		return -EBUSY;
  	}
- /*
-	 * Go over all buffers and their planes, comparing the given offset
-	 * with an offset assigned to each plane. If a match is found,
-	 * return its buffer and plane numbers.
-	 */
-	for (buffer = 0; buffer < q->num_buffers; ++buffer) {
-		vb = q->bufs[buffer];
+	/* Get buffer and plane from the offset */
+	buffer = (offset >> PLANE_INDEX_SHIFT) & BUFFER_INDEX_MASK;
+	*plane = (offset >> PAGE_SHIFT) & PLANE_INDEX_MASK;
- for (plane = 0; plane < vb->num_planes; ++plane) {
-			if (vb->planes[plane].m.offset == offset) {
-				*_buffer = buffer;
-				*_plane = plane;
-				return 0;
-			}
-		}
-	}
+	if (buffer >= q->num_buffers || *plane >= q->bufs[buffer]->num_planes)
+		return -EINVAL;
- return -EINVAL;
+	*vb = q->bufs[buffer];
+	return 0;
  }
int vb2_core_expbuf(struct vb2_queue *q, int *fd, unsigned int type,
@@ -2306,7 +2307,7 @@ int vb2_mmap(struct vb2_queue *q, struct vm_area_struct *vma)
  {
  	unsigned long offset = vma->vm_pgoff << PAGE_SHIFT;
  	struct vb2_buffer *vb;
-	unsigned int buffer = 0, plane = 0;
+	unsigned int plane = 0;
  	int ret;
  	unsigned long length;
@@ -2335,12 +2336,10 @@ int vb2_mmap(struct vb2_queue *q, struct vm_area_struct *vma)
  	 * Find the plane corresponding to the offset passed by userspace. This
  	 * will return an error if not MEMORY_MMAP or file I/O is in progress.
  	 */
-	ret = __find_plane_by_offset(q, offset, &buffer, &plane);
+	ret = __find_plane_by_offset(q, offset, &vb, &plane);
  	if (ret)
  		goto unlock;
- vb = q->bufs[buffer];
-
  	/*
  	 * MMAP requires page_aligned buffers.
  	 * The buffer length was page_aligned at __vb2_buf_mem_alloc(),
@@ -2368,7 +2367,7 @@ int vb2_mmap(struct vb2_queue *q, struct vm_area_struct *vma)
  	if (ret)
  		return ret;
- dprintk(q, 3, "buffer %d, plane %d successfully mapped\n", buffer, plane);
+	dprintk(q, 3, "buffer %u, plane %d successfully mapped\n", vb->index, plane);
  	return 0;
  }
  EXPORT_SYMBOL_GPL(vb2_mmap);
@@ -2382,7 +2381,7 @@ unsigned long vb2_get_unmapped_area(struct vb2_queue *q,
  {
  	unsigned long offset = pgoff << PAGE_SHIFT;
  	struct vb2_buffer *vb;
-	unsigned int buffer, plane;
+	unsigned int plane;
  	void *vaddr;
  	int ret;
@@ -2392,12 +2391,10 @@ unsigned long vb2_get_unmapped_area(struct vb2_queue *q,
  	 * Find the plane corresponding to the offset passed by userspace. This
  	 * will return an error if not MEMORY_MMAP or file I/O is in progress.
  	 */
-	ret = __find_plane_by_offset(q, offset, &buffer, &plane);
+	ret = __find_plane_by_offset(q, offset, &vb, &plane);
  	if (ret)
  		goto unlock;
- vb = q->bufs[buffer];
-
  	vaddr = vb2_plane_vaddr(vb, plane);
  	mutex_unlock(&q->mmap_lock);
  	return vaddr ? (unsigned long)vaddr : -EINVAL;
--
2.39.2





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