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

Benjamin Gaignard
2023-11-08T10:14:44Z

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@collabora.com>
>> ---
>> .../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
>>