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

[Hans Verkuil]

On 08/11/2023 11:14, Benjamin Gaignard wrote:
> 
> 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 ?

I prefer to not make changes in videodev2.h, to be honest. I think it is fine to
just add a check in videobuf2-core.c.

Regards,

	Hans

> 
> 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
>>>