Re: [PATCH 1/2] media: videobuf2: use dmabuf size for length

[Helen Koike <helen.koike@collabora.com>]

Hi John,

On 3/25/21 7:20 AM, John Cox wrote:
> Hi
> 
>> Always use dmabuf size when considering the length of the buffer.
>> Discard userspace provided length.
>> Fix length check error in _verify_length(), which was handling single and
>> multiplanar diferently, and also not catching the case where userspace
>> provides a bigger length and bytesused then the underlying buffer.
>>
>> Suggested-by: Hans Verkuil <hverkuil@xs4all.nl>
>> Signed-off-by: Helen Koike <helen.koike@collabora.com>
>> ---
>>
>> Hello,
>>
>> As discussed on
>> https://patchwork.linuxtv.org/project/linux-media/patch/gh5kef5bkeel3o6b2dkgc2dfagu9klj4c0@4ax.com/
>>
>> This patch also helps the conversion layer of the Ext API patchset,
>> where we are not exposing the length field.
>>
>> It was discussed that userspace might use a smaller length field to
>> limit the usage of the underlying buffer, but I'm not sure if this is
>> really usefull and just complicates things.
>>
>> If this is usefull, then we should also expose a length field in the Ext
>> API, and document this feature properly.
>>
>> What do you think?
>> ---
>> .../media/common/videobuf2/videobuf2-core.c   | 21 ++++++++++++++++---
>> .../media/common/videobuf2/videobuf2-v4l2.c   |  8 +++----
>> include/uapi/linux/videodev2.h                |  7 +++++--
>> 3 files changed, 27 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
>> index 02281d13505f..2cbde14af051 100644
>> --- a/drivers/media/common/videobuf2/videobuf2-core.c
>> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
>> @@ -1205,6 +1205,7 @@ static int __prepare_dmabuf(struct vb2_buffer *vb)
>>
>> 	for (plane = 0; plane < vb->num_planes; ++plane) {
>> 		struct dma_buf *dbuf = dma_buf_get(planes[plane].m.fd);
>> +		unsigned int bytesused;
>>
>> 		if (IS_ERR_OR_NULL(dbuf)) {
>> 			dprintk(q, 1, "invalid dmabuf fd for plane %d\n",
>> @@ -1213,9 +1214,23 @@ static int __prepare_dmabuf(struct vb2_buffer *vb)
>> 			goto err;
>> 		}
>>
>> -		/* use DMABUF size if length is not provided */
>> -		if (planes[plane].length == 0)
>> -			planes[plane].length = dbuf->size;
>> +		planes[plane].length = dbuf->size;
>> +		bytesused = planes[plane].bytesused ?
>> +			    planes[plane].bytesused : dbuf->size;
>> +
>> +		if (planes[plane].bytesused > planes[plane].length) {
>> +			dprintk(q, 1, "bytesused is bigger then dmabuf length for plane %d\n",
>> +				plane);
>> +			ret = -EINVAL;
>> +			goto err;
>> +		}
>> +
>> +		if (planes[plane].data_offset >= bytesused) {
>> +			dprintk(q, 1, "data_offset >= bytesused for plane %d\n",
>> +				plane);
>> +			ret = -EINVAL;
>> +			goto err;
>> +		}
>>
>> 		if (planes[plane].length < vb->planes[plane].min_length) {
>> 			dprintk(q, 1, "invalid dmabuf length %u for plane %d, minimum length %u\n",
>> diff --git a/drivers/media/common/videobuf2/videobuf2-v4l2.c b/drivers/media/common/videobuf2/videobuf2-v4l2.c
>> index 7e96f67c60ba..ffc7ed46f74a 100644
>> --- a/drivers/media/common/videobuf2/videobuf2-v4l2.c
>> +++ b/drivers/media/common/videobuf2/videobuf2-v4l2.c
>> @@ -98,14 +98,14 @@ static int __verify_length(struct vb2_buffer *vb, const struct v4l2_buffer *b)
>> 	unsigned int bytesused;
>> 	unsigned int plane;
>>
>> -	if (V4L2_TYPE_IS_CAPTURE(b->type))
>> +	/* length check for dmabuf is performed in _prepare_dmabuf() */
>> +	if (V4L2_TYPE_IS_CAPTURE(b->type) || b->memory == VB2_MEMORY_DMABUF)
>> 		return 0;
>>
>> 	if (V4L2_TYPE_IS_MULTIPLANAR(b->type)) {
>> 		for (plane = 0; plane < vb->num_planes; ++plane) {
>> -			length = (b->memory == VB2_MEMORY_USERPTR ||
>> -				  b->memory == VB2_MEMORY_DMABUF)
>> -			       ? b->m.planes[plane].length
>> +			length = b->memory == VB2_MEMORY_USERPTR
>> +				? b->m.planes[plane].length
>> 				: vb->planes[plane].length;
>> 			bytesused = b->m.planes[plane].bytesused
>> 				  ? b->m.planes[plane].bytesused : length;
>> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
>> index 8d15f6ccc4b4..79b3b2893513 100644
>> --- a/include/uapi/linux/videodev2.h
>> +++ b/include/uapi/linux/videodev2.h
>> @@ -968,7 +968,9 @@ struct v4l2_requestbuffers {
>> /**
>>   * struct v4l2_plane - plane info for multi-planar buffers
>>   * @bytesused:		number of bytes occupied by data in the plane (payload)
>> - * @length:		size of this plane (NOT the payload) in bytes
>> + * @length:		size of this plane (NOT the payload) in bytes. Filled
>> + *			by userspace for USERPTR and by the driver for DMABUF
>> + *			and MMAP.
>>   * @mem_offset:		when memory in the associated struct v4l2_buffer is
>>   *			V4L2_MEMORY_MMAP, equals the offset from the start of
>>   *			the device memory for this plane (or is a "cookie" that
>> @@ -1025,7 +1027,8 @@ struct v4l2_plane {
>>   * @m:		union of @offset, @userptr, @planes and @fd
>>   * @length:	size in bytes of the buffer (NOT its payload) for single-plane
>>   *		buffers (when type != *_MPLANE); number of elements in the
>> - *		planes array for multi-plane buffers
>> + *		planes array for multi-plane buffers. Filled by userspace for
>> + *		USERPTR and by the driver for DMABUF and MMAP.
>>   * @reserved2:	drivers and applications must zero this field
>>   * @request_fd: fd of the request that this buffer should use
>>   * @reserved:	for backwards compatibility with applications that do not know
> 
> I think this does what I want. But I'm going to restate my usage desires
> and check that you agree that it covers them.
> 
> I'm interested in passing compressed bitstreams to a decoder.  The size
> of these buffers can be very variable and the worst case will nearly
> always be much larger than the typical case and that size cannot be
> known in advance of usage.  It can be very wasteful to have to allocate
> buffers that are over an order of magnitude bigger than are likely to
> ever be used.  If you have a fixed pool of fixed size buffers allocated
> at the start of time this wastefulness is unavoidable, but dmabufs can
> be dynamically sized to be as big as required and so there should be no
> limitation on passing in buffers that are smaller than the maximum.  It

Do you mean that the kernel should re-allocate the buffer dynamically
without userspace intervention?
I'm not entirely sure if this would be possible.

Regards,
Helen


> also seems plausible that dmabufs that are larger than the maximum
> should be allowed as long as their bytesused is smaller or equal.
> 
> As an aside, even when using dynamically sized dmabufs they are often
> way larger than the data they contain and forcing cache flushes or maps
> of their entire length rather than just the used portion is also
> wasteful.  This might be a use for the incoming size field.
> 
> Regards
> 
> John Cox
>