Date:   Fri, 26 Mar 2021 15:22:56 +0000
From:   John Cox <jc@...esim.co.uk>
To:     Helen Koike <helen.koike@...labora.com>
Cc:     linux-media@...r.kernel.org, hverkuil@...all.nl,
        kernel@...labora.com, linux-kernel@...r.kernel.org,
        laurent.pinchart@...asonboard.com, dave.stevenson@...pberrypi.org,
        tfiga@...omium.org
Subject: Re: [PATCH 1/2] media: videobuf2: use dmabuf size for length

Hi Helen

>On 3/26/21 10:03 AM, John Cox wrote:
>> Hi Helen
>> 
>>> 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@...all.nl>
>>>>> Signed-off-by: Helen Koike <helen.koike@...labora.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.
>> 
>> No - I didn't mean that at all.  Any reallocation would be done by the
>> user.  I was just setting out why damabufs are different from (and more
>> useful than) MMAP buffers for bitstream-like purposes.
>
>Right, thanks for the clarification.
>
>> 
>> Regards
>> 
>> John Cox
>> 
>>> 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.
>
>If I understand correctly, the requirements would be:
>
>(consider maximum being the length/boundary provided by userspace).
>
>(1) bytesused <= maximum && bytesused <= dmabuf_length, this must always be true.
>(2) maximum <= dmabuf_length is always ok.
>(3) dmabuf_length <= maximum is ok as long (1) is still true.
>if dmabuf_length <= maximum, but bytesused > maximum, then it is not ok.
>
>Make sense?
>
>We could save in vb2:
>bytesused_max = maximum ? min(maximum, dmabuf_length) : dmabuf_length;
>
>Then drivers could check if if bytesused <= bytesused_max,
>and we don't need to check dma_length against the maximum value.
>
>Or maybe there is little value in letting userspace define a maximum.
>
>What do you think we should do? Remove the maximum (as implemented in this patch)?
>Or just comparing against bytesused_max is enough (which would keeping the boundary
>feature) ?
>
>I would prefer to remove the maximum if there is no value for userspace, since
>this would make things easier for the Ext API implementation.

From my personal PoV, for an OUTPUT buffer, as long as the data fits in
the buffer i.e. bytesused <= dmabuf_length then that is all I really
care about. Other peoples mileage may vary!

Thanks

JC


>>>>
>>>> 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.
>
>I guess this can be achieved using the bytesused field.
>
>Regards,
>Helen
>
>>>>
>>>> Regards
>>>>
>>>> John Cox
>>>>

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