tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
1.. SPDX-License-Identifier: GPL-2.0
2
3The Virtual Stateless Decoder Driver (visl)
4===========================================
5
6A virtual stateless decoder device for stateless uAPI development
7purposes.
8
9This tool's objective is to help the development and testing of
10userspace applications that use the V4L2 stateless API to decode media.
11
12A userspace implementation can use visl to run a decoding loop even when
13no hardware is available or when the kernel uAPI for the codec has not
14been upstreamed yet. This can reveal bugs at an early stage.
15
16This driver can also trace the contents of the V4L2 controls submitted
17to it. It can also dump the contents of the vb2 buffers through a
18debugfs interface. This is in many ways similar to the tracing
19infrastructure available for other popular encode/decode APIs out there
20and can help develop a userspace application by using another (working)
21one as a reference.
22
23.. note::
24
25 No actual decoding of video frames is performed by visl. The
26 V4L2 test pattern generator is used to write various debug information
27 to the capture buffers instead.
28
29Module parameters
30-----------------
31
32- visl_debug: Activates debug info, printing various debug messages through
33 dprintk. Also controls whether per-frame debug info is shown. Defaults to off.
34 Note that enabling this feature can result in slow performance through serial.
35
36- visl_transtime_ms: Simulated process time in milliseconds. Slowing down the
37 decoding speed can be useful for debugging.
38
39- visl_dprintk_frame_start, visl_dprintk_frame_nframes: Dictates a range of
40 frames where dprintk is activated. This only controls the dprintk tracing on a
41 per-frame basis. Note that printing a lot of data can be slow through serial.
42
43- keep_bitstream_buffers: Controls whether bitstream (i.e. OUTPUT) buffers are
44 kept after a decoding session. Defaults to false so as to reduce the amount of
45 clutter. keep_bitstream_buffers == false works well when live debugging the
46 client program with GDB.
47
48- bitstream_trace_frame_start, bitstream_trace_nframes: Similar to
49 visl_dprintk_frame_start, visl_dprintk_nframes, but controls the dumping of
50 buffer data through debugfs instead.
51
52What is the default use case for this driver?
53---------------------------------------------
54
55This driver can be used as a way to compare different userspace implementations.
56This assumes that a working client is run against visl and that the ftrace and
57OUTPUT buffer data is subsequently used to debug a work-in-progress
58implementation.
59
60Information on reference frames, their timestamps, the status of the OUTPUT and
61CAPTURE queues and more can be read directly from the CAPTURE buffers.
62
63Supported codecs
64----------------
65
66The following codecs are supported:
67
68- FWHT
69- MPEG2
70- VP8
71- VP9
72- H.264
73- HEVC
74
75visl trace events
76-----------------
77The trace events are defined on a per-codec basis, e.g.:
78
79.. code-block:: bash
80
81 $ ls /sys/kernel/debug/tracing/events/ | grep visl
82 visl_fwht_controls
83 visl_h264_controls
84 visl_hevc_controls
85 visl_mpeg2_controls
86 visl_vp8_controls
87 visl_vp9_controls
88
89For example, in order to dump HEVC SPS data:
90
91.. code-block:: bash
92
93 $ echo 1 > /sys/kernel/debug/tracing/events/visl_hevc_controls/v4l2_ctrl_hevc_sps/enable
94
95The SPS data will be dumped to the trace buffer, i.e.:
96
97.. code-block:: bash
98
99 $ cat /sys/kernel/debug/tracing/trace
100 video_parameter_set_id 0
101 seq_parameter_set_id 0
102 pic_width_in_luma_samples 1920
103 pic_height_in_luma_samples 1080
104 bit_depth_luma_minus8 0
105 bit_depth_chroma_minus8 0
106 log2_max_pic_order_cnt_lsb_minus4 4
107 sps_max_dec_pic_buffering_minus1 6
108 sps_max_num_reorder_pics 2
109 sps_max_latency_increase_plus1 0
110 log2_min_luma_coding_block_size_minus3 0
111 log2_diff_max_min_luma_coding_block_size 3
112 log2_min_luma_transform_block_size_minus2 0
113 log2_diff_max_min_luma_transform_block_size 3
114 max_transform_hierarchy_depth_inter 2
115 max_transform_hierarchy_depth_intra 2
116 pcm_sample_bit_depth_luma_minus1 0
117 pcm_sample_bit_depth_chroma_minus1 0
118 log2_min_pcm_luma_coding_block_size_minus3 0
119 log2_diff_max_min_pcm_luma_coding_block_size 0
120 num_short_term_ref_pic_sets 0
121 num_long_term_ref_pics_sps 0
122 chroma_format_idc 1
123 sps_max_sub_layers_minus1 0
124 flags AMP_ENABLED|SAMPLE_ADAPTIVE_OFFSET|TEMPORAL_MVP_ENABLED|STRONG_INTRA_SMOOTHING_ENABLED
125
126
127Dumping OUTPUT buffer data through debugfs
128------------------------------------------
129
130If the **VISL_DEBUGFS** Kconfig is enabled, visl will populate
131**/sys/kernel/debug/visl/bitstream** with OUTPUT buffer data according to the
132values of bitstream_trace_frame_start and bitstream_trace_nframes. This can
133highlight errors as broken clients may fail to fill the buffers properly.
134
135A single file is created for each processed OUTPUT buffer. Its name contains an
136integer that denotes the buffer sequence, i.e.:
137
138.. code-block:: c
139
140 snprintf(name, 32, "bitstream%d", run->src->sequence);
141
142Dumping the values is simply a matter of reading from the file, i.e.:
143
144For the buffer with sequence == 0:
145
146.. code-block:: bash
147
148 $ xxd /sys/kernel/debug/visl/bitstream/bitstream0
149 00000000: 2601 af04 d088 bc25 a173 0e41 a4f2 3274 &......%.s.A..2t
150 00000010: c668 cb28 e775 b4ac f53a ba60 f8fd 3aa1 .h.(.u...:.`..:.
151 00000020: 46b4 bcfc 506c e227 2372 e5f5 d7ea 579f F...Pl.'#r....W.
152 00000030: 6371 5eb5 0eb8 23b5 ca6a 5de5 983a 19e4 cq^...#..j]..:..
153 00000040: e8c3 4320 b4ba a226 cbc1 4138 3a12 32d6 ..C ...&..A8:.2.
154 00000050: fef3 247b 3523 4e90 9682 ac8e eb0c a389 ..${5#N.........
155 00000060: ddd0 6cfc 0187 0e20 7aae b15b 1812 3d33 ..l.... z..[..=3
156 00000070: e1c5 f425 a83a 00b7 4f18 8127 3c4c aefb ...%.:..O..'<L..
157
158For the buffer with sequence == 1:
159
160.. code-block:: bash
161
162 $ xxd /sys/kernel/debug/visl/bitstream/bitstream1
163 00000000: 0201 d021 49e1 0c40 aa11 1449 14a6 01dc ...!I..@...I....
164 00000010: 7023 889a c8cd 2cd0 13b4 dab0 e8ca 21fe p#....,.......!.
165 00000020: c4c8 ab4c 486e 4e2f b0df 96cc c74e 8dde ...LHnN/.....N..
166 00000030: 8ce7 ee36 d880 4095 4d64 30a0 ff4f 0c5e ...6..@.Md0..O.^
167 00000040: f16b a6a1 d806 ca2a 0ece a673 7bea 1f37 .k.....*...s{..7
168 00000050: 370f 5bb9 1dc4 ba21 6434 bc53 0173 cba0 7.[....!d4.S.s..
169 00000060: dfe6 bc99 01ea b6e0 346b 92b5 c8de 9f5d ........4k.....]
170 00000070: e7cc 3484 1769 fef2 a693 a945 2c8b 31da ..4..i.....E,.1.
171
172And so on.
173
174By default, the files are removed during STREAMOFF. This is to reduce the amount
175of clutter.