1 .. SPDX-License-Identifier: GPL-2.0
2
3 The Virtual Stateless Decoder Driver (visl)
4 ===========================================
5
6 A virtual stateless decoder device for stateless uAPI development
7 purposes.
8
9 This tool's objective is to help the development and testing of
10 userspace applications that use the V4L2 stateless API to decode media.
11
12 A userspace implementation can use visl to run a decoding loop even when
13 no hardware is available or when the kernel uAPI for the codec has not
14 been upstreamed yet. This can reveal bugs at an early stage.
15
16 This driver can also trace the contents of the V4L2 controls submitted
17 to it. It can also dump the contents of the vb2 buffers through a
18 debugfs interface. This is in many ways similar to the tracing
19 infrastructure available for other popular encode/decode APIs out there
20 and can help develop a userspace application by using another (working)
21 one 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
29 Module 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
52 - tpg_verbose: Write extra information on each output frame to ease debugging
53 the API. When set to true, the output frames are not stable for a given input
54 as some information like pointers or queue status will be added to them.
55
56 What is the default use case for this driver?
57 ---------------------------------------------
58
59 This driver can be used as a way to compare different userspace implementations.
60 This assumes that a working client is run against visl and that the ftrace and
61 OUTPUT buffer data is subsequently used to debug a work-in-progress
62 implementation.
63
64 Even though no video decoding is actually done, the output frames can be used
65 against a reference for a given input, except if tpg_verbose is set to true.
66
67 Depending on the tpg_verbose parameter value, information on reference frames,
68 their timestamps, the status of the OUTPUT and CAPTURE queues and more can be
69 read directly from the CAPTURE buffers.
70
71 Supported codecs
72 ----------------
73
74 The following codecs are supported:
75
76 - FWHT
77 - MPEG2
78 - VP8
79 - VP9
80 - H.264
81 - HEVC
82 - AV1
83
84 visl trace events
85 -----------------
86 The trace events are defined on a per-codec basis, e.g.:
87
88 .. code-block:: bash
89
90 $ ls /sys/kernel/tracing/events/ | grep visl
91 visl_av1_controls
92 visl_fwht_controls
93 visl_h264_controls
94 visl_hevc_controls
95 visl_mpeg2_controls
96 visl_vp8_controls
97 visl_vp9_controls
98
99 For example, in order to dump HEVC SPS data:
100
101 .. code-block:: bash
102
103 $ echo 1 > /sys/kernel/tracing/events/visl_hevc_controls/v4l2_ctrl_hevc_sps/enable
104
105 The SPS data will be dumped to the trace buffer, i.e.:
106
107 .. code-block:: bash
108
109 $ cat /sys/kernel/tracing/trace
110 video_parameter_set_id 0
111 seq_parameter_set_id 0
112 pic_width_in_luma_samples 1920
113 pic_height_in_luma_samples 1080
114 bit_depth_luma_minus8 0
115 bit_depth_chroma_minus8 0
116 log2_max_pic_order_cnt_lsb_minus4 4
117 sps_max_dec_pic_buffering_minus1 6
118 sps_max_num_reorder_pics 2
119 sps_max_latency_increase_plus1 0
120 log2_min_luma_coding_block_size_minus3 0
121 log2_diff_max_min_luma_coding_block_size 3
122 log2_min_luma_transform_block_size_minus2 0
123 log2_diff_max_min_luma_transform_block_size 3
124 max_transform_hierarchy_depth_inter 2
125 max_transform_hierarchy_depth_intra 2
126 pcm_sample_bit_depth_luma_minus1 0
127 pcm_sample_bit_depth_chroma_minus1 0
128 log2_min_pcm_luma_coding_block_size_minus3 0
129 log2_diff_max_min_pcm_luma_coding_block_size 0
130 num_short_term_ref_pic_sets 0
131 num_long_term_ref_pics_sps 0
132 chroma_format_idc 1
133 sps_max_sub_layers_minus1 0
134 flags AMP_ENABLED|SAMPLE_ADAPTIVE_OFFSET|TEMPORAL_MVP_ENABLED|STRONG_INTRA_SMOOTHING_ENABLED
135
136
137 Dumping OUTPUT buffer data through debugfs
138 ------------------------------------------
139
140 If the **VISL_DEBUGFS** Kconfig is enabled, visl will populate
141 **/sys/kernel/debug/visl/bitstream** with OUTPUT buffer data according to the
142 values of bitstream_trace_frame_start and bitstream_trace_nframes. This can
143 highlight errors as broken clients may fail to fill the buffers properly.
144
145 A single file is created for each processed OUTPUT buffer. Its name contains an
146 integer that denotes the buffer sequence, i.e.:
147
148 .. code-block:: c
149
150 snprintf(name, 32, "bitstream%d", run->src->sequence);
151
152 Dumping the values is simply a matter of reading from the file, i.e.:
153
154 For the buffer with sequence == 0:
155
156 .. code-block:: bash
157
158 $ xxd /sys/kernel/debug/visl/bitstream/bitstream0
159 00000000: 2601 af04 d088 bc25 a173 0e41 a4f2 3274 &......%.s.A..2t
160 00000010: c668 cb28 e775 b4ac f53a ba60 f8fd 3aa1 .h.(.u...:.`..:.
161 00000020: 46b4 bcfc 506c e227 2372 e5f5 d7ea 579f F...Pl.'#r....W.
162 00000030: 6371 5eb5 0eb8 23b5 ca6a 5de5 983a 19e4 cq^...#..j]..:..
163 00000040: e8c3 4320 b4ba a226 cbc1 4138 3a12 32d6 ..C ...&..A8:.2.
164 00000050: fef3 247b 3523 4e90 9682 ac8e eb0c a389 ..${5#N.........
165 00000060: ddd0 6cfc 0187 0e20 7aae b15b 1812 3d33 ..l.... z..[..=3
166 00000070: e1c5 f425 a83a 00b7 4f18 8127 3c4c aefb ...%.:..O..'<L..
167
168 For the buffer with sequence == 1:
169
170 .. code-block:: bash
171
172 $ xxd /sys/kernel/debug/visl/bitstream/bitstream1
173 00000000: 0201 d021 49e1 0c40 aa11 1449 14a6 01dc ...!I..@...I....
174 00000010: 7023 889a c8cd 2cd0 13b4 dab0 e8ca 21fe p#....,.......!.
175 00000020: c4c8 ab4c 486e 4e2f b0df 96cc c74e 8dde ...LHnN/.....N..
176 00000030: 8ce7 ee36 d880 4095 4d64 30a0 ff4f 0c5e ...6..@.Md0..O.^
177 00000040: f16b a6a1 d806 ca2a 0ece a673 7bea 1f37 .k.....*...s{..7
178 00000050: 370f 5bb9 1dc4 ba21 6434 bc53 0173 cba0 7.[....!d4.S.s..
179 00000060: dfe6 bc99 01ea b6e0 346b 92b5 c8de 9f5d ........4k.....]
180 00000070: e7cc 3484 1769 fef2 a693 a945 2c8b 31da ..4..i.....E,.1.
181
182 And so on.
183
184 By default, the files are removed during STREAMOFF. This is to reduce the amount
185 of clutter.
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