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diff --git a/Documentation/admin-guide/media/visl.rst b/Documentation/admin-guide/media/visl.rst new file mode 100644 index 0000000000..7d2dc78341 --- /dev/null +++ b/Documentation/admin-guide/media/visl.rst @@ -0,0 +1,175 @@ +.. SPDX-License-Identifier: GPL-2.0 + +The Virtual Stateless Decoder Driver (visl) +=========================================== + +A virtual stateless decoder device for stateless uAPI development +purposes. + +This tool's objective is to help the development and testing of +userspace applications that use the V4L2 stateless API to decode media. + +A userspace implementation can use visl to run a decoding loop even when +no hardware is available or when the kernel uAPI for the codec has not +been upstreamed yet. This can reveal bugs at an early stage. + +This driver can also trace the contents of the V4L2 controls submitted +to it. It can also dump the contents of the vb2 buffers through a +debugfs interface. This is in many ways similar to the tracing +infrastructure available for other popular encode/decode APIs out there +and can help develop a userspace application by using another (working) +one as a reference. + +.. note:: + + No actual decoding of video frames is performed by visl. The + V4L2 test pattern generator is used to write various debug information + to the capture buffers instead. + +Module parameters +----------------- + +- visl_debug: Activates debug info, printing various debug messages through + dprintk. Also controls whether per-frame debug info is shown. Defaults to off. + Note that enabling this feature can result in slow performance through serial. + +- visl_transtime_ms: Simulated process time in milliseconds. Slowing down the + decoding speed can be useful for debugging. + +- visl_dprintk_frame_start, visl_dprintk_frame_nframes: Dictates a range of + frames where dprintk is activated. This only controls the dprintk tracing on a + per-frame basis. Note that printing a lot of data can be slow through serial. + +- keep_bitstream_buffers: Controls whether bitstream (i.e. OUTPUT) buffers are + kept after a decoding session. Defaults to false so as to reduce the amount of + clutter. keep_bitstream_buffers == false works well when live debugging the + client program with GDB. + +- bitstream_trace_frame_start, bitstream_trace_nframes: Similar to + visl_dprintk_frame_start, visl_dprintk_nframes, but controls the dumping of + buffer data through debugfs instead. + +What is the default use case for this driver? +--------------------------------------------- + +This driver can be used as a way to compare different userspace implementations. +This assumes that a working client is run against visl and that the ftrace and +OUTPUT buffer data is subsequently used to debug a work-in-progress +implementation. + +Information on reference frames, their timestamps, the status of the OUTPUT and +CAPTURE queues and more can be read directly from the CAPTURE buffers. + +Supported codecs +---------------- + +The following codecs are supported: + +- FWHT +- MPEG2 +- VP8 +- VP9 +- H.264 +- HEVC + +visl trace events +----------------- +The trace events are defined on a per-codec basis, e.g.: + +.. code-block:: bash + + $ ls /sys/kernel/debug/tracing/events/ | grep visl + visl_fwht_controls + visl_h264_controls + visl_hevc_controls + visl_mpeg2_controls + visl_vp8_controls + visl_vp9_controls + +For example, in order to dump HEVC SPS data: + +.. code-block:: bash + + $ echo 1 > /sys/kernel/debug/tracing/events/visl_hevc_controls/v4l2_ctrl_hevc_sps/enable + +The SPS data will be dumped to the trace buffer, i.e.: + +.. code-block:: bash + + $ cat /sys/kernel/debug/tracing/trace + video_parameter_set_id 0 + seq_parameter_set_id 0 + pic_width_in_luma_samples 1920 + pic_height_in_luma_samples 1080 + bit_depth_luma_minus8 0 + bit_depth_chroma_minus8 0 + log2_max_pic_order_cnt_lsb_minus4 4 + sps_max_dec_pic_buffering_minus1 6 + sps_max_num_reorder_pics 2 + sps_max_latency_increase_plus1 0 + log2_min_luma_coding_block_size_minus3 0 + log2_diff_max_min_luma_coding_block_size 3 + log2_min_luma_transform_block_size_minus2 0 + log2_diff_max_min_luma_transform_block_size 3 + max_transform_hierarchy_depth_inter 2 + max_transform_hierarchy_depth_intra 2 + pcm_sample_bit_depth_luma_minus1 0 + pcm_sample_bit_depth_chroma_minus1 0 + log2_min_pcm_luma_coding_block_size_minus3 0 + log2_diff_max_min_pcm_luma_coding_block_size 0 + num_short_term_ref_pic_sets 0 + num_long_term_ref_pics_sps 0 + chroma_format_idc 1 + sps_max_sub_layers_minus1 0 + flags AMP_ENABLED|SAMPLE_ADAPTIVE_OFFSET|TEMPORAL_MVP_ENABLED|STRONG_INTRA_SMOOTHING_ENABLED + + +Dumping OUTPUT buffer data through debugfs +------------------------------------------ + +If the **VISL_DEBUGFS** Kconfig is enabled, visl will populate +**/sys/kernel/debug/visl/bitstream** with OUTPUT buffer data according to the +values of bitstream_trace_frame_start and bitstream_trace_nframes. This can +highlight errors as broken clients may fail to fill the buffers properly. + +A single file is created for each processed OUTPUT buffer. Its name contains an +integer that denotes the buffer sequence, i.e.: + +.. code-block:: c + + snprintf(name, 32, "bitstream%d", run->src->sequence); + +Dumping the values is simply a matter of reading from the file, i.e.: + +For the buffer with sequence == 0: + +.. code-block:: bash + + $ xxd /sys/kernel/debug/visl/bitstream/bitstream0 + 00000000: 2601 af04 d088 bc25 a173 0e41 a4f2 3274 &......%.s.A..2t + 00000010: c668 cb28 e775 b4ac f53a ba60 f8fd 3aa1 .h.(.u...:.`..:. + 00000020: 46b4 bcfc 506c e227 2372 e5f5 d7ea 579f F...Pl.'#r....W. + 00000030: 6371 5eb5 0eb8 23b5 ca6a 5de5 983a 19e4 cq^...#..j]..:.. + 00000040: e8c3 4320 b4ba a226 cbc1 4138 3a12 32d6 ..C ...&..A8:.2. + 00000050: fef3 247b 3523 4e90 9682 ac8e eb0c a389 ..${5#N......... + 00000060: ddd0 6cfc 0187 0e20 7aae b15b 1812 3d33 ..l.... z..[..=3 + 00000070: e1c5 f425 a83a 00b7 4f18 8127 3c4c aefb ...%.:..O..'<L.. + +For the buffer with sequence == 1: + +.. code-block:: bash + + $ xxd /sys/kernel/debug/visl/bitstream/bitstream1 + 00000000: 0201 d021 49e1 0c40 aa11 1449 14a6 01dc ...!I..@...I.... + 00000010: 7023 889a c8cd 2cd0 13b4 dab0 e8ca 21fe p#....,.......!. + 00000020: c4c8 ab4c 486e 4e2f b0df 96cc c74e 8dde ...LHnN/.....N.. + 00000030: 8ce7 ee36 d880 4095 4d64 30a0 ff4f 0c5e ...6..@.Md0..O.^ + 00000040: f16b a6a1 d806 ca2a 0ece a673 7bea 1f37 .k.....*...s{..7 + 00000050: 370f 5bb9 1dc4 ba21 6434 bc53 0173 cba0 7.[....!d4.S.s.. + 00000060: dfe6 bc99 01ea b6e0 346b 92b5 c8de 9f5d ........4k.....] + 00000070: e7cc 3484 1769 fef2 a693 a945 2c8b 31da ..4..i.....E,.1. + +And so on. + +By default, the files are removed during STREAMOFF. This is to reduce the amount +of clutter. |