去掉v4l2mpeg4dec插件的mpeg4解码能力

背景：去掉v4l2videodec的MPEG4的caps

MPEG4 caps的初始化调用栈

plugin_init在gst-plugins-good/sys/v4l2/gstv4l2.c文件中，是v4l2相关plugin的init函数，通过gst-inspect可以看到video4linux2对应一系列插件

video4linux2:  v4l2src: Video (video4linux2) Source
video4linux2:  v4l2sink: Video (video4linux2) Sink
video4linux2:  v4l2radio: Radio (video4linux2) Tuner
video4linux2:  v4l2deviceprovider (GstDeviceProviderFactory)
video4linux2:  v4l2jpegdec: V4L2 JPEG Decoder
video4linux2:  v4l2mpeg4dec: V4L2 MPEG4 Decoder
video4linux2:  v4l2h264dec: V4L2 H264 Decoder
video4linux2:  v4l2h265dec: V4L2 H265 Decoder
video4linux2:  v4l2vp9dec: V4L2 VP9 Decoder

plugin_init函数，v4l2videodec的调用栈：

plugin_init- gst_v4l2_probe_and_register- gst_v4l2_video_dec_register- gst_v4l2_probe_template_caps- gst_v4l2_object_v4l2fourcc_to_structure- gst_v4l2_object_v4l2fourcc_to_bare_struct

在gst_v4l2_object_v4l2fourcc_to_bare_struct中可以看到MPEG4的caps定义：

    case V4L2_PIX_FMT_MPEG4:case V4L2_PIX_FMT_XVID:structure = gst_structure_new ("video/mpeg","mpegversion", G_TYPE_INT, 4, "systemstream",G_TYPE_BOOLEAN, FALSE, NULL);break;

分析

gst_caps_merge调用gst_v4l2_probe_template_caps生成cpas

    sink_caps = gst_caps_merge (gst_v4l2_probe_template_caps (it->device_path,video_fd, V4L2_BUF_TYPE_VIDEO_OUTPUT),gst_v4l2_probe_template_caps (it->device_path, video_fd,V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE));

gst_v4l2_probe_template_caps中，通过ioctl查询支持的format，format是多个，format的原型定义如下：

struct v4l2_fmtdesc {__u32           index;             /* Format number      */__u32            type;              /* enum v4l2_buf_type */__u32           flags;__u8           description[32];   /* Description string */__u32            pixelformat;       /* Format fourcc      */__u32            reserved[4];
};

ioctl查询支持的format

    if (ioctl (video_fd, VIDIOC_ENUM_FMT, &format) < 0)break;                    /* end of enumeration */

然后根据format.pixelformat创建template

template = gst_v4l2_object_v4l2fourcc_to_structure (format.pixelformat);

可以看到V4L2_PIX_FMT_MPEG4部分创建template

static GstStructure *
gst_v4l2_object_v4l2fourcc_to_bare_struct (guint32 fourcc)
{GstStructure *structure = NULL;switch (fourcc) {case V4L2_PIX_FMT_MJPEG:   /* Motion-JPEG */case V4L2_PIX_FMT_PJPG:    /* Progressive-JPEG */case V4L2_PIX_FMT_JPEG:    /* JFIF JPEG */structure = gst_structure_new_empty ("image/jpeg");break;case V4L2_PIX_FMT_MPEG4:case V4L2_PIX_FMT_XVID:structure = gst_structure_new ("video/mpeg","mpegversion", G_TYPE_INT, 4, "systemstream",G_TYPE_BOOLEAN, FALSE, NULL);break;

ERROR1

sh-5.0# GST_DEBUG=5 gst-inspect v4l2mpeg4dec
westeros-sink: discover decoder: /dev/video26(gst-plugin-scanner:9700): GStreamer-CRITICAL **: 14:47:48.527: gst_debug_log_valist: assertion 'category != NULL' failed
(gst-plugin-scanner:9708): GStreamer-CRITICAL **: 14:47:48.930: gst_debug_log_valist: assertion 'category != NULL' failed

这个错误是因为加了一句log输出导致：

+++ b/sys/v4l2/gstv4l2videodec.c
@@ -1183,6 +1183,7 @@ G_STMT_START { \}if (codec_name) {
+    GST_WARNING ("plugin '%s'", codec_name);type_name = g_strdup_printf ("v4l2%sdec", codec_name);if (g_type_from_name (type_name) != 0) {g_free (type_name);

正确的做法应该是定义default category，不然GST_WARNING用的时候发现category为null，就报前面的错了。GST_WARNING在有些地方可以直接用，有些地方不可以，原因取决于有没有有效的GST_CAT_DEFAULT定义。

这个地方也可以用g_warning增加调试信息：

g_warning ("plugin codec name-: '%s'", codec_name);

$ gst-inspect v4l2mpeg4dec plugin codec name: 'jpeg'plugin codec name: 'mpeg4'plugin codec name: 'h264'plugin codec name: 'h265'plugin codec name: 'vp9'

v4l2mpeg4dec

Factory Details:Rank                     primary + 1 (257)Long-name                V4L2 MPEG4 DecoderKlass                    Codec/Decoder/Video/HardwareDescription              Decodes MPEG4 streams via V4L2 APIAuthor                   Nicolas Dufresne <nicolas.dufresne@collabora.com>

对应代码：

  cdata->longname = "V4L2 " codec " Decoder"; \cdata->description = "Decodes " codec " streams via V4L2 API"; \codec_name = g_ascii_strdown (codec, -1); \

这部分是和v4l2video26mpeg4dec共用的。

继承关系：

GObject+----GInitiallyUnowned+----GstObject+----GstElement+----GstVideoDecoder+----GstV4l2VideoDec+----v4l2mpeg4dec

v4l2video26mpeg4dec

Factory Details:Rank                     primary + 1 (257)Long-name                V4L2 MPEG4 DecoderKlass                    Codec/Decoder/Video/HardwareDescription              Decodes MPEG4 streams via V4L2 APIAuthor                   Nicolas Dufresne <nicolas.dufresne@collabora.com>

继承关系：

GObject+----GInitiallyUnowned+----GstObject+----GstElement+----GstVideoDecoder+----GstV4l2VideoDec+----v4l2video26mpeg4dec

v4l2video26mpeg4dec为什么加了就不见了？

v4l2video26mpeg4dec是video4linux2的一个feature

GST_REGISTRY gstregistrychunks.c:729:gst_registry_chunks_load_feature:Added feature v4l2mpeg4dec, plugin 0x1b23008 video4linux2GST_REGISTRY gstregistrychunks.c:571:gst_registry_chunks_load_feature:Plugin 'video4linux2' feature 'v4l2video26mpeg4dec' typename : 'GstElementFactory'

上面打印log的地方：

  GST_DEBUG ("Plugin '%s' feature '%s' typename : '%s'", plugin_name, feature_name, type_name);

registry里面对应的代码

gstreamer/gst/gstregistrychunks.c

_priv_gst_registry_chunks_load_plugin中load插件的feature

  /* Load plugin features */for (i = 0; i < n; i++) {if (G_UNLIKELY (!gst_registry_chunks_load_feature (registry, in, end,plugin))) {GST_ERROR ("Error while loading binary feature for plugin '%s'",GST_STR_NULL (plugin->desc.name));gst_registry_remove_plugin (registry, plugin);goto fail;}}

ERROR2

如果代码有错误，后面是0 features

gstregistrychunks.c:850:_priv_gst_registry_chunks_load_plugin: read strings for name='libgstvideo4linux2.so'
gstregistrychunks.c:851:_priv_gst_registry_chunks_load_plugin:   desc.description='Plugin for blacklisted file'
gstregistrychunks.c:852:_priv_gst_registry_chunks_load_plugin:   filename='/usr/lib/gstreamer-1.0/libgstvideo4linux2.so'
gstregistrychunks.c:853:_priv_gst_registry_chunks_load_plugin:   desc.version='0.0.0'
gstregistrychunks.c:854:_priv_gst_registry_chunks_load_plugin:   desc.license='BLACKLIST'
gstregistrychunks.c:855:_priv_gst_registry_chunks_load_plugin:   desc.source='BLACKLIST'
gstregistrychunks.c:856:_priv_gst_registry_chunks_load_plugin:   desc.package='BLACKLIST'
gstregistrychunks.c:857:_priv_gst_registry_chunks_load_plugin:   desc.origin='BLACKLIST'
gstregistrychunks.c:858:_priv_gst_registry_chunks_load_plugin:   desc.datetime=
gstregistry.c:474:gst_registry_add_plugin:<registry0> adding plugin 0xf46008 for filename "/usr/lib/gstreamer-1.0/libgstvideo4linux2.so"
gstregistry.c:487:gst_registry_add_plugin:<registry0> emitting plugin-added for filename "/usr/lib/gstreamer-1.0/libgstvideo4linux2.so"
gstregistrychunks.c:878:_priv_gst_registry_chunks_load_plugin: Added plugin 'libgstvideo4linux2.so' plugin with 0 features from binary regis

video4linux2

gst-inspect video4linux2
WARNING: terminal is not fully functional
Plugin Details:N)Name                     video4linux2Description              elements for Video 4 LinuxFilename                 /usr/lib/gstreamer-1.0/libgstvideo4linux2.soVersion                  1.16.3License                  LGPLSource module            gst-plugins-goodBinary package           GStreamer Good Plug-ins source releaseOrigin URL               Unknown package originv4l2src: Video (video4linux2) Sourcev4l2sink: Video (video4linux2) Sinkv4l2radio: Radio (video4linux2) Tunerv4l2deviceprovider: Video (video4linux2) Device Providerv4l2jpegdec: V4L2 JPEG Decoderv4l2mpeg4dec: V4L2 MPEG4 Decoderv4l2video26mpeg4dec: V4L2 MPEG4 Decoderv4l2h264dec: V4L2 H264 Decoderv4l2h265dec: V4L2 H265 Decoderv4l2vp9dec: V4L2 VP9 Decoder10 features:+-- 9 elements+-- 1 device providers

增加log输出v4l2dec注册的type_name

+GST_DEBUG_CATEGORY_EXTERN (v4l2_debug);
+#define GST_CAT_DEFAULT v4l2_debug
+voidgst_v4l2_video_dec_register (GstPlugin * plugin, const gchar * basename,const gchar * device_path, GstCaps * sink_caps, GstCaps * src_caps)
@@ -1210,6 +1214,7 @@ gst_v4l2_video_dec_register (GstPlugin * plugin, const gchar * basename,GType type, subtype;gchar *type_name;+    s = gst_caps_get_structure (sink_caps, i);cdata = g_new0 (GstV4l2VideoDecCData, 1);
@@ -1222,7 +1227,7 @@ gst_v4l2_video_dec_register (GstPlugin * plugin, const gchar * basename,gst_caps_set_features_simple(cdata->src_caps, gst_caps_features_from_string(GST_CAPS_FEATURE_MEMORY_DMABUF));gst_caps_append(cdata->src_caps, gst_caps_copy (src_caps));type_name = gst_v4l2_video_dec_set_metadata (s, cdata, basename);
-
+    GST_CAT_WARNING (GST_CAT_DEFAULT, "shizh: type_name: %s, caps size: %d", type_name, gst_caps_get_size (sink_caps));

去掉mpeg4的caps

v4l2 gstv4l2videodec.c:1230:gst_v4l2_video_dec_register: type_name: v4l2jpegdec, caps size: 5
v4l2 gstv4l2videodec.c:1230:gst_v4l2_video_dec_register: type_name: v4l2mpeg4dec, caps size: 5
v4l2 gstv4l2videodec.c:1230:gst_v4l2_video_dec_register: type_name: v4l2h264dec, caps size: 5
v4l2 gstv4l2videodec.c:1230:gst_v4l2_video_dec_register: type_name: v4l2h265dec, caps size: 5
v4l2 gstv4l2videodec.c:1230:gst_v4l2_video_dec_register: type_name: v4l2vp9dec, caps size: 5

所以通过ENABLE_MPEG4PART2去掉mpeg4的caps是可行的：

#ifdef ENABLE_MPEG4PART2structure = gst_structure_new ("video/mpeg","mpegversion", G_TYPE_INT, 4, "systemstream",G_TYPE_BOOLEAN, FALSE, NULL);
#endif

v4l2video26mpeg4dec的出处

因为g_type_from_name通过mpeg4的name去取，发现已经有了，所以又用basename和codec_name组合了一个新的name，basename为video26。codec_name为mpeg4，即新的名字v4l2+video26+mpeg4dec，就是v4l2video26mpeg4dec，去掉mpeg4的caps之后，就不会有v4l2video26mpeg4dec。

  if (codec_name) {type_name = g_strdup_printf ("v4l2%sdec", codec_name);if (g_type_from_name (type_name) != 0) {g_free (type_name);GST_WARNING ("plugin codec name: v4l2 %s %s dec'", basename, codec_name);type_name = g_strdup_printf ("v4l2%s%sdec", basename, codec_name);}

加上前面的log会输出：

gst_v4l2_video_dec_set_metadata: plugin codec name: v4l2 video26 mpeg4 dec'

mpeg1和mpeg2

V4L2_PIX_FMT_MPEG1和V4L2_PIX_FMT_MPEG2应该merge caps的时候被merge到一块了：

    case V4L2_PIX_FMT_MPEG1:structure = gst_structure_new ("video/mpeg","mpegversion", G_TYPE_INT, 1, NULL);break;case V4L2_PIX_FMT_MPEG2:structure = gst_structure_new ("video/mpeg","mpegversion", G_TYPE_INT, 2, NULL);

v4l2通过ioctl查询format信息

在gst-plugins-good/sys/v4l2/gstv4l2.c的gst_v4l2_probe_template_caps函数中，通过ioctl调用得到format：

    if (ioctl (video_fd, VIDIOC_ENUM_FMT, &format) < 0)break;                    /* end of enumeration */GST_LOG ("index:       %u", format.index);GST_LOG ("type:        %d", format.type);GST_LOG ("flags:       %08x", format.flags);GST_LOG ("description: '%s'", format.description);GST_LOG ("pixelformat: %" GST_FOURCC_FORMAT,GST_FOURCC_ARGS (format.pixelformat));

输出log如下：

v4l2 gstv4l2.c:75:gst_v4l2_probe_template_caps: Getting /dev/video26 format enumerations
v4l2 gstv4l2.c:75:gst_v4l2_probe_template_caps: Getting /dev/video26 format enumerations
v4l2 gstv4l2.c:89:gst_v4l2_probe_template_caps: index:       0
v4l2 gstv4l2.c:90:gst_v4l2_probe_template_caps: type:        10
v4l2 gstv4l2.c:91:gst_v4l2_probe_template_caps: flags:       00000001
v4l2 gstv4l2.c:92:gst_v4l2_probe_template_caps: description: 'H.264'
v4l2 gstv4l2.c:93:gst_v4l2_probe_template_caps: pixelformat: H264
v4l2 gstv4l2.c:89:gst_v4l2_probe_template_caps: index:       1
v4l2 gstv4l2.c:90:gst_v4l2_probe_template_caps: type:        10
v4l2 gstv4l2.c:91:gst_v4l2_probe_template_caps: flags:       00000001
v4l2 gstv4l2.c:92:gst_v4l2_probe_template_caps: description: 'HEVC'
v4l2 gstv4l2.c:93:gst_v4l2_probe_template_caps: pixelformat: HEVC
v4l2 gstv4l2.c:89:gst_v4l2_probe_template_caps: index:       2
v4l2 gstv4l2.c:90:gst_v4l2_probe_template_caps: type:        10
v4l2 gstv4l2.c:91:gst_v4l2_probe_template_caps: flags:       00000001
v4l2 gstv4l2.c:92:gst_v4l2_probe_template_caps: description: 'VP9'
v4l2 gstv4l2.c:93:gst_v4l2_probe_template_caps: pixelformat: VP90
v4l2 gstv4l2.c:89:gst_v4l2_probe_template_caps: index:       3
v4l2 gstv4l2.c:90:gst_v4l2_probe_template_caps: type:        10
v4l2 gstv4l2.c:91:gst_v4l2_probe_template_caps: flags:       00000001
v4l2 gstv4l2.c:92:gst_v4l2_probe_template_caps: description: 'MPEG-1 ES'
v4l2 gstv4l2.c:93:gst_v4l2_probe_template_caps: pixelformat: MPG1
v4l2 gstv4l2.c:89:gst_v4l2_probe_template_caps: index:       4
v4l2 gstv4l2.c:90:gst_v4l2_probe_template_caps: type:        10
v4l2 gstv4l2.c:91:gst_v4l2_probe_template_caps: flags:       00000001
v4l2 gstv4l2.c:92:gst_v4l2_probe_template_caps: description: 'MPEG-2 ES'
v4l2 gstv4l2.c:93:gst_v4l2_probe_template_caps: pixelformat: MPG2
v4l2 gstv4l2.c:89:gst_v4l2_probe_template_caps: index:       5
v4l2 gstv4l2.c:90:gst_v4l2_probe_template_caps: type:        10
v4l2 gstv4l2.c:91:gst_v4l2_probe_template_caps: flags:       00000001
v4l2 gstv4l2.c:92:gst_v4l2_probe_template_caps: description: 'MPEG-4 Part 2 ES'
v4l2 gstv4l2.c:93:gst_v4l2_probe_template_caps: pixelformat: MPG4
v4l2 gstv4l2.c:89:gst_v4l2_probe_template_caps: index:       6
v4l2 gstv4l2.c:90:gst_v4l2_probe_template_caps: type:        10
v4l2 gstv4l2.c:91:gst_v4l2_probe_template_caps: flags:       00000001
v4l2 gstv4l2.c:92:gst_v4l2_probe_template_caps: description: 'Motion-JPEG'
v4l2 gstv4l2.c:93:gst_v4l2_probe_template_caps: pixelformat: MJPG
v4l2 gstv4l2.c:89:gst_v4l2_probe_template_caps: index:       7
v4l2 gstv4l2.c:90:gst_v4l2_probe_template_caps: type:        10
v4l2 gstv4l2.c:91:gst_v4l2_probe_template_caps: flags:       00000001
v4l2 gstv4l2.c:92:gst_v4l2_probe_template_caps: description: 'AV1'
v4l2 gstv4l2.c:93:gst_v4l2_probe_template_caps: pixelformat: AV10
v4l2 gstv4l2object.c:1573:gst_v4l2_object_v4l2fourcc_to_bare_struct: Unsupported fourcc 0x30315641 AV10

通过v4l2插件播放

GST_DEBUG=v4l2*:5 gst-launch-1.0 filesrc location=/data/hbc9_h264.avi ! avidemux ! h264parse ! v4l2h264dec ! autovideosink

通过property控制

想了下如果通过property控制不可行，因为caps已经关掉了，没法在通过property增加caps，需要重新registry。

void gst_v4l2_object_install_properties_helper (GObjectClass * gobject_class, const char *default_device)
{g_object_class_install_property (gobject_class, PROP_DEVICE,g_param_spec_string ("device", "Device", "Device location",default_device, G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));g_object_class_install_property (gobject_class, PROP_DEVICE_NAME,g_param_spec_string ("device-name", "Device name","Name of the device", DEFAULT_PROP_DEVICE_NAME,G_PARAM_READABLE | G_PARAM_STATIC_STRINGS));

添加property

在gst-plugins-good/sys/v4l2/gstv4l2object.c中搜索g_object_class_install_property函数，可以参考添加property的代码

  g_object_class_install_property (gobject_class, PROP_FLAGS,g_param_spec_flags ("flags", "Flags", "Device type flags",GST_TYPE_V4L2_DEVICE_FLAGS, DEFAULT_PROP_FLAGS,G_PARAM_READABLE | G_PARAM_STATIC_STRINGS));

测试播放命令：

 DISABLE_MPEG4_DECODE=1 gst-play-1.0 /data/mpeg4.ts