修改vlcplayer源码，支持指定不同TV frontend设备号节点

次文章记录了在Android系统中使用vlcplayer播放DVB制式数字TV遇到的问题。
在Android上使用win-tv-975 电视棒播放实现中，win-tv-975的frontend节点有两个，ATSC/QAM时使用节点/dev/dvb/adapter0/frontend0，DVB制式时使用/dev/dvb/adapter0/frontend1，log如下：

06-08 08:03:08.477  4300  4300 I cx231xx 5-1: 1.1: DVB: registering adapter 0 frontend 0 (LG Electronics LGDT3306A VSB/QAM Frontend)...
06-08 08:03:08.478  4300  4300 I cx231xx 5-1: 1.1: DVB: registering adapter 0 frontend 1 (Silicon Labs Si2168)...

节点截图如下

但是vlcplayer不支持这种的。查看vlcplayer源码，其只定义了dvb-adapter和dvb-device两个参数，dmx节点和frontend节点使用dvb-device决定了demux、frontend设备号只能是一样的。

vlc/modules/access/dvb/linux_dvb.c
#define DMX      "/dev/dvb/adapter%d/demux%d"
#define FRONTEND "/dev/dvb/adapter%d/frontend%d" 
#define DVR      "/dev/dvb/adapter%d/dvr%d"

    i_adapter = var_GetInteger( p_access, "dvb-adapter" );
    i_device = var_GetInteger( p_access, "dvb-device" );
    snprintf( frontend, sizeof(frontend), FRONTEND, i_adapter, i_device );
    snprintf( dmx, sizeof(dmx), DMX, i_adapter, i_device );//dmx节点和frontend节点使用dvb-device

所以这里需要修改vlc库源码使他能动态配置demux、frontend的设备号。这里新增dvb-dmx-device和dvb-frontend-device配置项，默认值是-1

vlc/modules/access/dtv/access.c
    add_integer ("dvb-dmx-device", -1, DEVICE_TEXT, DEVICE_LONGTEXT, false)
        change_safe ()
    add_integer ("dvb-frontend-device", -1, DEVICE_TEXT, DEVICE_LONGTEXT, false)
        change_safe ()

判断如果app配置了dvb-frontend-device或者dvb-dmx-device参数，则获取并用之，否则默认使用dvb-device。这样实现demux和frontend的设备号可以不一样。
vlc源码中grep dvb-device一下，在使用dvb-device的地方需要修改，linux_dvb.c如下：

vlc/modules/access/dvb/linux_dvb.c
int FrontendOpen( vlc_object_t *p_access, dvb_sys_t *p_sys, const char *psz_access )
    unsigned int i_adapter, i_device,i_frotend_device;
    i_frotend_device = var_GetInteger( p_access, "dvb-frontend-device");
    i_adapter = var_GetInteger( p_access, "dvb-adapter" );
    i_device = var_GetInteger( p_access, "dvb-device" );
    if(i_frotend_device >= 0){
        i_device=i_frotend_device;
    }
int DMXSetFilter( vlc_object_t *p_access, int i_pid, int * pi_fd, int i_type )    
    i_dmx_device = var_GetInteger( p_access, "dvb-dmx-device");
    i_adapter = var_GetInteger( p_access, "dvb-adapter" );
    i_device = var_GetInteger( p_access, "dvb-device" );

    if(i_dmx_device>=0){
        i_device =i_dmx_device;
    }

linux.c中也有使用，修改如下：

vlc/modules/access/dtv/linux.c
    uint8_t frontend_device;
    uint8_t demux_device;
    d->demux_device = var_InheritInteger (obj, "dvb-dmx-device");
    d->frontend_device = var_InheritInteger (obj, "dvb-frontend-device");
    
/** Opens the DVB device node of the specified type */
static int dvb_open_node (dvb_device_t *d, const char *type, int flags)
{
    char path[strlen (type) + 4];
    uint8_t device = d->device;
    if(0==strcmp(type,"frontend")){
       if(d->frontend_device>=0){
          device= d->frontend_device;
       }
    }
    if(0==strcmp(type,"demux")){
       if(d->demux_device >=0){
          device= d->demux_device;
       }
    }
    snprintf (path, sizeof (path), "%s%u", type,device);

这样修改以后，重新编译出vlc player aar包，app中配置ATSC播放是配置–dvb-frontend-device=1而DVB播放时使用–dvb-frontend-device=0即可。

  private ArrayList<String> getOpt() {
        final ArrayList<String> args = new ArrayList<>();//VLC参数

        switch (norm) {
            case TYPE_ATSC:
                args.add("--live-caching=1000");
                args.add("--dvb-adapter=0");
                args.add("--file-caching=1000");
                args.add("--sub-track=0");
                args.add("--dvb-frontend-device=0");
                args.add("--dvb-dmx-device=0");
                break;
            case TYPE_DVB_C:
            case TYPE_DVB_S:
            case TYPE_DVB_T:
            case TYPE_DVB_S2:
            case TYPE_DVB_T2:
                args.add("--live-caching=1000");
                args.add("--file-caching=1000");
                args.add("--dvb-adapter=0");
                args.add("--sub-track=0");
                args.add("--dvb-frontend-device=1");
                args.add("--dvb-dmx-device=0");
                break;