Details on HCI filter socket options?

While I was testing some basic HCI commands using python's socket library, it seems that in order to get any socket traffic using AF_BLUETOOTH and BTPROTO_HCI a "pass all" filter needs to be set in the socket options:

from socket import socket, AF_BLUETOOTH, SOCK_RAW, BTPROTO_HCI, SOL_HCI, HCI_FILTER
from struct import pack

PASS_ALL = pack("IIIh2x", 0xffffffff, 0xffffffff, 0xffffffff, 0)

def open_socket_with_hci(dev_id: int):
    hci = socket(AF_BLUETOOTH, SOCK_RAW, BTPROTO_HCI)
    hci.bind((dev_id,))
    hci.setsockopt(SOL_HCI, HCI_FILTER, PASS_ALL)
    return hci

I found hci_filter struct defined as:

struct hci_filter {
    uint32_t type_mask;
    uint32_t event_mask[2];
    uint16_t opcode;
};

The opcode mask is straight forward enough. I'm assuming type_mask is a mask on values (code from hci.h):

/* HCI data types */
#define HCI_COMMAND_PKT     0x01
#define HCI_ACLDATA_PKT     0x02
#define HCI_SCODATA_PKT     0x03
#define HCI_EVENT_PKT       0x04
#define HCI_DIAG_PKT        0xf0
#define HCI_VENDOR_PKT      0xff

But could someone please explain the 2x event_masks? Is the 1st for HCI Event Type and the 2nd for Subevent Type (i.e. LE Meta Event)?

So, I was looking through the noble source and found this:

Hci.prototype.setSocketFilter = function() {
  var filter = new Buffer(14);
  var typeMask = (1 << HCI_COMMAND_PKT) | (1 << HCI_EVENT_PKT) | (1 << HCI_ACLDATA_PKT);
  var eventMask1 = (1 << EVT_DISCONN_COMPLETE) | (1 << EVT_ENCRYPT_CHANGE) | (1 << EVT_CMD_COMPLETE) | (1 << EVT_CMD_STATUS);
  var eventMask2 = (1 << (EVT_LE_META_EVENT - 32));
  var opcode = 0;

  filter.writeUInt32LE(typeMask, 0);
  filter.writeUInt32LE(eventMask1, 4);
  filter.writeUInt32LE(eventMask2, 8);
  filter.writeUInt16LE(opcode, 12);

  debug('setting filter to: ' + filter.toString('hex'));
  this._socket.setFilter(filter);
};

So the mask is defined as the bit shifted left by the constant's value defined in the Bluetooth Core Volume 2, Part E, Section 5 & 7.