python-can J1939 filter mask

Question

I have been working in a Raspberry with an MCP2515 CAN bus device for read entire values of J1939 messages on broadcasting with python .

I would like to filter the J1939 messages, but i'm not undestand the meaning of the can-mask and how I discorver that. In the docs of python-can says :

All messages that match at least one filter are returned. If filters is None or a zero length sequence, all messages are matched.

[{"can_id": 0x11, "can_mask": 0x21, "extended": False}]

Even though I understood this 'None' part, i didn't understand how identify the mask for my ID message

Example:

I would like to just get by script just the messages with ID "0xCF00400" and "0x18fee927"

import can

# CAN Setting
can_interface = 'can0'
bus = can.interface.Bus(can_interface, bustype='socketcan_native')

while True:
     message = bus.recv()

     bus.set_filters([{"can_id":0xF004 , "can_mask": ?? , "extended": True},
                      {"can_id":0xfee9 , "can_mask": ?? , "extended": True}])

How i should fill each variable and how I determine the mask of an ID?

UPDATING 03/10/2021

I have tried the code below, but still returning all messages

import can

# CAN Setting
can_interface = 'can0'

can_filters = [{"can_id":0xCF00400, "can_mask": 0, "extended": True},
               {"can_id":0x18fee927, "can_mask": 0, "extended": True}]

bus = can.interface.Bus(can_interface, bustype='socketcan_native',can_filters=can_filters)

while True:
     message = bus.recv()
     print(message)

Output:

Timestamp: 1615382511.238233    ID: 18fee500    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.238893    ID: 18fef100    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.247038    ID: 18fef100    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.247611    ID: 18fee500    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.248222    ID: 18fee900    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.248868    ID: 0cf00400    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.257056    ID: 0cf00400    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.257623    ID: 18fee900    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.258223    ID: 18fef100    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.258827    ID: 18fee500    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.267039    ID: 18fee500    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.267624    ID: 18fef100    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.268229    ID: 0cf00400    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.268835    ID: 18fee900    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.277035    ID: 18fee900    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.277620    ID: 0cf00400    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.278220    ID: 18fee500    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0
Timestamp: 1615382511.278823    ID: 18fef100    X                DLC:  8    ff ff ff ff ff ff ff ff     Channel: can0

Answer 1

您可以使用它"can_id":0xCF00400, "can_mask": 0xFFFFFFF来有效地过滤（在内核级别）所需的 cob ID，而无需掩码/过滤器。掩码0xFFFFFFF（所有掩码位设置为 1）要求与 can_id 完全匹配。

bus.set_filters([{"can_id":0xCF00400, "can_mask": 0xFFFFFFF, "extended": True},
                 {"can_id":0x18fee927, "can_mask": 0xFFFFFFF, "extended": True}])

例如，文档说：

当 时，过滤器匹配 <received_can_id> & can_mask == can_id & can_mask。如果extended也设置了，它只匹配 的消息 <received_is_extended> == extended。否则，它仅根据仲裁 ID 和掩码匹配每个消息。

举个例子：

# The following just equals zero
0xCF00400 & 0 == 0 # True

# The following equals 0xCF00400 (217056256 in decimal) exactly
0xCF00400 & 0xFFFFFFF == 0xCF00400 # True
0xCF00400 & 0xFFFFFFF == 217056256 # True

# The following can_id would not get through the filter + mask:
0x18fee500 & 0xFFFFFFF == 0xCF00400 & 0xFFFFFFF # False

# The following obviously would get through the filter + mask:
0xCF00400 & 0xFFFFFFF == 0xCF00400 & 0xFFFFFFF # True

我将把 放在循环bus.set_filters()之前while True和 之前bus.recv。这是一个设置，因此您只需在开始时设置一次即可。

最好在初始化总线时添加它，如下所示：

can_filters = [{"can_id":0xCF00400, "can_mask": 0xFFFFFFF, "extended": True},
               {"can_id":0x18fee927, "can_mask": 0xFFFFFFF, "extended": True}]

bus = can.Bus(
    interface="socketcan",
    channel="can0",
    can_filters=can_filters
)

另外，我相信bustype='socketcan_native'已被弃用，有利于interface="socketcan". 我已经成功使用后者相当长一段时间了，并且没有警告消息。