I hacked the code to set the DATA_FRAME_MAX_SIZE = 8500, half-way between 8000 and MTU 9000.
Then I made sure to set the send_frame_size=recv_frame_size=8700, which is equal, and over the MAX_SIZE.It still picks 8144. You can see below. I added a bunch of debug of the intermediate variables. $\~/uhd-4.6.0.0/host/build/examples$ ./tx_samples_c -a "type=x300,addr=192.168.40.2,send_frame_size=8700,recv_frame_size=8700" ``` Creating USRP with args "type=x300,addr=192.168.40.2,send_frame_size=8700,recv_frame_size=8700"... ``` ``` [INFO] [UHD] linux; GNU C++ version 9.4.0; Boost_107100; UHD_4.6.0.0-1-ga9f0b4c7 ``` ``` [INFO] [X300] X300 initialization sequence... ``` ``` [INFO] [X300] user_frame_size.send_frame_size: 8700 ``` ``` [INFO] [X300] x300::DATA_FRAME_MAX_SIZE: 8500 ``` ``` [INFO] [X300] std::min(user_frame_size.send_frame_size, x300::DATA_FRAME_MAX_SIZE): 8500 ``` ``` [INFO] [X300] max_send_frame_size: 8500 ``` ``` [INFO] [X300] min_recv_frame_size: 8144 ``` ``` [INFO] [X300] min_send_frame_size: 8144 ``` ``` [INFO] [X300] Maximum frame size: 8144 bytes. ``` ``` [WARNING] [X300] You requested a receive frame size of (8700) but your NIC's max frame size is (8144).Please verify your NIC's MTU setting using 'ip link' or set the recv_frame_size argument appropriately.UHD will use the auto-detected max frame size for this connection. ``` ``` [WARNING] [X300] You requested a send frame size of (8700) but your NIC's max frame size is (8144).Please verify your NIC's MTU setting using 'ip link' or set the send_frame_size argument appropriately.UHD will use the auto-detected max frame size for this connection. ``` ``` [WARNING] [X300] For the 192.168.40.2 connection, UHD recommends a send frame size of at least 8500 for best ``` ``` performance, but your configuration will only allow 8144.This may negatively impact your maximum achievable sample rate. ``` ``` Check the MTU on the interface and/or the send_frame_size argument. ``` ``` [WARNING] [X300] For the 192.168.40.2 connection, UHD recommends a receive frame size of at least 8500 for best ``` ``` performance, but your configuration will only allow 8144.This may negatively impact your maximum achievable sample rate. ``` ``` Check the MTU on the interface and/or the recv_frame_size argument. ``` ``` [INFO] [GPS] Found an internal GPSDO: LC_XO, Firmware Rev 0.929a ``` ``` [INFO] [X300] Radio 1x clock: 200 MHz ``` ``` Setting TX Rate: 200000000.000000... ``` ``` Actual TX Rate: 200000000.000000... ``` ``` Setting TX Gain: 0.000000 db... ```
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