On Tue, Jul 02, 2024 at 12:09:26AM -0400, Faizal Rahim wrote:
A large tx latency issue was discovered during testing when only QBV was enabled. The issue occurs because gtxoffset was not set when QBV is active, it was only set when launch time is active.
The patch "igc: Correct the launchtime offset" only sets gtxoffset when the launchtime_enable field is set by the user. Enabling launchtime_enable ultimately sets the register IGC_TXQCTL_QUEUE_MODE_LAUNCHT (referred to as LaunchT in the SW user manual).
Section 7.5.2.6 of the IGC i225/6 SW User Manual Rev 1.2.4 states: "The latency between transmission scheduling (launch time) and the time the packet is transmitted to the network is listed in Table 7-61."
However, the patch misinterprets the phrase "launch time" in that section by assuming it specifically refers to the LaunchT register, whereas it actually denotes the generic term for when a packet is released from the internal buffer to the MAC transmit logic.
This launch time, as per that section, also implicitly refers to the QBV gate open time, where a packet waits in the buffer for the QBV gate to open. Therefore, latency applies whenever QBV is in use. TSN features such as QBU and QAV reuse QBV, making the latency universal to TSN features.
Discussed with i226 HW owner (Shalev, Avi) and we were in agreement that the term "launch time" used in Section 7.5.2.6 is not clear and can be easily misinterpreted. Avi will update this section to: "When TQAVCTRL.TRANSMIT_MODE = TSN, the latency between transmission scheduling and the time the packet is transmitted to the network is listed in Table 7-61."
Fix this issue by using igc_tsn_is_tx_mode_in_tsn() as a condition to write to gtxoffset, aligning with the newly updated SW User Manual.
Tested:
Enrol taprio on talker board base-time 0 cycle-time 1000000 flags 0x2 index 0 cmd S gatemask 0x1 interval1 index 0 cmd S gatemask 0x1 interval2
Note: interval1 = interval for a 64 bytes packet to go through interval2 = cycle-time - interval1
Take tcpdump on listener board
Use udp tai app on talker to send packets to listener
Check the timestamp on listener via wireshark
Test Result: 100 Mbps: 113 ~193 ns 1000 Mbps: 52 ~ 84 ns 2500 Mbps: 95 ~ 223 ns
Note that the test result is similar to the patch "igc: Correct the launchtime offset".
Fixes: 790835fcc0cb ("igc: Correct the launchtime offset") Signed-off-by: Faizal Rahim faizal.abdul.rahim@linux.intel.com
Reviewed-by: Simon Horman horms@kernel.org