diff mbox series

[net,4/4] igc: Fix qbv tx latency by setting gtxoffset

Message ID 20240807231329.3827092-5-anthony.l.nguyen@intel.com (mailing list archive)
State Accepted
Delegated to: Netdev Maintainers
Headers show
Series Intel Wired LAN Driver Updates 2024-08-07 (igc) | expand

Checks

Context Check Description
netdev/series_format success Posting correctly formatted
netdev/tree_selection success Clearly marked for net
netdev/ynl success Generated files up to date; no warnings/errors; no diff in generated;
netdev/fixes_present success Fixes tag present in non-next series
netdev/header_inline success No static functions without inline keyword in header files
netdev/build_32bit success Errors and warnings before: 29 this patch: 29
netdev/build_tools success No tools touched, skip
netdev/cc_maintainers fail 1 blamed authors not CCed: pmenzel@molgen.mpg.de; 1 maintainers not CCed: pmenzel@molgen.mpg.de
netdev/build_clang success Errors and warnings before: 29 this patch: 29
netdev/verify_signedoff success Signed-off-by tag matches author and committer
netdev/deprecated_api success None detected
netdev/check_selftest success No net selftest shell script
netdev/verify_fixes success Fixes tag looks correct
netdev/build_allmodconfig_warn success Errors and warnings before: 29 this patch: 29
netdev/checkpatch success total: 0 errors, 0 warnings, 0 checks, 8 lines checked
netdev/build_clang_rust success No Rust files in patch. Skipping build
netdev/kdoc success Errors and warnings before: 0 this patch: 0
netdev/source_inline success Was 0 now: 0
netdev/contest success net-next-2024-08-10--18-00 (tests: 707)

Commit Message

Tony Nguyen Aug. 7, 2024, 11:13 p.m. UTC 
From: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>

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:
1. 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

2. Take tcpdump on listener board

3. Use udp tai app on talker to send packets to listener

4. 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>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
---
 drivers/net/ethernet/intel/igc/igc_tsn.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
diff mbox series

Patch

diff --git a/drivers/net/ethernet/intel/igc/igc_tsn.c b/drivers/net/ethernet/intel/igc/igc_tsn.c
index ada751430517..d68fa7f3d5f0 100644
--- a/drivers/net/ethernet/intel/igc/igc_tsn.c
+++ b/drivers/net/ethernet/intel/igc/igc_tsn.c
@@ -61,7 +61,7 @@  void igc_tsn_adjust_txtime_offset(struct igc_adapter *adapter)
 	struct igc_hw *hw = &adapter->hw;
 	u16 txoffset;
 
-	if (!is_any_launchtime(adapter))
+	if (!igc_tsn_is_tx_mode_in_tsn(adapter))
 		return;
 
 	switch (adapter->link_speed) {