Re: [Intel-wired-lan] [PATCH iwl-net v1 4/4] igc: Fix qbv tx latency by setting gtxoffset

[Simon Horman <horms@kernel.org>]

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:
> 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>