RE: [PATCH net 1/2] igb: Preserve RXPBS.CFG_TS_EN in igb_setup_tx_mode

-----Original Message-----
From: Pascal Kneuper <redacted> 
Sent: Thursday, September 10, 2026 4:39 AM
To: Nguyen, Anthony L <anthony.l.nguyen@intel.com>; Kitszel, Przemyslaw <przemyslaw.kitszel@intel.com>; Andrew Lunn <andrew+netdev@lunn.ch>; David S . Miller <davem@davemloft.net>; Eric Dumazet <redacted>; Jakub Kicinski <kuba@kernel.org>; Paolo Abeni <pabeni@redhat.com>
Cc: Baldin, Daniel <redacted>; intel-wired-lan@lists.osuosl.org; netdev@vger.kernel.org; linux-kernel@vger.kernel.org; Kneuper, Paskal <redacted>
Subject: [PATCH net 1/2] igb: Preserve RXPBS.CFG_TS_EN in igb_setup_tx_mode

Commit 05f9d3e1ae6e ("igb: Add support for CBS offload") restores the i210 Rx packet buffer size with an unmasked write to RXPBS. Bit 31 of RXPBS is CFG_TS_EN, which enables per-packet Rx timestamping. Writing the register unconditionally clears this bit on every igb_up() (link down/up, MTU change, or driver reset).

Like other drivers (igc, i40e, ice), igb intends timestamping settings to persist across link changes: igb_reset() calls igb_ptp_reset() to re-apply tstamp_config and set CFG_TS_EN with queues stopped. igb_up() then clears it immediately afterwards, undoing its own restore.

igb_ptp_hwtstamp_get() returns cached settings and never reads the register, masking the fallback to software timestamps. On an i210, a link flap increases PTP path delay from <1us to tens of microseconds.

Fix by masking the buffer size field, as the FQTSS branch already does.

Fixes: 05f9d3e1ae6e ("igb: Add support for CBS offload")
Signed-off-by: Pascal Kneuper <redacted>
---
 drivers/net/ethernet/intel/igb/igb_main.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

Tested-by: Alexander Nowlin <redacted>