RE: [Intel-wired-lan] [PATCH iwl] ice: Restore Ordered MMIO Writes for Tx Doorbells

[Nowlin, Alexander]

> -----Original Message-----
> From: Intel-wired-lan <intel-wired-lan-bounces@xxxxxxxxxx> On Behalf Of Bryan Fraschetti via Intel-wired-lan
> Sent: Monday, August 3, 2026 8:29 AM
> To: Nguyen, Anthony L <anthony.l.nguyen@xxxxxxxxx>; Kitszel, Przemyslaw <przemyslaw.kitszel@xxxxxxxxx>; Andrew Lunn <andrew+netdev@xxxxxxx>; David S. Miller <davem@xxxxxxxxxxxxx>; Eric Dumazet <edumazet@xxxxxxxxxx>; Jakub Kicinski <kuba@xxxxxxxxxx>; Paolo Abeni <pabeni@xxxxxxxxxx>; Alice Michael <alice.michael@xxxxxxxxx>; Greenwalt, Paul <paul.greenwalt@xxxxxxxxx>; Fijalkowski, Maciej <maciej.fijalkowski@xxxxxxxxx>; Loktionov, Aleksandr <aleksandr.loktionov@xxxxxxxxx>
> Cc: Bryan Fraschetti <bryan.fraschetti@xxxxxxxxxxxxx>; intel-wired-lan@xxxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: [Intel-wired-lan] [PATCH iwl] ice: Restore Ordered MMIO Writes for Tx Doorbells
> 
> The DQL accounting state which tracks the number of bytes queued for the NIC to transmit, namely dql->num_queued, is updated by the ICE driver when it invokes __netdev_tx_sent_queue(). Subsequently the > driver updates the hardware queue tail allowing the NIC to begin processing the work queue. The queue accounting update should be ordered before the NIC begins transmitting descriptors.
> 
> The transmit path currently updates the hardware doorbell using writel_relaxed(), which (on arm64) does not provide the same ordering guarantees between writes to normal memory and writes to MMIO registers that writel() does. This introduces a potential race where the NIC begins transmitting descriptors before the dql->num_queued update is globally visible. If the NIC finishes before the update is observed > by dql_completed(), it detects an invalid state where more bytes have been completed than have been queued. When this happens the following BUG_ON is triggered.
> 
>   BUG_ON(count > num_queued - dql->num_completed);
> 
> This has been observed and manifests as the following crash (note that the trace has been trimmed) in an environment with sustained network load that uses an Intel Corporation Ethernet Controller E810-XXV > for SFP (rev 02) on an arm64 machine. Replacing writel_relaxed() with writel() in a test kernel eliminated the crash in the user's workload, which previously reproduced the issue reliably.
> 
>   kernel BUG at lib/dynamic_queue_limits.c:99
> 
> Internal error: Oops - BUG: 00000000f2000800 [#1] SMP pc : dql_completed+0x268/0x2a0 lr : ice_clean_tx_irq+0x1d4/0x620 [ice] Call trace:
> dql_completed+0x268/0x2a0 (P)
> ice_napi_poll+0x94/0x520 [ice]
> __napi_poll+0x48/0x3f0
> net_rx_action+0x194/0x420
> 
> This restores the behaviour prior to commit ccde82e90946 ("ice: add E830 Earliest TxTime First Offload support"), which changed the notification mechanism from writel() to writel_relaxed().
> 
> Link: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2161572
> Fixes: ccde82e90946 ("ice: add E830 Earliest TxTime First Offload support")
> Signed-off-by: Bryan Fraschetti <bryan.fraschetti@xxxxxxxxxxxxx>
> ---
>  drivers/net/ethernet/intel/ice/ice_txrx.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

Tested-by: Alexander Nowlin <alexander.nowlin@xxxxxxxxx>