Subject: Re: [PATCH iwl-net v3] ice: support SBQ posted writes with non-posted support for CGU

From: Simon Horman <[email-masked]>
Date: Tue, 26 May 2026 18:06:42 +0100

On Wed, May 20, 2026 at 12:52:03PM +0200, Przemyslaw Korba wrote:
> From: Karol Kolacinski <[email-masked]>
> 
> Sideband queue (SBQ) is a HW queue with very short completion time. All
> SBQ writes were posted by default, which means that the driver did not
> have to wait for completion from the neighbor device, because there was
> none. This introduced unnecessary delays, where only those delays were
> "ensuring" that the command is "completed" and this was a potential race
> condition.
> 
> Add the possibility to perform non-posted writes where it's necessary to
> wait for completion, instead of relying on fake completion from the FW,
> where only the delays are guarding the writes.
> 
> Flush the SBQ by reading address 0 from the PHY 0 before issuing SYNC
> command to ensure that writes to all PHYs were completed and skip SBQ
> message completion if it's posted.
> 
> E810 only supports opcode 0x01, but its FW always sends completion
> responses for this opcode, so the driver waits for each write to complete.
> This makes E810 writes synchronous and eliminates the need for SBQ flush.
> 
> To analyze if delays are gone, look for and compare time spent in
> ice_sq_send_cmd — posted writes should return immediately after the wr32.
> That can be done for example by adjusting phc time with phc_ctl on E830
> device, for less than 2 seconds to use this new mechanism. Without it,
> command below will fail.
> 
> Reproduction steps:
> phc_ctl eth13 adj 1
> phc_ctl[4478170.994]: adjusted clock by 1.000000 seconds
> 
> Check trace for timing for comparisions:
> echo ice_sbq_send_cmd > /sys/kernel/debug/tracing/set_ftrace_filter
> echo function_graph > /sys/kernel/debug/tracing/current_tracer
> cat /sys/kernel/debug/tracing/trace
> 
> Tested on:
>   - Intel E830 NIC (FW version 1.00)
>   - Kernel 6.19.0+
> 
> Fixes: 8f5ee3c477a8 ("ice: add support for sideband messages")
> Signed-off-by: Karol Kolacinski <[email-masked]>
> Signed-off-by: Przemyslaw Korba <[email-masked]>
> Reviewed-by: Aleksandr Loktionov <[email-masked]>
> Reviewed-by: Arkadiusz Kubalewski <[email-masked]>
> ---
> v3:
> - include information in comments and commit message about different
> E810 behavior
> v2:
> https://lore.kernel.org/intel-wired-lan/[email-masked]/

Thanks for the update.

Reviewed-by: Simon Horman <[email-masked]>