Re: [PATCH iwl-next v9 10/14] igc: add support for frame preemption verification

[Vladimir Oltean <vladimir.oltean@nxp.com>]

On Sun, Mar 09, 2025 at 06:46:44AM -0400, Faizal Rahim wrote:
> This patch implements the "ethtool --set-mm" callback to trigger the
> frame preemption verification handshake.
> 
> Uses the MAC Merge Software Verification (mmsv) mechanism in ethtool
> to perform the verification handshake for igc.
> The structure fpe.mmsv is set by mmsv in ethtool and should remain
> read-only for the driver.
> 
> Other mmsv callbacks:
> a) configure_tx() -> not used yet at this point
>    - igc lacks registers to configure FPE in the transmit direction, so
>      this API is not utilized for now. When igc supports preemptible queue,
>      driver will use this API to manage its configuration.
> 
> b) configure_pmac() -> not used
>    - this callback dynamically controls pmac_enabled at runtime. For
>      example, mmsv calls configure_pmac() and disables pmac_enabled when
>      the link partner goes down, even if the user previously enabled it.
>      The intention is to save power but it is not feasible in igc
>      because it causes an endless adapter reset loop:
> 
>    1) Board A and Board B complete the verification handshake. Tx mode
>       register for both boards are in TSN mode.
>    2) Board B link goes down.
> 
>    On Board A:
>    3) mmsv calls configure_pmac() with pmac_enabled = false.
>    4) configure_pmac() in igc updates a new field based on pmac_enabled.
>       Driver uses this field in igc_tsn_new_flags() to indicate that the
>       user enabled/disabled FPE.
>    5) configure_pmac() in igc calls igc_tsn_offload_apply() to check
>       whether an adapter reset is needed. Calls existing logic in
>       igc_tsn_will_tx_mode_change() and igc_tsn_new_flags().
>    6) Since pmac_enabled is now disabled and no other TSN feature is
>       active, igc_tsn_will_tx_mode_change() evaluates to true because Tx
>       mode will switch from TSN to Legacy.
>    7) Driver resets the adapter.
>    8) Registers are set, and Tx mode switches to Legacy.
>    9) When link partner is up, steps 3–8 repeat, but this time with
>       pmac_enabled = true, reactivating TSN.
>       igc_tsn_will_tx_mode_change() evaluates to true again, since Tx
>       mode will switch from Legacy to TSN.
>   10) Driver resets the adapter.
>   11) Adapter reset completes, registers are set, and Tx mode switches to
>       TSN.
> 
>   On Board B:
>   12) Adapter reset on Board A at step 10 causes it to detect its link
>       partner as down.
>   13) Repeats steps 3–8.
>   14) Once reset adapter on Board A is completed at step 11, it detects
>       its link partner as up.
>   15) Repeats steps 9–11.
> 
>    - this cycle repeats indefinitely. To avoid this issue, igc only uses
>      mmsv.pmac_enabled to track whether FPE is enabled or disabled.
> 
> Co-developed-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
> Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
> Co-developed-by: Choong Yong Liang <yong.liang.choong@linux.intel.com>
> Signed-off-by: Choong Yong Liang <yong.liang.choong@linux.intel.com>
> Co-developed-by: Chwee-Lin Choong <chwee.lin.choong@intel.com>
> Signed-off-by: Chwee-Lin Choong <chwee.lin.choong@intel.com>
> Signed-off-by: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
> ---

Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>