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

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Message-ID: <20250312155245.fdntan3nml424ufe@skbuf>
Date: Wed, 12 Mar 2025 17:52:45 +0200
From: Vladimir Oltean <vladimir.oltean@....com>
To: Faizal Rahim <faizal.abdul.rahim@...ux.intel.com>
Cc: Tony Nguyen <anthony.l.nguyen@...el.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S . Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Simon Horman <horms@...nel.org>,
	Russell King <linux@...linux.org.uk>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>,
	Furong Xu <0x1207@...il.com>,
	Russell King <rmk+kernel@...linux.org.uk>,
	Serge Semin <fancer.lancer@...il.com>,
	Xiaolei Wang <xiaolei.wang@...driver.com>,
	Suraj Jaiswal <quic_jsuraj@...cinc.com>,
	Kory Maincent <kory.maincent@...tlin.com>,
	Gal Pressman <gal@...dia.com>,
	Jesper Nilsson <jesper.nilsson@...s.com>,
	Choong Yong Liang <yong.liang.choong@...ux.intel.com>,
	Chwee-Lin Choong <chwee.lin.choong@...el.com>,
	Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>,
	Vinicius Costa Gomes <vinicius.gomes@...el.com>,
	intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org, bpf@...r.kernel.org
Subject: Re: [PATCH iwl-next v9 10/14] igc: add support for frame preemption
 verification

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@...el.com>
> Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@...el.com>
> Co-developed-by: Choong Yong Liang <yong.liang.choong@...ux.intel.com>
> Signed-off-by: Choong Yong Liang <yong.liang.choong@...ux.intel.com>
> Co-developed-by: Chwee-Lin Choong <chwee.lin.choong@...el.com>
> Signed-off-by: Chwee-Lin Choong <chwee.lin.choong@...el.com>
> Signed-off-by: Faizal Rahim <faizal.abdul.rahim@...ux.intel.com>
> ---

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