netdev - Re: [PATCH v2 net-next 2/9] net: enetc: report mm tx-active based on tx-enabled and verify-status

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Date:   Thu, 20 Apr 2023 16:40:59 +0200
From:   Simon Horman <simon.horman@...igine.com>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Michal Kubecek <mkubecek@...e.cz>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Xiaoliang Yang <xiaoliang.yang_1@....com>,
        Petr Machata <petrm@...dia.com>,
        Danielle Ratson <danieller@...dia.com>,
        Pranavi Somisetty <pranavi.somisetty@....com>,
        Harini Katakam <harini.katakam@....com>,
        Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Gerhard Engleder <gerhard@...leder-embedded.com>,
        Ferenc Fejes <ferenc.fejes@...csson.com>,
        Aaron Conole <aconole@...hat.com>,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 net-next 2/9] net: enetc: report mm tx-active based on
 tx-enabled and verify-status

On Tue, Apr 18, 2023 at 02:14:52PM +0300, Vladimir Oltean wrote:
> The MMCSR register contains 2 fields with overlapping meaning:
> 
> - LPA (Local preemption active):
> This read-only status bit indicates whether preemption is active for
> this port. This bit will be set if preemption is both enabled and has
> completed the verification process.
> - TXSTS (Merge status):
> This read-only status field provides the state of the MAC Merge sublayer
> transmit status as defined in IEEE Std 802.3-2018 Clause 99.
> 00 Transmit preemption is inactive
> 01 Transmit preemption is active
> 10 Reserved
> 11 Reserved
> 
> However none of these 2 fields offer reliable reporting to software.
> 
> When connecting ENETC to a link partner which is not capable of Frame
> Preemption, the expectation is that ENETC's verification should fail
> (VSTS=4) and its MM TX direction should be inactive (LPA=0, TXSTS=00)
> even though the MM TX is enabled (ME=1). But surprise, the LPA bit of
> MMCSR stays set even if VSTS=4 and ME=1.
> 
> OTOH, the TXSTS field has the opposite problem. I cannot get its value
> to change from 0, even when connecting to a link partner capable of
> frame preemption, which does respond to its verification frames (ME=1
> and VSTS=3, "SUCCEEDED").
> 
> The only option with such buggy hardware seems to be to reimplement the
> formula for calculating tx-active in software, which is for tx-enabled
> to be true, and for the verify-status to be either SUCCEEDED, or
> DISABLED.
> 
> Without reliable tx-active reporting, we have no good indication when
> to commit the preemptible traffic classes to hardware, which makes it
> possible (but not desirable) to send preemptible traffic to a link
> partner incapable of receiving it. However, currently we do not have the
> logic to wait for TX to be active yet, so the impact is limited.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>

Reviewed-by: Simon Horman <simon.horman@...igine.com>