linux-kernel - Re: [PATCH net-next v1 3/5] net: stmmac: support fp parameter of tc-taprio

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Message-ID: <20240801232937.rmkv3er5cc2lykwf@skbuf>
Date: Fri, 2 Aug 2024 02:29:37 +0300
From: Vladimir Oltean <olteanv@...il.com>
To: Furong Xu <0x1207@...il.com>
Cc: Andrew Lunn <andrew@...n.ch>, "David S. Miller" <davem@...emloft.net>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Jose Abreu <joabreu@...opsys.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Joao Pinto <jpinto@...opsys.com>, netdev@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	xfr@...look.com, rock.xu@....com
Subject: Re: [PATCH net-next v1 3/5] net: stmmac: support fp parameter of
 tc-taprio

On Wed, Jul 31, 2024 at 06:43:14PM +0800, Furong Xu wrote:
> tc-taprio can select whether traffic classes are express or preemptible.
> 
> After some traffic tests, MAC merge layer statistics are all good.
> 
> Local device:
> ethtool --include-statistics --json --show-mm eth1
> [ {
>         "ifname": "eth1",
>         "pmac-enabled": true,
>         "tx-enabled": true,
>         "tx-active": true,
>         "tx-min-frag-size": 60,
>         "rx-min-frag-size": 60,
>         "verify-enabled": true,
>         "verify-time": 100,
>         "max-verify-time": 128,
>         "verify-status": "SUCCEEDED",
>         "statistics": {
>             "MACMergeFrameAssErrorCount": 0,
>             "MACMergeFrameSmdErrorCount": 0,
>             "MACMergeFrameAssOkCount": 0,
>             "MACMergeFragCountRx": 0,
>             "MACMergeFragCountTx": 1398,
>             "MACMergeHoldCount": 15783

In order for readers to really understand this output (including me),
could you also post the associated tc-taprio command, please?

You deleted the code that treated the Set-And-Hold-MAC GCL command -
and according to 802.1Q, that is the only source of Hold requests.
I _think_ that as a side effect of your reimplementation, every time the
gate for TC 0 opens, the HoldCount bumps by one. Would that be a correct
description?

The more unfortunate part is that I haven't yet come across a NIC
hardware design that would behave completely as you'd expect w.r.t. Hold
requests. In the case of DWMAC, I would expect that with a taprio
schedule that lacks any Set-And-Hold-MAC command, the HoldCount would
stay at zero. I'm not sure, given the way they piggy back onto gate 0
for Hold/Release, that this is possible :(

At least HoldCount stays constant with a tc-mqprio offload, right?

>         }
>     } ]