netdev - Re: [PATCH v2 3/3] net: stmmac: Bring down the clocks to lower frequencies when mac link goes down

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Date: Wed, 26 Jun 2024 17:10:35 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Andrew Halaney <ahalaney@...hat.com>
Cc: Sagar Cheluvegowda <quic_scheluve@...cinc.com>,
	Vinod Koul <vkoul@...nel.org>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Jose Abreu <joabreu@...opsys.com>,
	"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>,
	Russell King <linux@...linux.org.uk>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Bhupesh Sharma <bhupesh.sharma@...aro.org>, kernel@...cinc.com,
	linux-arm-msm@...r.kernel.org, netdev@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org
Subject: Re: [PATCH v2 3/3] net: stmmac: Bring down the clocks to lower
 frequencies when mac link goes down

> I'm still curious if any of the netdev folks have any opinion on scaling
> things down like this on link down.

It does make sense, in that there are no frames to process, so the
clock can be reduced. But i also think it is a bit of a workaround for
poor hardware design. Often you can tell the MAC the link is down, and
it can shut down a lot more, and even turn all the clocks off.

I also wounder if there are going to be any side effects of this. Some
Ethernet MACs export a clock to the PHY. Is that clock going to
change? I don't think it will, because we are changing to a valid MAC
speed, not 0. So the PHY has to work at this speed clock.

But to make it easier to find issues like this, open() should probably
set the clocks to a low speed until the link is up. That way, if there
are going to be problems, the link should never come up, as opposed to
the link never comes up after being lost the first time...

     Andrew