netdev - Re: [net-next,v4] net: ethernet: rtsn: Add support for Renesas Ethernet-TSN

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Date: Fri, 10 May 2024 14:38:56 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Niklas Söderlund <niklas.soderlund+renesas@...natech.se>
Cc: Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, linux-renesas-soc@...r.kernel.org
Subject: Re: [net-next,v4] net: ethernet: rtsn: Add support for Renesas
 Ethernet-TSN

On Fri, May 10, 2024 at 12:07:20AM +0200, Niklas Söderlund wrote:
> On 2024-05-09 23:12:43 +0200, Andrew Lunn wrote:
> > > +	/* Link verification */
> > > +	rtsn_modify(priv, MLVC, MLVC_PLV, MLVC_PLV);
> > > +	ret = rtsn_reg_wait(priv, MLVC, MLVC_PLV, 0);
> > > +	if (ret)
> > > +		return ret;
> > 
> > What is link verification?
> 
> It's a part of the TSN capabilities, it verifies if the link is capable 
> of preemption. AFIK it's described in IEEE802.1Qbu Frame Preemption 
> Protocol.
> 
> For this device in particular, this register writes enables the Link 
> Verification process. Which in turn allows the IP to transmitting Verify 
> frames, which if replied too with a valid Reply frame by the other node 
> lets the IP know the link supports preemption.

Thanks for the explanation.

I've seen some hardware which polls the PHY for link status. Such
hardware assumes it has exclusive access to the PHY, which is
incorrect in Linux. hwmon might of swapped the PHY to a different page
in order to read a temperature sensor etc. If the hardware were to
poll at that point, rather than getting the BMSR it reads some other
random register...

I just wanted to check it was not something like this.

	Andrew