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

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Message-ID: <ba68dd2b-b605-435e-8f6b-457bf8af08c6@lunn.ch>
Date: Tue, 16 Apr 2024 15:05:31 +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] net: ethernet: rtsn: Add support for Renesas
 Ethernet-TSN

> > > +	if (!of_property_read_u32(np, "rx-internal-delay-ps", &delay))
> > > +		if (delay)
> > > +			val |= GPOUT_RDM;
> > > +
> > > +	/* Valid values are 0 and 2000, according to DT bindings */
> > > +	if (!of_property_read_u32(np, "tx-internal-delay-ps", &delay))
> > > +		if (delay)
> > > +			val |= GPOUT_TDM;
> > > +
> > > +	rtsn_write(priv, GPOUT, val);
> > 
> > So you seem to be using it as bool?
> 
> Yes.
> 
> > That is wrong. It is a number of pico seconds!
> 
> The issue is that the hardware only supports no delay or a fixed delay 
> that can depend on electric properties of the board.

The general convention is that the MAC does not add delays, it leaves
it to the PHY. Probably 95% of boards are like this, and many MAC
drivers don't even add support for configuring their hardware delays,
it is not needed. Those that do, it is generally for fine tuning the
delays, being able to add/remove 100s of pico seconds, not the full
2us. This hardware cannot do that.

So i suggest you drop all this code, and just hard code the delay to
0ps.

	Andrew