netdev - Re: [PATCH v3 net-next 05/10] net: dsa: microchip: add DSA support for microchip lan937x

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Date:   Mon, 2 Aug 2021 22:47:16 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Prasanna Vengateshan <prasanna.vengateshan@...rochip.com>,
        netdev@...r.kernel.org, robh+dt@...nel.org,
        UNGLinuxDriver@...rochip.com, Woojung.Huh@...rochip.com,
        hkallweit1@...il.com, linux@...linux.org.uk, davem@...emloft.net,
        kuba@...nel.org, linux-kernel@...r.kernel.org,
        vivien.didelot@...il.com, f.fainelli@...il.com,
        devicetree@...r.kernel.org
Subject: Re: [PATCH v3 net-next 05/10] net: dsa: microchip: add DSA support
 for microchip lan937x

On Mon, Aug 02, 2021 at 04:59:11PM +0300, Vladimir Oltean wrote:
> On Mon, Aug 02, 2021 at 03:13:01PM +0200, Andrew Lunn wrote:
> > In general, the MAC does nothing, and passes the value to the PHY. The
> > PHY inserts delays as requested. To address Vladimir point,
> > PHY_INTERFACE_MODE_RGMII_TXID would mean the PHY adds delay in the TX
> > direction, and assumes the RX delay comes from somewhere else,
> > probably the PCB.
> 
> For the PHY, that is the only portion where things are clear.
> 
> > I only recommend the MAC adds delays when the PHY cannot, or there is
> > no PHY, e.g. SoC to switch, or switch to switch link. There are a few
> > MAC drivers that do add delays, mostly because that is how the vendor
> > crap tree does it.
> > 
> > So as i said, what you propose is O.K, it follows this general rule of
> > thumb.
> 
> The "rule of thumb" for a MAC driver is actually applied in reverse by
> most MAC drivers compared to what Russell described should be happening.
> For example, mv88e6xxx_port_set_rgmii_delay():
> 
> 	switch (mode) {
> 	case PHY_INTERFACE_MODE_RGMII_RXID:
> 		reg |= MV88E6XXX_PORT_MAC_CTL_RGMII_DELAY_RXCLK;
> 
> The mv88e6xxx is a MAC, so when it has a phy-mode = "rgmii-rxid", it
> should assume it is connected to a link partner (PHY or otherwise) that
> has applied the RXCLK delay already. So it should only be concerned with
> the TXCLK delay. That is my point. I am just trying to lay out the
> points to Prasanna that would make a sane system going forward. I am not
> sure that we actually have an in-tree driver that is sane in that
> regard.

It is a can or worms. For the used use case for the mv88e6xxx, it is a
DSA link, so there isn't really one side MAC and the other side
PHY. And if i remember correctly, both sides use rgmii-rxid.

     Andrew