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

[Prasanna Vengateshan <prasanna.vengateshan@microchip.com>]

On Mon, 2021-08-02 at 16:59 +0300, Vladimir Oltean wrote:
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> 
> 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.
> 
> That discussion, and Russell's point, was here, btw:
> https://patchwork.ozlabs.org/project/netdev/patch/20200616074955.GA9092@laureti-dev/#2461123

Thanks Vladimir & Andrew for the right pointers and info. The thread talks about
"rgmii-*" are going to be applied by the PHY only as per the doc. For fixed-
link, MAC needs to add the delay. This fixed-link can be No-PHY or MAC-MAC or
MAC to in-accessible PHY. In such case, i am not convinced in using rgmii-tx-
delay-ps & rgmii-rx-delay-ps on the MAC side and apply delay. I still think
proposed code in earlier mail thread should still be okay.