netdev - Re: [PATCH RESEND net-next v2 1/8] dt-bindings: net: dwmac-sun8i: Clean up clock delay chain descriptions

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Message-ID: <20180513202749.GG12738@lunn.ch>
Date:   Sun, 13 May 2018 22:27:49 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Icenowy Zheng <icenowy@...c.io>
Cc:     linux-arm-kernel@...ts.infradead.org, Chen-Yu Tsai <wens@...e.org>,
        devicetree <devicetree@...r.kernel.org>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        netdev <netdev@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Corentin Labbe <clabbe.montjoie@...il.com>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>
Subject: Re: [PATCH RESEND net-next v2 1/8] dt-bindings: net: dwmac-sun8i:
 Clean up clock delay chain descriptions

On Mon, May 14, 2018 at 04:10:58AM +0800, Icenowy Zheng wrote:
> 
> 
> 于 2018年5月14日 GMT+08:00 上午4:05:29, Andrew Lunn <andrew@...n.ch> 写到:
> >> > Hi Chen-Yu
> >> >
> >> > Are these delays the MAC applies? Not the PHY. It would be good to
> >> > make it clear here these are MAC imposed delays.
> >> 
> >> Yes these are applied on the MAC side. Being described in the device
> >> tree bindings for the MAC, I thought this was implied to be the case?
> >> Are there known exceptions?
> >
> >There is frequent confusion with this. Most of the time, the PHY does
> >the delay, not the MAC, based on the phy-mode. So the MAC doing it is
> >an exception in itself.
> >
> >Do you actually need these delays for the board you adding support
> >for? Does the PHY not support adding the needed delays? If you don't
> >need the delays, i would not even implement them.
> 
> They are needed if the rx/tx have not equal length.
> 

> For several boards if the delay is not present, the network
> is unusable. (I have at least met this on Banana Pi M3, an
> Allwinner A83T SBC w/ RTL8211E)

The RTL8211E does not appear to support adding delays, via
registers. You can do it with strapping resistors. The RTL8211F does
support delays via a register. So for the Banana Pi M3 it is
justified.

However, in general, i would recommend letting the PHY do it, if it
can.

	Andrew