linux-kernel - Re: [PATCH 2/2] net: ethernet: stmicro: stmmac: dwmac-rk: Add rv1126 support

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Date:   Thu, 1 Sep 2022 14:12:36 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Jagan Teki <jagan@...eble.ai>
Cc:     Anand Moon <anand@...eble.ai>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Sugar Zhang <sugar.zhang@...k-chips.com>,
        David Wu <david.wu@...k-chips.com>, netdev@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] net: ethernet: stmicro: stmmac: dwmac-rk: Add rv1126
 support

On Thu, Sep 01, 2022 at 12:56:09PM +0530, Jagan Teki wrote:
> On Mon, 29 Aug 2022 at 18:40, Andrew Lunn <andrew@...n.ch> wrote:
> >
> > On Mon, Aug 29, 2022 at 06:50:42AM +0000, Anand Moon wrote:
> > > Rockchip RV1126 has GMAC 10/100/1000M ethernet controller
> > > via RGMII and RMII interfaces are configured via M0 and M1 pinmux.
> > >
> > > This patch adds rv1126 support by adding delay lines of M0 and M1
> > > simultaneously.
> >
> > What does 'delay lines' mean with respect to RGMII?
> 
> These are MAC receive clock delay lengths.
> 
> >
> > The RGMII signals need a 2ns delay between the clock and the data
> > lines. There are three places this can happen:
> >
> > 1) In the PHY
> > 2) Extra long lines on the PCB
> > 3) In the MAC
> >
> > Generally, 1) is used, and controlled via phy-mode. A value of
> > PHY_INTERFACE_MODE_RGMII_ID passed to the PHY driver means it will add
> > these delays.
> >
> > You don't want both the MAC and the PHY adding delays.
> 
> Yes, but these are specific to MAC, not related to PHY delays. Similar
> to what is there in other Rockchip SoC families like RK3366, 3368,
> 3399, 3128, but these MAC clock delay lengths are grouped based on the
> iomux group in RV1126. We have iomux group 0 (M0) and group 1 (M1), so
> the rgmii has to set these lengths irrespective of whether PHY add's
> or not.

So this is just fine tuning, in the order of pico seconds?

If that is all it is, then this is fine. It becomes a problem when it
is 2ns.

	Andrew