netdev - Re: [PATCH RFT] ravb: Mask PHY mode to avoid inserting delays twice

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Date:   Sat, 30 May 2020 21:51:02 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     geert+renesas@...der.be
Cc:     sergei.shtylyov@...entembedded.com, kuba@...nel.org,
        andrew@...n.ch, linux@...pel-privat.de,
        philippe.schenker@...adex.com, f.fainelli@...il.com,
        hkallweit1@...il.com, linux@...linux.org.uk,
        kazuya.mizuguchi.ks@...esas.com, grygorii.strashko@...com,
        wsa+renesas@...g-engineering.com, netdev@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFT] ravb: Mask PHY mode to avoid inserting delays twice

From: Geert Uytterhoeven <geert+renesas@...der.be>
Date: Fri, 29 May 2020 14:25:40 +0200

> Until recently, the Micrel KSZ9031 PHY driver ignored any PHY mode
> ("RGMII-*ID") settings, but used the hardware defaults, augmented by
> explicit configuration of individual skew values using the "*-skew-ps"
> DT properties.  The lack of PHY mode support was compensated by the
> EtherAVB MAC driver, which configures TX and/or RX internal delay
> itself, based on the PHY mode.
> 
> However, now the KSZ9031 driver has gained PHY mode support, delays may
> be configured twice, causing regressions.  E.g. on the Renesas
> Salvator-X board with R-Car M3-W ES1.0, TX performance dropped from ca.
> 400 Mbps to 0.1-0.3 Mbps, as measured by nuttcp.
> 
> As internal delay configuration supported by the KSZ9031 PHY is too
> limited for some use cases, the ability to configure MAC internal delay
> is deemed useful and necessary.  Hence a proper fix would involve
> splitting internal delay configuration in two parts, one for the PHY,
> and one for the MAC.  However, this would require adding new DT
> properties, thus breaking DTB backwards-compatibility.
> 
> Hence fix the regression in a backwards-compatibility way, by letting
> the EtherAVB driver mask the PHY mode when it has inserted a delay, to
> avoid the PHY driver adding a second delay.  This also fixes messages
> like:
> 
>     Micrel KSZ9031 Gigabit PHY e6800000.ethernet-ffffffff:00: *-skew-ps values should be used only with phy-mode = "rgmii"
> 
> as the PHY no longer sees the original RGMII-*ID mode.
> 
> Solving the issue by splitting configuration in two parts can be handled
> in future patches, and would require retaining a backwards-compatibility
> mode anyway.
> 
> Fixes: bcf3440c6dd78bfe ("net: phy: micrel: add phy-mode support for the KSZ9031 PHY")
> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>

Applied to net-next, thank you.