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

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Message-ID: <20200529142823.GC869823@lunn.ch>
Date:   Fri, 29 May 2020 16:28:23 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Geert Uytterhoeven <geert+renesas@...der.be>
Cc:     Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Oleksij Rempel <linux@...pel-privat.de>,
        Philippe Schenker <philippe.schenker@...adex.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Kazuya Mizuguchi <kazuya.mizuguchi.ks@...esas.com>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Wolfram Sang <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

On Fri, May 29, 2020 at 02:25:40PM +0200, Geert Uytterhoeven wrote:
> 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>

Reviewed-by: Andrew Lunn <andrew@...n.ch>

    Andrew