netdev - Re: [net-next PATCH v1 0/2] stmmac: dwmac-meson8b: configurable RGMII TX delay

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Date:   Thu, 24 Nov 2016 18:05:22 +0100
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     Jerome Brunet <jbrunet@...libre.com>, f.fainelli@...il.com
Cc:     linux-amlogic@...ts.infradead.org, devicetree@...r.kernel.org,
        netdev@...r.kernel.org, davem@...emloft.net, khilman@...libre.com,
        mark.rutland@....com, robh+dt@...nel.org,
        linux-arm-kernel@...ts.infradead.org, alexandre.torgue@...com,
        peppe.cavallaro@...com, carlo@...one.org
Subject: Re: [net-next PATCH v1 0/2] stmmac: dwmac-meson8b: configurable RGMII
 TX delay

On Thu, Nov 24, 2016 at 4:56 PM, Jerome Brunet <jbrunet@...libre.com> wrote:
> On Thu, 2016-11-24 at 15:34 +0100, Martin Blumenstingl wrote:
>> Currently the dwmac-meson8b stmmac glue driver uses a hardcoded 1/4
>> cycle TX clock delay. This seems to work fine for many boards (for
>> example Odroid-C2 or Amlogic's reference boards) but there are some
>> others where TX traffic is simply broken.
>> There are probably multiple reasons why it's working on some boards
>> while it's broken on others:
>> - some of Amlogic's reference boards are using a Micrel PHY
>> - hardware circuit design
>> - maybe more...
>>
>> This raises a question though:
>> Which device is supposed to enable the TX delay when both MAC and PHY
>> support it? And should we implement it for each PHY / MAC separately
>> or should we think about a more generic solution (currently it's not
>> possible to disable the TX delay generated by the RTL8211F PHY via
>> devicetree when using phy-mode "rgmii")?
>
> Actually you can skip the part which activate the Tx-delay on the phy
> by setting "phy-mode = "rgmii-id" instead of "rgmii"
>
> phy->interface will no longer be PHY_INTERFACE_MODE_RGMII
> but PHY_INTERFACE_MODE_RGMII_ID.
unfortunately this is not true for RTL8211F (I did my previous tests
with the same expectation in mind)!
the code seems to suggest that TX-delay is disabled whenever mode !=
PHY_INTERFACE_MODE_RGMII.
BUT: on my device RTL8211F_TX_DELAY is set even before
"phy_write(phydev, 0x11, reg);"!

Based on what I found it seems that rgmii-id, rgmii-txid and
rgmii-rxid are supposed to be handled by the PHY.
That would mean that we have two problems here:
1) drivers/net/phy/realtek.c:rtl8211f_config_init should check for
PHY_INTERFACE_MODE_RGMII_ID or PHY_INTERFACE_MODE_RGMII_TXID and
enable the TX-delay in that case - otherwise explicitly disable it
2) dwmac-meson8b.c should only use the configured TX-delay for
PHY_INTERFACE_MODE_RGMII
@Florian: could you please share your thoughts on this (who handles
the TX delay in which case)?


Regards,
Martin