netdev - Re: [PATCH net-next v4 2/2] net: stmmac: dwmac-meson8b: make the RGMII TX delay configurable

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Message-ID: <CAFBinCDZ_mqt8q2rG6zDy8ke5sX6L+4sVYOOhWUdO3apwFiRYw@mail.gmail.com>
Date:   Sun, 18 Dec 2016 17:13:50 +0100
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, devicetree@...r.kernel.org,
        linux-amlogic@...ts.infradead.org, robh+dt@...nel.org,
        mark.rutland@....com, carlo@...one.org, khilman@...libre.com,
        peppe.cavallaro@...com, alexandre.torgue@...com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH net-next v4 2/2] net: stmmac: dwmac-meson8b: make the
 RGMII TX delay configurable

On Sun, Dec 18, 2016 at 4:49 PM, David Miller <davem@...emloft.net> wrote:
> From: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
> Date: Sat, 17 Dec 2016 19:21:19 +0100
>
>> Prior to this patch we were using a hardcoded RGMII TX clock delay of
>> 2ns (= 1/4 cycle of the 125MHz RGMII TX clock). This value works for
>> many boards, but unfortunately not for all (due to the way the actual
>> circuit is designed, sometimes because the TX delay is enabled in the
>> PHY, etc.). Making the TX delay on the MAC side configurable allows us
>> to support all possible hardware combinations.
>>
>> This allows fixing a compatibility issue on some boards, where the
>> RTL8211F PHY is configured to generate the TX delay. We can now turn
>> off the TX delay in the MAC, because otherwise we would be applying the
>> delay twice (which results in non-working TX traffic).
>>
>> Signed-off-by: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
>> Tested-by: Neil Armstrong <narmstrong@...libre.com>
>
> Is this really the safest thing to do?
>
> If you say the existing hard-coded setting of 1/4 cycle works on most
> boards, and what you're trying to do is override it with an OF
> property value for boards where the existing setting does not work,
> then you _must_ use a default value that corresponds to what the
> existing code does not when you don't see this new OF property.
it's a bit more complicated in reality: 1/4 cycle works when the TX
delay of the RTL8211F PHY is turned off (until recently it was always
enabled for phy-mode RGMII).

> So please retain the current behavior of the 1/4 cycle TX delay
> setting when you don't see the amlogic,tx-delay-ns property.
>
> I really think you risk breaking existing boards by not doing so,
> unless you can have this patch tested on every such board that exists
> and I don't think you really can feasibly and rigorously do that.
there's a patch in my follow-up series which adds the 2ns to the .dts
for all RGMII based boards: [0] (and I would keep these even if we had
a default value, just to make it explicit and thus easier to
understand for other people).
however, we can add the 2ns default back (I can do this if you want -
Rob Herring was unhappy with the missing documentation of this default
value [1] - so note to myself: take care of that as well). but then we
have to decide when to apply this default value: only when we're in
RGMII mode or also in any of the RGMII_*ID modes?

please let me know how we should proceed


Regards,
Martin


[0] http://lists.infradead.org/pipermail/linux-amlogic/2016-December/001838.html
[1] http://lists.infradead.org/pipermail/linux-amlogic/2016-November/001817.html