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

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Message-ID: <CAFBinCBvDddU0DDcDfBRPkS+mWi61C0ON5_fneog-jnjOdevXA@mail.gmail.com>
Date:   Fri, 20 Jan 2017 14:47:58 +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

Hi David,

On Sun, Dec 18, 2016 at 5:13 PM, Martin Blumenstingl
<martin.blumenstingl@...glemail.com> wrote:
> 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?
could you please let me know if I should add a a fallback (2ns which
was the old hardcoded value) to the driver or if I should simply leave
it out (that's the way it is right now).
I'm fine with either way, I would just like to avoid duplicate work.
So please let me know how you prefer it.


Regards,
Martin