Date:   Thu, 18 Jun 2020 12:02:13 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Helmut Grohne <helmut.grohne@...enta.de>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: macb: reject unsupported rgmii delays



On 6/18/2020 11:49 AM, Russell King - ARM Linux admin wrote:
> On Thu, Jun 18, 2020 at 11:06:43AM -0700, Florian Fainelli wrote:
>>
>>
>> On 6/18/2020 1:45 AM, Russell King - ARM Linux admin wrote:
>>> On Thu, Jun 18, 2020 at 10:14:33AM +0200, Helmut Grohne wrote:
>>>> On Wed, Jun 17, 2020 at 02:08:09PM +0200, Russell King - ARM Linux admin wrote:
>>>>> With a fixed link, we could be in either a MAC-to-PHY or MAC-to-MAC
>>>>> setup; we just don't know.  However, we don't have is access to the
>>>>> PHY (if it exists) in the fixed link case to configure it for the
>>>>> delay.
>>>>
>>>> Let me twist that a little: We may have access to the PHY, but we don't
>>>> always have access. When we do have access, we have a separate device
>>>> tree node with another fixed-link and another phy-mode. For fixed-links,
>>>> we specify the phy-mode for each end.
>>>
>>> If you have access to the PHY, you shouldn't be using fixed-link. In
>>> any case, there is no support for a fixed-link specification at the
>>> PHY end in the kernel.  The PHY binding doc does not allow for this
>>> either.
>>>
>>>>> In the MAC-to-MAC RGMII setup, where neither MAC can insert the
>>>>> necessary delay, the only way to have a RGMII conformant link is to
>>>>> have the PCB traces induce the necessary delay. That errs towards
>>>>> PHY_INTERFACE_MODE_RGMII for this case.
>>>>
>>>> Yes.
>>>>
>>>>> However, considering the MAC-to-PHY RGMII fixed link case, where the
>>>>> PHY may not be accessible, and may be configured with the necessary
>>>>> delay, should that case also use PHY_INTERFACE_MODE_RGMII - clearly
>>>>> that would be as wrong as using PHY_INTERFACE_MODE_RGMII_ID would
>>>>> be for the MAC-to-MAC RGMII with PCB-delays case.
>>>>
>>>> If you take into account that the PHY has a separate node with phy-mode
>>>> being rgmii-id, it makes a lot more sense to use rgmii for the phy-mode
>>>> of the MAC. So I don't think it is that clear that doing so is wrong.
>>>
>>> The PHY binding document does not allow for this, neither does the
>>> kernel.
>>>
>>>> In an earlier discussion Florian Fainelli said:
>>>> https://patchwork.ozlabs.org/project/netdev/patch/20190410005700.31582-19-olteanv@gmail.com/#2149183
>>>> | fixed-link really should denote a MAC to MAC connection so if you have
>>>> | "rgmii-id" on one side, you would expect "rgmii" on the other side
>>>> | (unless PCB traces account for delays, grrr).
>>>>
>>>> For these reasons, I still think that rgmii would be a useful
>>>> description for the fixed-link to PHY connection where the PHY adds the
>>>> delay.
>>>
>>> I think Florian is wrong; consider what it means when you have a
>>> fixed-link connection between a MAC and an inaccessible PHY and
>>> the link as a whole is operating in what we would call "rgmii-id"
>>> mode if the PHY were accessible.
>>>
>>> Taking Florian's stance, it basically means that DT no longer
>>> describes the hardware, but how we have chosen to interpret the
>>> properties in _one_ specific case in a completely different way
>>> to all the other cases.
>>
>> How do you ensure that a MAC to MAC connection using RGMII actually
>> works if you do not provide a 'phy-mode' for both sides right now?
>>
>> Yes this is more of a DT now describes a configuration choice rather
>> than the hardware but given that Ethernet MACs are usually capable of
>> supporting all 4 possible RGMII modes, how do you propose to solve the
>> negotiation of the appropriate RGMII mode here?
> 
> This is actually answered by yourself below.
> 
>>>>> So, I think a MAC driver should not care about the specific RGMII
>>>>> mode being asked for in any case, and just accept them all.
>>>>
>>>> I would like to agree to this. However, the implication is that when you
>>>> get your delays wrong, your driver silently ignores you and you never
>>>> notice your mistake until you see no traffic passing and wonder why.
>>>
>>> So explain to me this aspect of your reasoning:
>>>
>>> - If the link is operating in non-fixed-link mode, the rgmii* PHY modes
>>>   describe the delay to be applied at the PHY end.
>>> - If the link is operating in fixed-link mode, the rgmii* PHY modes
>>>   describe the delay to be applied at the MAC end.
>>>
>>> That doesn't make sense, and excludes properly describing a MAC-to-
>>> inaccessible-PHY setup.
>>
>> The point with a fixed link is that it does not matter what is the other
>> end, it could be a MAC, it could be a PHY, it could go nowhere, it just
>> does not matter, the only thing that you can know is you configure your
>> side of the fixed link.
> 
> That is *exactly* my point.
> 
> Today, with a PHY on the other end, the four RGMII modes tell you how
> the PHY is to be configured, not the MAC.  The documentation states
> this.
> 
> So, why should a fixed-link be any different?

It should not, but that means that when you describe the fixed-link, the
'phy-mode' within the local fixed-link node is meant to describe how the
other side is configured not how you are configured. For some reason I
did not consider that to be intuitive when I sent out my response, but I
suppose spelling it out would greatly help.
-- 
Florian

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