Re: [PATCH 5/8] net: ethernet: ti: am65-cpsw: Add support for fixed-link configuration

From: Siddharth Vadapalli <s-vadapalli@ti.com>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: <davem@davemloft.net>, <edumazet@google.com>, <kuba@kernel.org>,
	<pabeni@redhat.com>, <robh+dt@kernel.org>,
	<krzysztof.kozlowski@linaro.org>,
	<krzysztof.kozlowski+dt@linaro.org>, <vladimir.oltean@nxp.com>,
	<grygorii.strashko@ti.com>, <vigneshr@ti.com>, <nsekhar@ti.com>,
	<netdev@vger.kernel.org>, <devicetree@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>,
	<linux-arm-kernel@lists.infradead.org>, <kishon@ti.com>,
	<s-vadapalli@ti.com>
Subject: Re: [PATCH 5/8] net: ethernet: ti: am65-cpsw: Add support for fixed-link configuration
Date: Fri, 16 Sep 2022 15:25:32 +0530	[thread overview]
Message-ID: <5be6de72-ba90-472e-7714-93940260b2ae@ti.com> (raw)
In-Reply-To: <YyQ+h4o9hqO+paUL@shell.armlinux.org.uk>

Hello Russell,

On 16/09/22 14:44, Russell King (Oracle) wrote:
> On Fri, Sep 16, 2022 at 02:33:23PM +0530, Siddharth Vadapalli wrote:
>> Hello Russell,
>>
>> On 16/09/22 12:50, Russell King (Oracle) wrote:
>>> SGMII is designed for the setup in the diagram I provided in my previous
>>> email. It is not designed for two MACs to talk direct to each other
>>> without any ethernet PHY because of the asymmetric nature of the control
>>> word.
>>>
>>> The PHY sends e.g. a control word of 0x9801 for 1G full duplex. On
>>> reception of that, the MAC responds with 0x4001. Finally, the PHY
>>> responds with 0xd801 to acknowledge the receipt of the MAC response.
>>>
>>> If both ends of the link are SGMII, both ends will be waiting for
>>> the control word from a PHY which is not present, and the link will
>>> not come up.
>>>
>>> 1000base-X is a symmetric protocol where both ends of the link
>>> advertise their capabilities, acknowledge each others abilities and
>>> resolve the duplex and pause settings.
>>>
>>> SGMII is a Cisco proprietary modification of 1000base-X designed for
>>> communicating the results of media autonegotiation between an
>>> ethernet PHY and ethernet MAC.
>>
>> I will try to implement and test SGMII mode in the conventional way with
>> both the MAC and the PHY present. If I am unable to do so, I will revert
>> to the current set of patches for the special case where the MAC
>> emulates a PHY, and mention this setup in the commit message of the v2
>> series. I hope this approach would be fine to proceed with. Please let
>> me know in case of any suggestions.
> 
> What exact setups are you trying to support with this patch set?
> 
> If you're looking to only add support for SGMII, then all you need
> to do is to make sure it works with a PHY. Fixed-link in SGMII only
> makes sense if you're directly connected to something like a network
> switch, but even then, network switches tend to use 1000base-X in a
> fixed-link mode rather than SGMII.

I plan to support the standard MAC2PHY based SGMII mode. However, the
SGMII ethernet PHY that I have with me has issues which is why I had
tried the unconventional fixed-link SGMII mode with the MAC emulating
the ethernet PHY. I will try to obtain a functional SGMII ethernet PHY
and test the standard SGMII mode.

Thank you for patiently answering my questions and helping me understand
the SGMII mode better :)

Regards,
Siddharth.