Date:   Fri, 16 Sep 2022 14:33:23 +0530
From:   Siddharth Vadapalli <s-vadapalli@...com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
CC:     <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
        <pabeni@...hat.com>, <robh+dt@...nel.org>,
        <krzysztof.kozlowski@...aro.org>,
        <krzysztof.kozlowski+dt@...aro.org>, <vladimir.oltean@....com>,
        <grygorii.strashko@...com>, <vigneshr@...com>, <nsekhar@...com>,
        <netdev@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <kishon@...com>,
        <s-vadapalli@...com>
Subject: Re: [PATCH 5/8] net: ethernet: ti: am65-cpsw: Add support for
 fixed-link configuration

Hello Russell,

On 16/09/22 12:50, Russell King (Oracle) wrote:
> On Fri, Sep 16, 2022 at 10:24:48AM +0530, Siddharth Vadapalli wrote:
>> On 15/09/22 15:37, Russell King (Oracle) wrote:
>>> Hi,
>>>
>>> On Thu, Sep 15, 2022 at 02:58:52PM +0530, Siddharth Vadapalli wrote:
>>>> Hello Russell,
>>>>
>>>> On 14/09/22 21:39, Russell King (Oracle) wrote:
>>>>> On Wed, Sep 14, 2022 at 03:20:50PM +0530, Siddharth Vadapalli wrote:
>>>>>> Check for fixed-link in am65_cpsw_nuss_mac_config() using struct
>>>>>> am65_cpsw_slave_data's phy_node property to obtain fwnode. Since
>>>>>> am65_cpsw_nuss_mac_link_up() is not invoked in fixed-link mode, perform
>>>>>> the relevant operations in am65_cpsw_nuss_mac_config() itself.
>>>>>
>>>>> Further to my other comments, you also fail to explain that, when in
>>>>> fixed-link SGMII mode, you _emulate_ being a PHY - which I deduce
>>>>> since you are sending the duplex setting and speed settings via the
>>>>> SGMII control word. Also, as SGMII was invented for a PHY to be able
>>>>> to communicate the media negotiation resolution to the MAC, SGMII
>>>>> defines that the PHY fills in the speed and duplex information in
>>>>> the control word to pass it to the MAC, and the MAC acknowledges this
>>>>> information. There is no need (and SGMII doesn't permit) the MAC to
>>>>> advertise what it's doing.
>>>>>
>>>>> Maybe this needs to be explained in the commit message?
>>>>
>>>> I had tested SGMII fixed-link mode using a bootstrapped ethernet layer-1
>>>> PHY. Based on your clarification in the previous mails that there is an
>>>> issue with the fixed-link mode which I need to debug, I assume that what
>>>> you are referring to here also happens to be a consequence of that.
>>>> Please let me know if I have misunderstood what you meant to convey.
>>>
>>> I think what you're saying is that you have this setup:
>>>
>>>   ethernet MAC <--SGMII link--> ethernet PHY <---> media
>>>
>>> which you are operating in fixed link mode?
>>
>> Yes, and the other end is connected to my PC's ethernet port.
>>
>>>
>>> From the SGMII specification: "This is achieved by using the Auto-
>>> Negotiation functionality defined in Clause 37 of the IEEE
>>> Specification 802.3z. Instead of the ability advertisement, the PHY
>>> sends the control information via its tx_config_Reg[15:0] as specified
>>> in Table 1 whenever the control information changes. Upon receiving
>>> control information, the MAC acknowledges the update of the control
>>> information by asserting bit 14 of its tx_config_reg{15:0] as specified
>>> in Table 1."
>>>
>>> For the control word sent from the MAC to the PHY, table 1 specifies a
>>> value of 0x4001. All the zero bits in that word which are zero are
>>> marked as "Reserved for future use." There are no fields for speed and
>>> duplex in this acknowledgement word to the PHY.
>>>
>>> I hope this clears up my point.
>>
>> Thank you for the detailed explanation. After reading the above, my
>> understanding is that even in the fixed-link mode, the ethernet MAC is
>> not supposed to advertise the speed and duplex settings. The ethernet
>> MACs present on both ends of the connection are supposed to be set to
>> the same speed and duplex settings via the devicetree node. Thus, only
>> for my setup which happens to be a special case of fixed-link mode where
>> the ethernet PHY is present, I am having to send the control word due to
>> the presence of a PHY in between.
> 
> In SGMII, the control word is only passed between the ethernet MAC and
> the ethernet PHY. It is not conveyed across the media.
> 
>> And, I am supposed to mention this in
>> the commit message, which I haven't done. Please let me know if this is
>> what I was supposed to understand.
> 
> If you implement this conventionally, then you don't need to mention it
> in the commit message, because you're following the standard.
> 
>> I am planning to change to a proper fixed-link setup without any
>> ethernet PHY between the MACs, for debugging the driver's fixed-link
>> mode where the "mac_link_up()" is not invoked.
> 
> 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.

Regards,
Siddharth.

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