netdev - Re: [PATCH v5 3/6] net: phy: at803x: add QCA8084 ethernet phy support

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Date: Fri, 24 Nov 2023 17:47:04 +0800
From: Jie Luo <quic_luoj@...cinc.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
CC: Andrew Lunn <andrew@...n.ch>, <davem@...emloft.net>, <edumazet@...gle.com>,
        <kuba@...nel.org>, <pabeni@...hat.com>, <robh+dt@...nel.org>,
        <krzysztof.kozlowski+dt@...aro.org>, <conor+dt@...nel.org>,
        <hkallweit1@...il.com>, <corbet@....net>, <netdev@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v5 3/6] net: phy: at803x: add QCA8084 ethernet phy support



On 11/23/2023 8:01 PM, Russell King (Oracle) wrote:
> On Thu, Nov 23, 2023 at 06:57:59PM +0800, Jie Luo wrote:
>> On 11/21/2023 7:52 PM, Russell King (Oracle) wrote:
>>> Ultimately, you will need a way to use inband signalling with Cisco
>>> SGMII for 10M/100M/1G speeds, and then switch to 2500base-X when
>>> operating at 2.5G speeds, and that is done via the PHY driver
>>> updating phydev->interface.
>>>
>>> What we do need is some way for the PHY to also tell the PCS/MAC
>>> whether inband should be used. This is something I keep bringing up
>>> and now that we have PCS drivers revised to use the value from
>>> phylink_pcs_neg_mode() _and_ a consistent implementation amongst them
>>> we can now think about signalling to PCS drivers whether inband mode
>>> needs to be turned off when switching between modes.
>>
>> Yes, we can switch the interface mode according to the current link
>> speed in the pcs driver.
>> but the issue is that the phy-mode i specified for the PHYLINK,
>> if phy-mode is sgmii, the support capability is limited to maximum
>> capability 1G during the PHYLINK setup and i can't configure it to 2.5G
>> dynamically, if the phy-mode is 2500base-x, then PHY capability will
>> be modified to only support 2.5G, other speeds can't be linked up.
> 
> So you need my patches that add "possible_interfaces" to phylib so you
> can tell phylink that you will be switching between SGMII and
> 2500base-X. Please see the RFC posting of those patches I sent
> yesterday and try them out - you will need to modify your phylib
> driver to fill in phydev->possible_interfaces.

Your patches work on my board, thanks Russell.

> 
>>> There have been patches in the past that allow inband mode to be
>>> queried from phylib, and this is another important component in
>>> properly dealing with PHYs that need to use inband signalling with
>>> Cisco SGMII, but do not support inband signalling when operating at
>>> 2.5G speeds. The problem when operating at 2.5G speed is that the
>>> base-X protocols are normally for use over fibre, which is the media,
>>> and therefore the ethtool Autoneg bit should define whether inband
>>> gets used or not. However, in the case of a PHY using 2500base-X,
>>> the Autoneg bit continues to define whether autonegotiation should
>>> be used on the media, and in this case it's the media side of the
>>> PHY rather than the 2500base-X link.
>>>
>>> So, when using a 2500base-X link to a PHY, we need to disregard the
>>> Autoneg bit, but that then raises the question about how we should
>>> configure it - and one solution to that would be to entire of phylib
>>> what the PHY wants to do. Another is to somehow ask the PCS driver
>>> whether it supports inband signalling at 2500base-X, and resolve
>>> those capabilities.
>>
>> For the qca808x PHY, when it is linked in 2.5G, the autoneg is also
>> disabled in PCS hardware, so the sgmii+ of qca808x PHY is almost
>> same as 2500base-X.
> 
> Not "almost". It _is_ the same. This is the point I've been trying
> to get across to you. Without inband signalling, 1000base-X and SGMII
> (when operating at 1G) are _identical_ and entirely compatible.
> 
> You've said that your 2.5G "SGMII" mode has inband signalling disabled,
> and thus it without inband signalling, 2500base-X and this 2.5G mode
> are again identical and entirely compatible. There's no "almost" about
> it.
> 
> 
Yes, confirmed with HW guy, they work on the same way.