Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.31 as permitted sender) client-ip=23.128.96.31;
Date:   Thu, 23 Nov 2023 12:01:03 +0000
From:   "Russell King (Oracle)" <linux@armlinux.org.uk>
To:     Jie Luo <quic_luoj@quicinc.com>
Cc:     Andrew Lunn <andrew@lunn.ch>, davem@davemloft.net,
        edumazet@google.com, kuba@kernel.org, pabeni@redhat.com,
        robh+dt@kernel.org, krzysztof.kozlowski+dt@linaro.org,
        conor+dt@kernel.org, hkallweit1@gmail.com, corbet@lwn.net,
        netdev@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org
Subject: Re: [PATCH v5 3/6] net: phy: at803x: add QCA8084 ethernet phy support
Message-ID: <ZV8+/4eNzLpLzSDG@shell.armlinux.org.uk>
References: <20231118062754.2453-1-quic_luoj@quicinc.com>
 <20231118062754.2453-4-quic_luoj@quicinc.com>
 <1eb60a08-f095-421a-bec6-96f39db31c09@lunn.ch>
 <ZVkRkhMHWcAR37fW@shell.armlinux.org.uk>
 <eee39816-b0b8-475c-aa4a-8500ba488a29@lunn.ch>
 <fef2ab86-ccd7-4693-8a7e-2dac2c80fd53@quicinc.com>
 <1d4d7761-6b42-48ec-af40-747cb4b84ca5@lunn.ch>
 <316fb626-4dc3-4540-9cc4-e45840e36f77@quicinc.com>
 <ZVyZ+8Q2eNfAKjO/@shell.armlinux.org.uk>
 <d2ac542c-aae3-49ae-ae2b-9defc4ca98eb@quicinc.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <d2ac542c-aae3-49ae-ae2b-9defc4ca98eb@quicinc.com>
Sender: Russell King (Oracle) <linux@armlinux.org.uk>
Precedence: bulk

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.

> > 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.


-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!