linux-kernel - Re: [PATCH net-next v3 1/7] net: dsa: mt7530: empty default case on mt7530_setup

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Date: Fri, 2 Feb 2024 18:05:36 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Arınç ÜNAL <arinc.unal@...nc9.com>
Cc: Daniel Golle <daniel@...rotopia.org>, DENG Qingfang <dqfext@...il.com>,
	Sean Wang <sean.wang@...iatek.com>, Andrew Lunn <andrew@...n.ch>,
	Florian Fainelli <f.fainelli@...il.com>,
	Vladimir Oltean <olteanv@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	mithat.guner@...ont.com, erkin.bozoglu@...ont.com,
	Bartel Eerdekens <bartel.eerdekens@...stell8.be>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH net-next v3 1/7] net: dsa: mt7530: empty default case on
 mt7530_setup_port5()

On Fri, Feb 02, 2024 at 08:44:39PM +0300, Arınç ÜNAL wrote:
> On 2.02.2024 14:40, Russell King (Oracle) wrote:
> > While reviewing this change, but not related to it, I notice that this
> > function sets the TX delay based on the RGMII interface mode. This isn't
> > correct. I've explained why this is this many times in the past, but
> > essentially it comes down to the model:
> > 
> > 
> > phy-mode in NIC node	Network driver	PCB		PHY
> > rgmii			no delays	delays		no delays
> > rgmii-id		no delays	no delays	tx/rx delays
> > rgmii-txid		no delays	no delays	tx delays
> > rgmii-rxid		no delays	no delays	rx delays
> > 
> > Then we have rx-internal-delay-ps and tx-internal-delay-ps in the NIC
> > node which define the RGMII delays at the local end and similar
> > properties for the PHY node.
> > 
> > 
> > So, if we take the view that, when a switch is connected to a NIC in
> > RGMII mode, then the phy-mode specified delays still should not impact
> > the local NIC.
> > 
> > Now, for the switch, we specify the phy-mode in the port node as well.
> > Consider the case of a switch port connected to a RGMII PHY. This has
> > to operate in exactly the same way as a normal NIC - that is, the
> > RGMII delays at the port should be ignored as it's the responsibility
> > of a PHY.
> > 
> > The final scenario to examine is the case of a RGMII switch port
> > connected to a NIC. The NIC's phy-mode has no way to be communicated
> > to DSA or vice versa, so neither phy-mode can impact the other side
> > of the RGMII link, but should only place the link into RGMII mode.
> > Given everything I've said above, the only way to configure RGMII
> > delays is via the rx-internal-delay-ps and tx-internal-delay-ps
> > properties. So, DSA drivers should _not_ be configuring their ports
> > with RGMII delays based on the RGMII phy interface mode.
> > 
> > The above is my purely logically reasoned point of view on this
> > subject. Others may have other (to me completely illogical)
> > interpretations that can only lead to interoperability issues.
> 
> I will address this with the next patch series. Thank you for explaining it
> in detail.

This is a good time to point out not to rush with the next patch
series, as my email will _likely_ provoke some additional discussion
from Andrew and/or Vladimir. So please give it a few days (maybe
around the middle of next week) to give them time to consider my
email and respond.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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