netdev - Re: [PATCH RFC net-next 1/5] net: stmmac: call phylink_start() and phylink

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Message-ID: <Z8D9AiZRPp_oyKi_@shell.armlinux.org.uk>
Date: Fri, 28 Feb 2025 00:02:10 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Andrew Lunn <andrew@...n.ch>
Cc: Heiner Kallweit <hkallweit1@...il.com>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Jon Hunter <jonathanh@...dia.com>,
	linux-arm-kernel@...ts.infradead.org,
	linux-stm32@...md-mailman.stormreply.com,
	Maxime Coquelin <mcoquelin.stm32@...il.com>, netdev@...r.kernel.org,
	Paolo Abeni <pabeni@...hat.com>,
	Thierry Reding <treding@...dia.com>
Subject: Re: [PATCH RFC net-next 1/5] net: stmmac: call phylink_start() and
 phylink_stop() in XDP functions

On Thu, Feb 27, 2025 at 11:27:57PM +0100, Andrew Lunn wrote:
> On Thu, Feb 27, 2025 at 03:05:02PM +0000, Russell King (Oracle) wrote:
> > Phylink does not permit drivers to mess with the netif carrier, as
> > this will de-synchronise phylink with the MAC driver. Moreover,
> > setting and clearing the TE and RE bits via stmmac_mac_set() in this
> > path is also wrong as the link may not be up.
> > 
> > Replace the netif_carrier_on(), netif_carrier_off() and
> > stmmac_mac_set() calls with the appropriate phylink_start() and
> > phylink_stop() calls, thereby allowing phylink to manage the netif
> > carrier and TE/RE bits through the .mac_link_up() and .mac_link_down()
> > methods.
> > 
> > Note that RE should only be set after the DMA is ready to avoid the
> > receive FIFO between the MAC and DMA blocks overflowing, so
> > phylink_start() needs to be placed after DMA has been started.
> 
> Sorry, i don't know enough about XDP to review this :-(

I suspect there aren't many people who could review it.

However, delving into the history, it seems that this commit was
responsible for introducing stmmac_xdp_{open,release}, thus
intrducing the fidding of the netif carrier which is prohibited
by phylink:

commit ac746c8520d9d056b6963ecca8ff1da9929d02f1
Author: Ong Boon Leong <boon.leong.ong@...el.com>
Date:   Thu Nov 11 22:39:49 2021 +0800

    net: stmmac: enhance XDP ZC driver level switching performance

but that commit was wrong for this very reason.

Didn't phylib used to not renegotiate the link if nothing changed
across a phy_stop()..phy_start() ?

I'm wondering whether my commit is in essence reverting this commit.


-- 
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