linux-kernel - Re: [PATCH net-next v1 3/3] net: dsa: microchip: Fix PHY loopback configuration for KSZ8794 and KSZ8873

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Message-ID: <20231206155440.GA1324895@pengutronix.de>
Date:   Wed, 6 Dec 2023 16:54:40 +0100
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Andrew Lunn <andrew@...n.ch>,
        Eric Dumazet <edumazet@...gle.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Woojung Huh <woojung.huh@...rochip.com>,
        Arun Ramadoss <arun.ramadoss@...rochip.com>,
        kernel@...gutronix.de, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, UNGLinuxDriver@...rochip.com
Subject: Re: [PATCH net-next v1 3/3] net: dsa: microchip: Fix PHY loopback
 configuration for KSZ8794 and KSZ8873

On Wed, Dec 06, 2023 at 05:14:06PM +0200, Vladimir Oltean wrote:
> On Wed, Dec 06, 2023 at 09:55:20AM +0100, Oleksij Rempel wrote:
> > On Thu, Nov 23, 2023 at 11:52:57AM +0100, Paolo Abeni wrote:
> > > Hi,
> > > 
> > > On Tue, 2023-11-21 at 16:24 +0100, Oleksij Rempel wrote:
> > > > Correct the PHY loopback bit handling in the ksz8_w_phy_bmcr and
> > > > ksz8_r_phy_bmcr functions for KSZ8794 and KSZ8873 variants in the ksz8795
> > > > driver. Previously, the code erroneously used Bit 7 of port register 0xD
> > > > for both chip variants, which is actually for LED configuration. This
> > > > update ensures the correct registers and bits are used for the PHY
> > > > loopback feature:
> > > > 
> > > > - For KSZ8794: Use 0xF / Bit 7.
> > > > - For KSZ8873: Use 0xD / Bit 0.
> > > > 
> > > > Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
> > > 
> > > This looks like a bugfix, so possibly worth a Fixes tag? Given the
> > > dependency on the previous refactor, I think we can take it via net-
> > > next.
> > > 
> > > @Andrew, Florian, Vladimir: do you have any specific preference here?
> > 
> > I do not think any one cares about supporting this switch variant in
> > stable :)
> > 
> > Regards,
> > Oleksij
> 
> Sorry, this simply fell through the cracks.
> 
> How is PHY loopback even supposed to be triggered? User space flips
> NETIF_F_LOOPBACK on the netdev, driver ndo_set_features() catches it and
> calls phy_loopback() and this calls into phylib's phydev->drv->set_loopback()
> or the generic genphy_loopback()?

correct.

> I don't see DSA implementing ndo_set_features(), nor offering NETIF_F_LOOPBACK.
> The PHY is integrated, so DSA is the only relevant netdev driver. Is
> there any other way to test this functionality?

yes - net_selftest()

> If not, I think it's a case of "tree falling in the woods and nobody
> hearing it". Not "stable" material. But it definitely has nothing to do
> with not caring about the switch variant.

Sorry, my intention is not to criticize anyone. I am not getting
feedbacks or bug reports for ksz88xx variants, so it seems like not many
people use it in upstream.

When I have time slots to work on this driver, I try to use them to do
fixes and also clean up the code. Since there is some sort of fog of
uncertainty about when I get the next time slot, or even if I get it at
all, I am trying to push both fixes and cleanups together.

But, you are right, it is not a good reason for not caring about stable :)

What is the decision about this patch set?

Regards,
Oleksij
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