Re: [PATCH net-next v2] net: phylink: add phylink_pcs_loopback() method for PCS loopback support

[Oleksij Rempel]

On Tue, Jul 28, 2026 at 10:41:12AM +0200, Maxime Chevallier wrote:
> Hi,
> 
> On 7/27/26 14:07, Zxyan Zhu wrote:
> > Some Ethernet MAC drivers (such as stmmac) need to perform PCS-level
> > loopback for selftest purposes when no external PHY is present. Add a
> > new phylink_pcs_loopback() method that allows MAC drivers to request
> > PCS loopback mode through the phylink framework.
> > 
> > Implement the pcs_loopback callback in the stmmac integrated PCS
> > using the GMAC_AN_CTRL_ELE register bit. Add a helper function
> > stmmac_selftest_get_pcs() in stmmac selftests to unify PCS lookup
> > across glue driver PCS and integrated PCS. Update the selftest
> > enable/disable paths to use the new helper.
> > 
> > Signed-off-by: Zxyan Zhu <zxyan0222@xxxxxxxxx>
> 
> I'm actually seriously wondering if we shouldn't just stick to MAC loopback only
> for stmmac selftests. I even have a patch ready to send for that, as I've
> been struggling with selftests in the past days because some PHYs will
> either stop their RXC while in loopback, or bring the carrier down which
> makes the dev_xmit drop the packets we're sending to ourselves.
> 
> Using an external loopnback (in PCS, in PHY, somewhere else...) doesn't
> bring anything to the table given what we are testing today, which is
> internal stmmac behaviour. The MAC loopback (on dwmac1000 and dwmac4 at
> least) happends at the GMII level, so everything we are currently testing
> happens before that.
> 
> If the goal is to validate what comes after (RGMII, PCS behaviour and so on),
> I think we should instead add some other, more generic selftests that aren't
> stmmac specific, that we could run on any netdev, such as the ones Oleksij
> has added in:
> 
>   3e1e58d64c3d net: add generic selftest support

Ack, it is better to push the effort towards generic selftests.

Best Regards,
Oleksij
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