Re: [PATCH net-next v1 2/3] dt-bindings: net: micrel: Add keep-preamble-before-sfd

[Stefan Eichenberger <eichest@gmail.com>]

On Wed, Dec 17, 2025 at 07:55:19AM -0600, Rob Herring wrote:
> On Wed, Dec 17, 2025 at 01:21:19PM +0100, Stefan Eichenberger wrote:
> > On Wed, Dec 17, 2025 at 10:58:54AM +0100, Stefan Eichenberger wrote:
> > > On Mon, Dec 15, 2025 at 08:03:30AM -0600, Rob Herring wrote:
> > > > On Fri, Dec 12, 2025 at 09:46:17AM +0100, Stefan Eichenberger wrote:
> > > > > From: Stefan Eichenberger <stefan.eichenberger@toradex.com>
> > > > > 
> > > > > Add a property to activate a Micrel PHY feature that keeps the preamble
> > > > > enabled before the SFD (Start Frame Delimiter) is transmitted.
> > > > > 
> > > > > This allows to workaround broken Ethernet controllers as found on the
> > > > > NXP i.MX8MP. Specifically, errata ERR050694 that states:
> > > > > ENET_QOS: MAC incorrectly discards the received packets when Preamble
> > > > > Byte does not precede SFD or SMD.
> > > > 
> > > > It doesn't really work right if you have to change the DT to work-around 
> > > > a quirk in the kernel. You should have all the information needed 
> > > > already in the DT. The compatible string for the i.MX8MP ethernet 
> > > > controller is not sufficient? 
> > > 
> > > Is doing something like this acceptable in a phy driver?
> > > if (of_machine_is_compatible("fsl,imx8mp")) {
> > > ...
> > > }
> > > 
> > > That would be a different option, rather than having to add a new DT
> > > property. Unfortunately, the workaround affects the PHY rather than the
> > > MAC driver. This is why we considered adding a DT property.
> > 
> > Francesco made a good point about this. The i.MX8MP has two MACs, but
> > only one of them is affected. Therefore, checking the machine's
> > compatible string would not be correct. As far as I know, checking the
> > MAC's compatible string from within the PHY driver is also not good
> > practice, is it?
> 
> It's not great, but probably what you need to do. The 2 MACs are the 
> same (compatible) or different? As that only works if they are 
> different. I suppose you need to only check the MAC the PHY is connected 
> to.
> 
> I think the ideal implementation would be the MAC driver calling some 
> phy API to apply the quirk, and then that gets passed on to the phy 
> driver. Surely this isn't the first case of a MAC-PHY combination 
> needing to go fiddle with some special setting.

I was also hoping to find something like that, but I couldn't really
find it. However, I will try looking in that direction. At least we can
identify the broken MAC via the compatible string of the MAC driver, as
they use two different compatibles: 'fsl,imx8mp-fec' (fine) and
'nxp,imx8mp-dwmac-eqos' (affected by the errata).

Regards,
Stefan