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

[Stefan Eichenberger <eichest@gmail.com>]

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.

> > 
> > The bit which disables this feature is not documented in the datasheet
> > from Micrel, but has been found by NXP and Micrel following this
> > discussion:
> > https://community.nxp.com/t5/i-MX-Processors/iMX8MP-eqos-not-working-for-10base-t/m-p/2151032
> > 
> > It has been tested on Verdin iMX8MP from Toradex by forcing the PHY to
> > 10MBit. Withouth this property set, no packets are received. With this
> > property set, reception works fine.
> 
> What's the impact of just unconditionally setting this bit? Seems like 
> any impact would be minimal given 10MBit is probably pretty rare now.

In theory it shouldn't have any negative impact. According to the
Errata:
  The IEEE 802.3 standard states that, in MII/GMII modes, the byte
  preceding the SFD (0xD5), SMD-S (0xE6,0x4C, 0x7F, or 0xB3), or SMD-C
  (0x61, 0x52, 0x9E, or 0x2A) byte can be a non-PREAMBLE byte or there
  can be no preceding preamble byte. The MAC receiver must successfully
  receive a packet without any preamble(0x55) byte preceding the SFD,
  SMD-S, or SMD-C byte.

However, since Micrel didn't document this bit and because the driver is
already older, we are afraid to break something for other users if we
enable it unconditionally.

Regards,
Stefan