On Thu, 18 Sep 2025 17:04:55 +0200
Andrew Lunn [off-list ref] wrote:

quoted

Yes, unfortunately the SJA1105Q does not support PAUSE frames, and the i.MX8MP
FEC isn't able to sustain 1000Mbps (only about 400ish) due to insufficient
internal bus bandwidth. It will generate PAUSE frames, but the SJA1105Q
ignores these, leading to packet loss, which is obviously worse than
restricting this link to 100Mbps. Ironically both chips are from the same
manufacturer, yet are incompatible in this regard.  

Thanks for the explanation. Maybe add a comment that the bandwidth is
limited due to the lack of flow control resulting in packet loss in
the FEC.

Anything which looks odd deserves a comment, otherwise somebody will
question it....

Yes! This is a golden tip. Ironically what I said above is incorrect. Sorry
for the noise.

Ftr: I wrote this DT about 4 years ago, so my memory failed me, and a comment
in the code would have saved me this embarrassment ;-)

The comment above applies to the i.MX6 SoC's which had this limitation. On the
i.MX8MP we had a different problem that also caused the SJA1105Q not to work
reliably at 1000Mbps either. We haven't been able to find the issue, but so far
this switch hasn't been able to work at 1000Mbps reliable on any platform,
possibly for different reasons in each case.

Best regards,

-- 
David Jander