Are there any known issues with running the imx8 mac at 100mbps with the ptp features? On the imx8 eval board I am able to get the ptp4l from linuxptp to measure a rms master offset of about 30ns in 1gbps ethernet. The imx is connected directly via a 1gbps connection to a ptp master.
ptp4l: rms 32 max 86 freq -471 +/- 28 delay 574 +/- 7
When the same port is directly connected via a 100mbps to ptp master the rms offset is much worse, on the order of 800ns:
ptp4l: rms 837 max 447 freq -29414 +/- 356 delay 864 +/- 284
Any ideas on why the ptp offset is so much worse at 100mbps?
It appears from the IMX8 processor reference manual that the RGMII 1gbps mode is the only mode that sets the speed bit of the Ethernet Control Register (ENET_ECR). For all other configurations, 10mbps and 100mbps, that bit would not be set. That is about the only indication from the refreence manual that the mac might be clocked at something different for 1gbps.
The eval kit is based on the nxp imx linux kernel rel_imx_4.14.78_1.0.0_ga.