dpaa2: Avoid DPMAC link toggling

[dsl]

Summary

Postpone the media state update to avoid DPMAC link toggling. Should fix https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=279352.

Test Plan

This is how "dpaa2_ni_miibus_statchg" is called on my Honeycomb LX2 under load (building ports with poudriere) before the patch:

dmesg_dpaa2_mii_trace_filtered.txt6 KBDownload

dmesg_dpaa2_mii_trace2_filtered.txt8 KBDownload

The same after the patch:

dmesg_dpaa2_mii_trace3.txt4 KBDownload

[bz]

You'll have to explain WHY this is possibly fixing the problem. Usually deferred contexts are not the right solutions to fix ordering problems (if that's what it would be)?

[dsl]

In D61079#1400373, @bz wrote:

You'll have to explain WHY this is possibly fixing the problem. Usually deferred contexts are not the right solutions to fix ordering problems (if that's what it would be)?

I'm not exactly sure this problem is about ordering, but rather a lack of proper MAC<->PHY link configuration. And the result of the improper configuration is the "dpaa2_ni_miibus_statchg" behavior I observed in dmesg_dpaa2_mii_trace_filtered.txt and dmesg_dpaa2_mii_trace2_filtered.txt. i.e.

[13357.646529] dpaa2_ni0: dpaa2_ni_miibus_statchg: miibus state:
mii_media_status=1, 
mii_media_active=100026, 
ifm_media=0
[13357.646679] dpaa2_ni0: dpaa2_ni_miibus_statchg: miibus state:
mii_media_status=3, 
mii_media_active=100036, 
ifm_media=0
[13357.655337] dpaa2_ni0: dpaa2_ni_miibus_statchg: miibus state:
mii_media_status=3, 
mii_media_active=100130, 
ifm_media=0

which leads to the DPMAC link state toggling. I'm not sure why the PHY driver reports changes on the MII bus so quickly, but I don't think it's a good idea to call DPAA2_CMD_MAC_SET_LINK_STATE on each and every one of them. Hence 2s callout.