When turning off connector polling in drm_kms_helper using the poll=N module parameter HDMI hot plugging does not work anymore. Quoting Alex Deucher:
See this discussion for details and some logs:
This isn't really specific to radeon.
Freedesktop's Bugzilla instance is EOLed and open bugs are about to be migrated to http://gitlab.freedesktop.org.
To avoid migrating out of date bugs, I am now closing all the bugs that did not see any activity in the past year. If the issue is still happening, please create a new bug in the relevant project at https://gitlab.freedesktop.org/drm (use misc by default).
Sorry about the noise!