But looking at the code for 21.02.1 I can see that the config should be different (from https://github.com/openwrt/openwrt/blob/v21.02.1/target/linux/ipq806x/base-files/etc/board.d/02_network):
So, should the R7800 support DSA? How should one even tell if it is supported on a given device? Why am I getting eth0/1 rather than lan0/1/2/3? Is there anything I can do to resolve this?
DSA for ipq806x has not been merged into master yet (much less the stable builds that you are using). See PR 4036. I've found that DSA is functional and stable on the r7500v2, but apparently there is some addition work needed before DSA will be merged (for example: PRs 4716 & 4717)
That really does, thank you. I had looked for a list of supported platforms but couldn't find one, nor a way to interrogate* the running system for support. Is the right answer to search the PRs?
For the stable builds, refer to the 21.02.0 release notes:
add DSA support for the platform/device (like the mentioned PR)
build or download, and install a firmware image with DSA support
configure the device starting from default settings, or at least avoid restoring the network configuration, which is incompatible between swconfig and DSA.
If you want to test the code under development and perhaps report your findings, then yes.
Otherwise, stay with snapshot or even stable releases.
You did what I would do which is ask on the forums. IRC can be good as well, but that is hit or miss depending on who is on.
FWIW, I follow openwrt development for ipq806x fairly closely and I still have trouble knowing what feature is available and working (e.g. i'm pretty sure airtime fairness is not enabled on my r7500v2 but likely is working on the r7800).