Currently there is no explicit cfg80211/nl80211 API for triggering a
channel survey. It seems that all mainline drivers which implement the
dump_survey rdev op manage to gather survey information during a normal
scan. While it seems to work perfectly fine in these cases, this
assumption cannot be made for all devices.
For some context: I am looking into adding survey support for the
brcmfmac driver, and the firmware interface has a separate, explicit API
for triggering a survey and reading back the results. There appears to
be no way to gather survey data through an ordinary scan.
Adding a dump_survey op to brcmfmac is not impossible within the bounds
of the current cfg80211 API, but it does present some issues. For
example, some variants of the out-of-tree bcmdhd driver simply trigger
the survey in the dump_survey function. But doing it like this means
that a typical survey will hold the RTNL mutex for about 5 seconds - yikes!
So my question is: could the API be extended to support explicitly
triggering a survey? And if so, how should it look?
The original commit (61fa713c7516 "cfg80211: return channel noise via
survey API") introducing the dump_survey API notes:
> In future, there will either be a survey-trigger command --- or the
> scan-trigger command will be enhanced. This will allow user-space to
> request survey for arbitrary channels.
The easiest solution would be just to add an attribute to
CMD_TRIGGER_SCAN - say NL80211_ATTR_SCAN_SURVEY - to indicate to the
driver that it should collect survey information if possible. Existing
drivers would be unaffected (since they already do this implicitly)
while a driver like brcmfmac could tell the firmware to do an explicit
survey instead. The only issue I see with this is that there would be no
new scan results to send back. I wonder if that matters?
Otherwise a new CMD_TRIGGER_SURVEY could be added, and the driver could
emit CMD_NEW_SURVEY_RESULTS when the survey is complete. I guess the
implementation here would look a lot like what we already have for
scanning. I'm not sure how far to take this though - should cfg80211
also cache the survey results like it does for scans? In that case it
might also require changes to existing drivers which currently populate
the survey dump on demand.
I'm quite happy to send some patches for this and I just wanted to get
some feedback first. I'm less confident about implementing the latter
approach, particularly if it involves sending patches for drivers I
cannot test (and for a rather niche feature at that). But if I can get
some more clarity on what such an interface should look like, I'll see
what I can do.
Thanks in advance.