On Tue, 07 Jul 2020 14:22:42 +0200, Bjørn Mork said:
> I have "always" tested simple patches by rebuilding just the affected
> driver on whatever kernel I happen to run.  Like this:

>  make -C /lib/modules/4.19.0-9-amd64/build M=$(pwd)/drivers/net/usb qmi_wwan.ko

I suspect the problem is that M=  is causing everything in that directory to be
rebuilt, possibly because it points at itself rather than an output directory elsewhere.

Because it points at itself, why are you even specifying it, instead of just saying
make -C /lib/modules/4.19.0-9-amd64/build drivers/net/usb/qmi_wwan.ko

(For that matter, using a 4.19 directory for -C when you're apparently
working with a git tree is probably rather sketchy as well...