In order to be able to fix your kernel bug we need relevant information.
$ reportbug kernel
as this will include various system information in your report. Also see the Required information section in the kernel handbook's chapter on bugs.
This may not be sufficient and we may ask for more in followups, please prepare to be responsive or otherwise the bug report won't help. A big and easy step is to verify if the issue is fixed in a newer kernel.
Check out unstable and experimental for a newer linux-image and see if your issue is fixed. If it is be happy and be assured that soon that image will be pushed to Testing. If it still does not fix the issue report upstream in bugzilla.kernel.org.
The most common case for wanting newer upstream is better and newer driver support. The DebianKernel experimental linux images install just fine in Testing or Unstable.
If an oops is reproducible on latest please notify upstream via email. Another way to notify upstream is to file a bug in https://bugzilla.kernel.org/.
Report package versions
Often bug reports come in that list versions like "4.19.0-9-amd64" or "5.7.0-3". This is useful information, but it doesn't uniquely identify a Debian version. Please also include the package version. The package version looks more like "4.19.118-2+deb10u1" or "5.7.17-1". The package version for the running kernel can be found by running:
$ uname -v
To find the package versions for other installed kernels, run:
$ dpkg -l 'linux-image-*' | grep ^ii
Identifying when the bug was introduced
If the problem does not occur on one version but does occur on another, please clearly include this information in your report, for example:
- This problem occurs in linux-image-4.19.0-10-amd64 version 4.19.132-1, but does not occur in linux-image-5.7.0-3-amd64 version 5.7.17-1.
Also, if there were versions in between the two you've tested, it helps to test those too to help us reduce the possibilities. You can find older versions of the kernel under https://snapshot.debian.org/package/linux/.
One of the best pieces of information you can provide is the specific changeset that introduced or resolved your bug. A good method for doing this is git bisect
Report upstream issues upstream
If you are able to reproduce your problem on the latest upstream kernel version, please report the problem upstream and link the Debian bug to the upstream bug using the "forwarded" command, e.g. (install devscripts for the bts command):
$ bts forwarded 123456 https://bugzilla.kernel.org/show_bug.cgi?id=1234
This will allow us to be automatically notified when the bug is resolved upstream.
The kernel handbook also has a section on Filing a bug against a kernel package.