[kwlug-disc] Why would an LTS install non-LTS kernels?

[Khalid Baheyeldin]

Finally got around to the task of reinstalling my laptop, using the
warranty replaced SSD disk from OCZ.

After installing Kubuntu 14.04, guvcview would not work with the built
in webcam, but another USB webcam (actually a microscope of sorts).
Both would give a black screen (no video).

The USB webcam is supported by uvc (supposedly).

usb 2-2: New USB device found, idVendor=1871, idProduct=7670
usb 2-2: New USB device strings: Mfr=2, Product=3, SerialNumber=0
usb 2-2: Product: USB2.0 UVC PC Camera
usb 2-2: Manufacturer: GenesysLogic Technology Co., Ltd.


Both were working fine with guvcview on the previous install (Ubuntu
Server 14.04, then KDE Plasma Desktop on top, so should not be much
different).

A lengthy investigation ensued, using programs like Cheese, vlc,
qtv4l2 test, ...etc. Some of them (e.g. Cheese) would work with the
built in web cam, but not the USB webcam. Others (e.g. vlc) could work
with the USB video, but NOT using the uvcvideo driver (e.g. v4l).

So there was something with the uvcvideo kernel module for the USB
webcam with an error like this:

kernel: [  100.912205] uvcvideo: Failed to query (GET_DEF) UVC control
11 on unit 3: -32 (exp. 1).

The reason I say uvc was the culprit is that other applications using
other drivers do work.

Luckily, I have the old disk lying around in the state it was in
before I removed it from the laptop. I probed around, and found that
the new Kubuntu install has a 3.19 kernel, while the old one Ubuntu
Server LTS was 3.13.

So, I installed the Trusty generic image:

$ sudo aptitude install linux-image-generic-lts-trusty

And purged the vivid ones:

$ sudo aptitude purge linux-generic-lts-vivid
linux-image-generic-lts-vivid linux-image-3.19.0-56-generic
linux-image-3.19.0-25-generic linux-image-extra-3.19.0-25-generic
linux-image-extra-3.19.0-56-generic

After a reboot, both webcams work fine, as things were before.

There is still the same error in dmesg, but things work as I want them too.

So, the big questions are:

1. Why would something that works stop working in a point release of
the kernel (remember that most devices have drivers written once, and
then not touched at all).

2. Why would an LTS release install a non-LTS kernel?
-- 
Khalid M. Baheyeldin
2bits.com, Inc.
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