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

[Darcy Casselman]

The LTS point releases ship updated kernels for hardware enablement.  The
process is described here: https://wiki.ubuntu.com/Kernel/LTSEnablementStack

Generally I'd expect that things that work shouldn't stop working.  But I
know from experience that that assumption is naive.  It's software.  Stuff
breaks.  Also, it's the kernel, and they do strange things sometimes.

Darcy.

On Tue, Mar 15, 2016 at 5:03 PM, Khalid Baheyeldin <kb at 2bits.com> wrote:

> 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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