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media-libs/libv4l dependency on kernel source? [SOLVED]
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tld
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PostPosted: Mon Oct 09, 2017 5:15 pm    Post subject: media-libs/libv4l dependency on kernel source? [SOLVED] Reply with quote

I've been unable to find a clear answer on this one: I recently switched from using LIRC on my MythTV frontend to using ir-keytable, which required these:
Code:
Calculating dependencies... done!
[ebuild  N     ] media-libs/libv4l-1.10.1::gentoo  USE="jpeg" 1,321 KiB
[ebuild  N     ] media-tv/v4l-utils-1.6.2::gentoo  USE="udev -qt4" 1,122 KiB
It's clear during the build of media-tv/v4l-utils that it's finding the current kernel source based on my symbolic link at /usr/src/linux in addition to the appropriate /lib/modules directory. What I'm unclear on it whether this implies a need to re-emerge media-tv/v4l-utils for any kernel upgrade. The only similar situations I've run into are for example, lirc in older versions that had their own kernel modules, or nvidia-drivers...both of which actually installed modules, which is clearly not the case with media-tv/v4l-utils.

I'm assuming this is primarily for kernel headers and the like. I just upgraded from sys-kernel/gentoo-sources-4.12.5 to sys-kernel/gentoo-sources-4.12.12 and recompiled it to be safe, but I'd like to better understand the actual dependency there.

Thanks!
Tom


Last edited by tld on Tue Oct 10, 2017 8:09 pm; edited 1 time in total
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muhlemmer
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PostPosted: Tue Oct 10, 2017 2:49 pm    Post subject: Reply with quote

Usually these are pre-merge checks portage is doing. I don't have V4L, but many other packages that do this checks during emerge. It will just check if you have sane settings in your /usr/src/linux/.config file. I assume you have a message like this:
Code:
>>> Running pre-merge checks for x11-drivers/xf86-input-evdev-2.10.5
 * Determining the location of the kernel source code
 * Found kernel source directory:
 *     /usr/src/linux
 * Found kernel object directory:
 *     /lib/modules/4.13.5-gentoo/build
 * Found sources for kernel version:
 *     4.13.5-gentoo
 * Checking for suitable kernel configuration options...


This process normally will spawn a warning if your kernel is mis-configured wrt the package. It does not affect the actual build. I had plenty of times that through emerge --updates and --depclean, the /usr/src/linux symlink was moved to sources which did not have a .config file in place. Then emerge will complain a lot during pre-merge checks, but continues to build. When emerge finishes, it prints the warnings encountered and leaves it up to you to fix it:
Code:
 * Messages for package x11-drivers/xf86-input-evdev-2.10.5:

 * Unable to check for the following kernel config options due
 * to absence of any configured kernel sources or compiled
 * config:
 *  - INPUT_EVDEV
 * You're on your own to make sure they are set if needed.


So, to answer your question: you don't need to rebuild packages after upgrading the kernel. And even if it complains, if your running kernel was configured correctly before and the .config doesn't exist anymore, you won't be in trouble either. Rebuild could be needed for some packages only after upgrading sys-kernel/linux-headers. I suppose that the rebuild will be handled by portage automatically in that case: https://wiki.gentoo.org/wiki/Sub-slots_and_Slot-Operators
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NeddySeagoon
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PostPosted: Tue Oct 10, 2017 3:42 pm    Post subject: Reply with quote

tld,

There was a debate on the -dev ML a few years ago about kernel config checks, what they should check and if they should cause build fails.

It was eventually decided that /usr/src/linux should be checked because /proc/config.gz may not exist and you may not be building for the running kernel anyway.
That, and the cross compile case, when neither /usr/src/linux, nor /proc/config.gz matter, determined that the checks should be warnings only.

There is no need to rebuild the running kernel, unless it does not satisfy the warnings and only you know that.
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tld
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PostPosted: Tue Oct 10, 2017 8:08 pm    Post subject: Reply with quote

Thanks for the replies! That answers it for sure. That does appear to be the same check I was referring to:
Code:
>>> Emerging (1 of 1) media-libs/libv4l-1.10.1::gentoo
 * v4l-utils-1.10.1.tar.bz2 SHA256 SHA512 WHIRLPOOL size ;-) ...                                                                                                        [ ok ]
 * Determining the location of the kernel source code
 * Found kernel source directory:
 *     /usr/src/linux
 * Found kernel object directory:
 *     /lib/modules/4.12.12-gentoo/build
 * Found sources for kernel version:
 *     4.12.12-gentoo
 * Checking for suitable kernel configuration options...                                                                                                                [ ok ]
>>> Unpacking source...
Thanks again!

Tom
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