[EdLUG] Module parameters are different from cmdline

Edinburgh Linux Users Group edlug at lists.edlug.org.uk
Thu Dec 29 16:15:12 UTC 2016


Hi Roy,

Thanks for your help! Actually the info you provided was very useful. Using
it I found how i915 is loaded, where it's located and finally figured out
that there's an issue in this specific module.

It's located in initrd and kernel passes module parameters correctly. I see
that other parameters for i915 are correctly handled. All except this
particular one: enable_execlists.

Looking further into that module's source code for my distro, I found a
function called "intel_sanitize_enable_execlists". Based on its source code
it seems like it can silently change that parameter's value without any
notification in the logs.

So at least now it's clear what's the root cause of such weird parameter
handling.

Again, thanks a lot for your advises!

Best Regards,
Maxim

On Thu, Dec 29, 2016 at 3:06 PM, Edinburgh Linux Users Group <
edlug at lists.edlug.org.uk> wrote:

> On 2016.12.28 23:56, Edinburgh Linux Users Group wrote:
> > Hi Roy,
> >
> > After some investigation it turned out that Ubuntu packs both -
> > gzipped
> > initrd.img and Intel's microcode into one initrd file. That's why I
> > couldn't initially extract the whole Init ramfs.
> >
> > Now I see that i915.ko module is included into initrd and that
> > /etc/modprobe.d actually has i915.conf with my options inside. So,
> > it's
> > clear now that i915 is a module and where its configuration comes
> > from.
> > It's also confirmed by "dmesg" output where i915 reports setting of
> > the
> > "dangerous options".
> >
> > The weird thing now is - provided all of the facts above, the module
> > still
> > ignores the config. It still continues to use some default settings.
> >
> > There's no other place except /etc/modprobe.d in initrd which contains
> > or
> > may override "enable_execlists" setting (checked by 'grepping' all of
> > the
> > extracted contents)
> >
> > Best Regards,
> > Maxim
> >
> [snip]
>
> Maxim,
>
> I'm stretching my module loading knowledge a bit now.
> There are several mechanisms for loading kernel modules.
> They don't all respect module parameter settings, as you
> appear to be discovering.
>
> How you address this for your use case with Ubuntu is
> outside my experience. If you can unload the module, its trivial.
> You can only do that while the module is not used though.
>
> Having helped with identifying the problem, I'll leave it to
> someone who knows Ubuntu to point the way to a Ubuntu
> solution.
>
> The backstop is to rebuild your kernel with i915 set to =y.
> Then it will respond to kernel command line parameters.
>
> Regards,
>
> Roy Bamford.
>
>
>
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