[Bug 214001] [bisected][regression] After commit "drm/ttm: Initialize debugfs from ttm_global_init()" kernels without debugfs explicitly set to 'allow all' fail to boot

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=214001

--- Comment #3 from Linux_Chemist (untaintableangel@hotmail.co.uk) ---
Thanks for your comment, Duncan!
Yes, I'm on a customised kernel that has a lot removed (including debugfs as
you can tell) and also amdgpu (RX 5700). 
There's usually a bug in a testing RC every few releases, I just report them
here after bisecting; seems the right place for it even if it's not lol  
Caught a nice bug last release cycle with the memory reservation for the bios
(https://www.phoronix.com/scan.php?page=news_item&px=Linux-Always-Reserve-1MB)

(I wasn't sure to file this one under an AMD ("non-intel") specific 'video' bug
but the commit was for 'drivers/gpu/drm/ttm' which I assume is agnostic. I
don't know what it's for or whether only amdgpu/radeon makes use of it to say
but it is interesting that the 3 of us have similar hardware.)
I can confirm all my .configs have had CONFIG_VGA_CONSOLE=y in it (though a lot
of fallback stuff pulled out that probably stops me getting the legacy low-res
VGA mode you mention, c'est la vie)

But anyways as you say, the ability to create a bootable kernel only becomes an
issue from the commit in question when not having CONFIG_DEBUG_FS=y (and
CONFIG_DEBUG_FS_ALLOW_ALL=y along with that)

Don't get me wrong, it's not a showstopper 'massive bug' because you can always
put debugfs + 'allow all' into your kernel, I did so and am happily on rc5 now,
but that's why I'd like a consensus to be known or shared (i.e. change the
wording for the kconfig options) about whether a lot of things are expecting
debugfs to be there in some form now - is it now an 'essential' part of the
kernel? Or should things that rely on it fail gracefully if they don't find it?
Either it's essential and this isn't a bug and there needs to be clarification
that debugfs should always be there in some form, or this is a bug and the
commit needs tweaked to account for debugfs not being there or there in a
diminished capacity.    

It is a bit silly that even CONFIG_DEBUG_FS_ALLOW_NONE wouldn't work for this
bug because that seems like it should be providing that 'fail gracefully'
mechanism to debugfs being 'there' but 'don't bother with it'.

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