Re: LinuxKPI debugfs Port

On Tue, Jun 28, 2022 at 03:38:48PM -0500, Jake Freeland wrote:
> Hi there,
> 
> I am working on porting Intel's igt-gpu-tools drm graphics driver testing
> suite to FreeBSD and I ran into some issues regarding debugfs. I spoke
> to manu@ who told me that CONFIG_DEBUG_FS is required for the
> testing to work properly. I started working on a debugfs port and quickly
> got confused about what manu@ meant by implementing "CONFIG_DEBUG_FS".

I would guess that he meant to compile the i915kms driver with
CONFIG_DEBUGFS configured.  I'm not sure what the "right" way is to do
that, but adding

KCONFIG+= DEBUGFS

to the end of kconfig.mk in the drm-kmod repository seems to do the
trick for me, with the caveat that the driver doesn't compile. :)

I think the task is to get it to compile by extending the existing
debugfs shims in the LinuxKPI, but see my comments below.

There is also a typo in the makefile there, see
https://github.com/freebsd/drm-kmod/pull/185

> Some quick internet searching says that CONFIG_DEBUG_FS is a
> Linux kernel configuration flag. I am curious how I would go about
> implementing this into FreeBSD. I copied the Linux debugfs source
> code into a new repository and attempted to compile it on FreeBSD
> as a kernel module:
> 
> https://github.com/jakesfreeland/debugfs-freebsd
> 
> Of course I was met with many, many incompatibility errors. I proceeded
> to copy the required `sys/compat/linuxkpi/common/include/linux` headers
> into my repository and I was met with two options:

Yes, that's not going to work and isn't the right approach.

> 1. continue modifying the LinuxKPI headers and commit my modifications
> to src.
> 
> 2. re-engineer the debugfs source code to comply with the preexisting
> LinuxKPI headers.
> 
> Many problems come with both approaches. First, if I modify the LinuxKPI
> headers, I'd be "copying" over some code from Linux's GPLv2 headers.
> I do not know how much I can "copy" before legal issues arise. Second,
> if I re-engineer the debugfs source code, I am revolting against what
> LinuxKPI
> stands for: running Linux code with little-to-no modification. I don't know
> what the correct approach is here.

The purpose of the LinuxKPI is to allow Linux _drivers_ to run (mostly)
unmodified on FreeBSD.  Generic kernel components like debugfs itself
are not meant to run under the LinuxKPI and are out of its scope.

debugfs provides a set of interfaces used by drivers, including i915, to
export debug info.  The LinuxKPI provides a partial implementation of
that interface already, in debugfs.h and lindebugfs.c, and I think the
task ahead of you is to extend it to allow i915 to compile with DEBUGFS
configured.  (But please double check that!)

> I also discovered that lindebugfs, a curtailed version of Linux's debugfs,
> already exists in FreeBSD's src. I am led to believe that this is
> exclusively
> used under the Linuxulator so it wouldn't help me. Is this correct?

No, I don't think that's accurate.  It's just another pseudo filesystem,
it can be used by anything that knows how to open and read files.

> Thank you so much,
> Jake Freeland