Re: [Help] Linux low level data structures < - > FreeBSD low level data structures

Hi Owen,

The most comprehensive open-source wrappers for Linux kernel APIs in the 
FreeBSD kernel is found at:

https://github.com/FreeBSDDesktop/freebsd-base-graphics/tree/drm-next/sys/compat/linuxkpi

The most comprehensive open-source wrappers for Linux kernel APIs in 
FreeBSD user-space is found at:

http://www.freshports.org/multimedia/webcamd

I recommend you start with webcamd. Download the source code and look at 
the sources. See if there is something you think you can improve. It is 
much easier to debug using GDB and if something goes wrong it can easily 
be restarted.

When you have a working solution for webcamd, try to use the same 
solution with the kernel.

There is no simple 1:1 mapping for Linux APIs to FreeBSD APIs. Take for 
example linux/list.h . You might think that a list can be re-implemented 
and mapped to our sys/queue.h. But oh-no. Linux kernel code depends on 
all the characteristics of list.h, how the structure members are stored 
and named, and how foreach() macros complete with NULL or non-NULL in 
the iterator. Further you'll find resistance among the Linux kernel 
developers to support compilers different from GCC. I recently found a 
piece of code in a header file, which works with GCC and causes a 
panic() when compiled with clang. I reported it to one of the Linux 
kernel team members and they didn't care about it. Even if you get 
everything compiling it doesn't it will work :-(

--HPS

On 06/04/17 16:35, blubee blubeeme wrote:
> Hi Julian
> 
> My goals are to port the Linux graphics stack over to FreeBSD w/o relying
> too heavily on the linuxkpi stuff. That's cool for a lot of use cases but
> it just seems a bit too brittle.
> 
> It is a very large I understand the task will not be easy but I am willing
> to do the work, even from scratch if necessary although some help would be
> appreciated.
> 
> I've been watching the Linux DRM project grow and while the top levels has
> changed, it's been a very long time since the actual low level stuff has
> been changed. Most of the diffs have shown changes in the
> [linux/driver/gpu/drm] layer which relies heavily on the
> [linux/include/drm] that does a lot of the heavy lifting here's a link to
> the latest version files:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/drm?h=v4.12-rc3
> 
> here's a diff of the latest version of the [linux/driver/gpu/drm] :
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/diff/?id=v4.12-rc3&id2=v4.12-rc2&dt=2
> 
> The diffs in the drivers change constantly but the lower level stuff hasn't
> changed as much.
> 
> Doing some of the lower level translation to native FreeBSD style data
> structures then the upper part could be easily migrated, even with
> something as using AST to translate the headers to their FreeBSD equivalent
> without worrying about inadvertently breaking something or having major
> diffs that needs people to actively look at maintaining.
> 
> That's a high level overview of my plan and what I'd like to achieve. Will
> it be easy, most likely not but once it's done FreeBSD will be just fine.
> 
> Hope that helps clarify things for anyone who is interested. Any assistance
> would be greatly appreciated.
> 
> Best,
> Owen
> 
> On Sun, Jun 4, 2017 at 9:26 PM, Julian Elischer <julian_at_elischer.org> wrote:
> 
>> On 4/6/17 7:07 pm, blubee blubeeme wrote:
>>
>>> Hello
>>>
>>> Is there anyone on either of these lists that have experience with both
>>> linux low level data structures and their equivalents on FreeBSD?
>>>
>>> For instance the linux header file:
>>> <linux/agp_backend.h>
>>>
>>> which includes the header file:
>>> <linux/list.h>
>>>
>>> Then looking at that file:
>>> <linux/types.h>
>>> <linux/stddef.h>
>>> <linux/poison.h>
>>> <linux/const.h>
>>> <linux/kernel.h>
>>>
>>
>> You are going to have to be a lot more specific about this.
>> I have worked in several places where they use s shim layer to make Linux
>> basic services work on freeBSD.
>> usually  a mix of functions, macros and inlines.
>> However you need to narrow down your questions a bit as the POSSIBLE scope
>> of your question is too large for anyone to attempt an answer.
>>
>> Remember that both systems are POSIX inspired so outside the kernel there
>> are many more simlarities than one might be led to expect,
>>   but you need to be way more specific.
>> It's even possible to write kernel code to run on both, but it is usually
>> domain specific.
>>
>>
>>
>>> I'll be doing a lot of work trying to find these FreeBSD equivalent of
>>> these types of files to port some code.
>>>
>>> Does anyone here have experience with something like this? Is there any
>>> other projects that maps these low level data structures from
>>> Linux <-> FreeBSD, etc?
>>>
>>> Best,
>>> Owen