Re: Extra DRM sources

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

> Date: Thu, 19 May 2022 19:36:50 +0100
> From: Robert Swindells <rjs%fdy2.co.uk@localhost>
> 
> I would like to import the source to some extra DRM drivers from Linux,
> would do lima, panfrost and vc4.
> 
> The layout that I'm using in my own tree is to put the GPL ones in a
> sys/external/gpl2/drm2 tree that mirrors the existing bsd/drm2 tree.
> 
> The lima sources apart from lima_regs.h are dual licenced so will put
> them under bsd/drm2.

If lima_regs.h is actually GPL, then it needs to be reimplemented from
scratch, cannot be just put in a directory called `gpl2' and included
in non-GPL code -- annoying as that is for a file of register
definitions.

Alternatively, you could contact the author and see if they would be
willing to release it under a permissive licence, or clarify that that
was the original intent.

Everything under sys/external/gpl2 needs to be done as an optionally
loadable module, not statically linked into the kernel.

> All files will be from the same v5.6-rc3 branch of Linux as the current
> drm sources.
> 
> Do I need to do a vendor import of this or just add the files?

Yes, this absolutely must be vendored, and must have lima2netbsd and
panfrost2netbsd scripts like radeon, nouveau, &c., to be run after the
top-level drm prepare-import.sh to adapt a clean tree for import --
otherwise anything done here will be discarded on the next drm update.

You should draft any work in git first (and generally do development
on this in git like I do), with a vendor branch and vendor tag in git
-- as far as git is concerned, these are just regular branches or tags
but I keep a convention of labelling them with `vendor/' in:

https://github.com/riastradh/netbsd-src

The upstream files should be committed on the branch vendor/linux-drm
(on top of the one in that git tree) in the appropriate subtree of
src, and the commit should be tagged vendor/linux-drm-v5.6-rc3-nb1.

The next step is tricky: you have to find the git commit corresponding
to the cvs merge of the 5.6-rc3 drm, and use `git replace --graft' to
make its parents be:

(a) the parent in the regular cvs->git history, and
(b) the commit tagged vendor/linux-drm-v5.6-rc3.

I think the commit you have to do git-replace on is currently this
one:

commit 677dec6e0973de7db21722260f425a64696d5fec
Author: riastradh <riastradh%NetBSD.org@localhost>
Date:   Sat Dec 18 23:44:57 2021 +0000

    Merge linux-drm-v5-6-rc3

Then you can `git merge vendor/linux-drm-v5.6-rc3-nb1' and the merge
should apply cleanly.  Mark the merge as the base point, say
lima-base, and work on a branch, say lima.  Later on, you can do the
cvs import, and then git-cvsexportcommit everything in lima-base..lima
on top of CVS, and the patches will generally all apply cleanly.

You can do all this in git in a private tree that you can blow away
and start over if you make mistakes.  (I might have identified the
cvs->git merge commit wrong, so it might take some playing around with
git-replace.)  All this is necessary to do a 3-way merge for the next
drm update; updates without 3-way merges are effectively impossible
and will invariably lead to throwing work out and starting over.