[PATCH libdrm] headers: Add README file

[Alex Deucher]

On Thu, Nov 10, 2016 at 11:44 AM, Emil Velikov <emil.l.velikov at gmail.com> wrote:
> From: Emil Velikov <emil.velikov at collabora.com>
>
> Since we're trying to standardise and make things more consistent in
> the area, add a basic README which covers some of the more popular
> topics.
>
> Cc: Dave Airlie <airlied at redhat.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Signed-off-by: Emil Velikov <emil.l.velikov at gmail.com>
> ---
> Dave, did I get it right on the "why only drm files should live here" ?
>
> Dave, Daniel, which trees/branches [in drm-misc] we can use as reference
> point here ?
> ---
>  include/drm/README | 72 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 72 insertions(+)
>  create mode 100644 include/drm/README
>
> diff --git a/include/drm/README b/include/drm/README
> new file mode 100644
> index 0000000..2f80c15
> --- /dev/null
> +++ b/include/drm/README
> @@ -0,0 +1,72 @@
> +What are these headers ?
> +------------------------
> +This is the canonical source of drm headers that user space should use for
> +communicating with the kernel DRM subsystem.
> +
> +They flow from the kernel, thus any changes must be merged there first.
> +Do _not_ attempt to "fix" these by deviating from the kernel ones !
> +
> +
> +Non-linux platforms - changes/patches
> +-------------------------------------
> +If your platform has local changes, please send them upstream for inclusion.
> +Even if your patches don't get accepted in their current form, devs will
> +give you feedback on how to address things properly.
> +
> +git send-email --subject-prefix="PATCH libdrm" your patches to dri-devel
> +mailing list.
> +
> +Before doing so, please consider the following:
> + - Have the [libdrm vs kernel] headers on your platform deviated ?
> +Consider unifying them first.
> +
> + - Have you introduced additional ABI that's not available in Linux ?
> +Propose it for [Linux kernel] upstream inclusion.
> +If that doesn't work out (hopefully it never does), move it to another header
> +and/or keep the change(s) local ?
> +
> + - Are your changes DRI1/UMS specific ?
> +There is virtually no interest/power in keeping those legacy interfaces. They
> +are around due to the kernel "thou shalt not break existing user space" rule.
> +
> +Consider porting the driver to DRI2/KMS - all (almost?) sensible hardware is
> +capable of supporting those.
> +
> +
> +Which headers go where ?
> +------------------------
> +A snipped from the, now removed, Makefile.am used to state:
> +
> +  XXX airlied says, nothing besides *_drm.h and drm*.h should be necessary.
> +  however, r300 and via need their reg headers installed in order to build.
> +  better solutions are welcome.
> +
> +Obviously the r300 and via headers are no longer around ;-)
> +
> +Reason behind is that the drm headers can be used as a basic communications
> +channel with the respective kernel modules. If more advanced functionality is
> +required one can pull the specific libdrm_$driver which is free to pull
> +additional files from the kernel.
> +
> +For example: nouveau has nouveau/nvif/*.h while vc4 has vc4/*.h
> +
> +If your driver is still in prototyping/staging state, consider moving the
> +$driver_drm.h into $driver and _not_ installing it. An header providing opaque
> +definitions and access [via $driver_drmif.h or similar] would be better fit.
> +
> +
> +When and how to update these files
> +----------------------------------
> +Ideally on each libdrm release these will be kept in sync, with the latest
> +released kernels. This way users won't need to provide local definitions.
> +
> +In order to update the files do the following:
> + - Switch to a Linux kernel tree/branch which is not rebased.
> +For example: airlied/drm-next, drm-misc/XXX

If we just want to update it to the latest released kernel, why not
just specify Linus' tree?  There's a chance there may be flux in -next
that you wouldn't necessarily want in libdrm.  Also, I think
generally, it would be the individual driver maintainers or people
working on specific core features that do this.  Does it really make
sense to update these en masse regularly?

Alex

> + - Install the headers via `make headers_install' to a separate location.
> + - Copy the drm header[s] + git add + git commit.
> + - Note: Your commit message must include:
> +   a) Brief summary on the delta. If there's any change that looks like an
> +API/ABI break one _must_ explicitly state why it's safe to do so.
> +   b) "Generated using make headers_install."
> +   c) "Generated from $tree/branch commit $sha"
> --
> 2.9.3
>
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