Re: [RFC] drm: Start documenting userspace ABI

[Thierry Reding <thierry.reding@gmail.com>]

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On Tue, Nov 18, 2014 at 03:27:27PM +0100, Daniel Vetter wrote:
> On Tue, Nov 18, 2014 at 03:19:33PM +0100, Thierry Reding wrote:
> > From: Thierry Reding <treding@nvidia.com>
> > 
> > After seeing how the DRM_IOCTL_MODE_CREATE_DUMB was implemented with
> > different semantics on different drivers it seems like a good idea to
> > start to more rigorously document userspace ABI to avoid these things
> > in the future.
> > 
> > This is a first draft of what such documentation could look like. Not
> > all IOCTLs are documented and some explanation about the other system
> > calls (mmap(), poll(), read(), ...) would be good too. But I wanted to
> > send this out for early review to see if the direction is reasonable.
> > If so my plan is to continue to chip away at this as I find time.
> > 
> > Signed-off-by: Thierry Reding <treding@nvidia.com>
> 
> Imo for ioctls the right thing to do is having proper manpages, not
> kerneldoc or DocBook sections. manpages really lend themselves well to
> specify all the different facets of a single interface.

I don't think I've ever seen manpages document IOCTLs at this level of
detail. The intention of this is to add documentation about the IOCTLs
at the kernel/userspace transition. Keeping this in the kernel has the
advantage that it's a whole lot easier to keep updated, much like what
we do with the other kerneldoc.

That doesn't mean we shouldn't have manpages, but I think both are for
the most part orthogonal, even though they may describe various facets
of the same interfaces.

> Also, we already have some skeleton of that in libdrm, so I think
> extending that would be best.

One other reason why I don't think libdrm is the best fit for this is
that libdrm is just one userspace implementation abstracting away the
interfaces that this describes. Not everyone will use libdrm. So in my
opinion its great if libdrm documents the API that it exposes, but I
don't think it should document the kernel interfaces that it uses. The
kernel exposes them, so it should provide the documentation for it as
well.

Thierry

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