[Mesa-dev] rules for merging patches to libdrm

[Thierry Reding]

On Mon, Nov 18, 2013 at 11:21:36AM -0500, Rob Clark wrote:
> On Mon, Nov 18, 2013 at 10:23 AM, Thierry Reding
> <thierry.reding at gmail.com> wrote:
> > On Mon, Nov 18, 2013 at 10:17:47AM -0500, Rob Clark wrote:
> >> On Mon, Nov 18, 2013 at 8:29 AM, Thierry Reding
> >> <thierry.reding at gmail.com> wrote:
> >> > On Sat, Nov 09, 2013 at 01:26:24PM -0800, Ian Romanick wrote:
> >> >> On 11/09/2013 12:11 AM, Dave Airlie wrote:
> >> >> >>> How does this interact with the rule that kernel interfaces require an
> >> >> >>> open source userspace? Is "here are the mesa/libdrm patches that use
> >> >> >>> it" sufficient to get the kernel interface merged?
> >> >> >>
> >> >> >> That's my understanding: open source userspace needs to exist, but it
> >> >> >> doesn't need to be merged upstream yet.
> >> >> >
> >> >> > Having an opensource userspace and having it committed to a final repo
> >> >> > are different things, as Daniel said patches on the mesa-list were
> >> >> > sufficient, they're was no hurry to merge them considering a kernel
> >> >> > release with the code wasn't close, esp with a 3 month release window
> >> >> > if the kernel merge window is close to that anyways.
> >> >> >
> >> >> >>> libdrm is easy to change and its releases are cheap. What problem does
> >> >> >>> committing code that uses an in-progress kernel interface to libdrm
> >> >> >>> cause? I guess I'm not understanding something.
> >> >> >>
> >> >> >
> >> >> > Releases are cheap, but ABI breaks aren't so you can't just go release
> >> >> > a libdrm with an ABI for mesa then decide later it was a bad plan.
> >> >> >
> >> >> >> Introducing new kernel API usually involves assigning numbers for things
> >> >> >> - a new ioctl number, new #defines for bitfield members, and so on.
> >> >> >>
> >> >> >> Multiple patches can be in flight at the same time.  For example, Abdiel
> >> >> >> and I both defined execbuf2 flags:
> >> >> >>
> >> >> >> #define I915_EXEC_RS (1 << 13)     (Abdiel's code)
> >> >> >> #define I915_EXEC_OA (1 << 13)     (my code)
> >> >> >>
> >> >> >> These obviously conflict.  One of the two will land, and the second
> >> >> >> patch author will need to switch to (1 << 14) and resubmit.
> >> >> >>
> >> >> >> If we both decide to push to libdrm, we might get the order backwards,
> >> >> >> or maybe one series won't get pushed at all (in this case, I'm planning
> >> >> >> to drop my patch).  Waiting until one lands in the kernel avoids that
> >> >> >> problem.  Normally, I believe we copy the kernel headers to userspace
> >> >> >> and fix them up a bit.
> >> >> >>
> >> >> >> Dave may have other reasons; this is just the one I thought of.
> >> >> >
> >> >> > But mostly this, we've been stung by this exact thing happening
> >> >> > before, and we made the process to stop it from happening again.
> >> >>
> >> >> Then in all honestly, commits to libdrm should be controlled by either a
> >> >> single person or a small cabal... just like the kernel and the xserver.
> >> >>  We're clearly in an uncomfortable middle area where we have a stringent
> >> >> set of restrictions but no way to actually enforce them.
> >> >
> >> > That doesn't sound like a bad idea at all. It obviously causes more work
> >> > for whoever will be the gatekeeper(s).
> >> >
> >> > It seems to me that libdrm is currently more of a free-for-all type of
> >> > project, and whoever merges some new feature required for a particular X
> >> > or Mesa driver cuts a new release so that the version number can be used
> >> > to track the dependency.
> >> >
> >> > I wonder if perhaps tying the libdrm releases more tightly to Linux
> >> > kernel releases would help. Since there already is a requirement for new
> >> > kernel APIs to be merged before the libdrm equivalent can be merged,
> >> > then having both release cycles in lockstep makes some sense.
> >>
> >> Not sure about strictly tying it to kernel releases would be ideal.
> >> Not *everything* in libdrm is about new kernel APIs.  It tends to be
> >> the place for things needed both by xorg ddx and mesa driver, which I
> >> suppose is why it ends up a bit of a free-for-all.
> >
> > I didn't mean that every release would need to be tied to the Linux
> > kernel. But whenever a new Linux kernel release was made, relevant
> > changes from the public headers could be pulled into libdrm and a
> > release be made. I could even imagine a matching of version numbers.
> > libdrm releases could be numbered using the same major and minor as
> > Linux kernels that they support. Micro version numbers could be used
> > in intermediate releases.
> 
> maybe an update-kernel-headers.sh script to grab the headers from
> drm-next and update libdrm wouldn't be a bad idea?

Perhaps. But I think it could even be a manual step. It's not something
that one person should be doing alone, but rather something that driver
maintainers should be doing, since they know best what will be needed
in a new version of libdrm.

Like I mentioned in another subthread, I think a subtree-oriented model
could work well.

Thierry
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