[Mesa-dev] [PATCH 00/53] i965: Eat libdrm_intel for breakfast

[Daniel Vetter]

On Wed, Apr 05, 2017 at 04:38:25PM +0100, Emil Velikov wrote:
> Hi Ken,
> 
> On 5 April 2017 at 01:09, Kenneth Graunke <kenneth at whitecape.org> wrote:
> > Hello,
> >
> > This series imports libdrm_intel into the i965 driver, hacks and
> > slashes it down to size, and greatly simplifies our relocation
> > handling.
> >
> > Some of the patches may be held for moderation.  You can find the
> > series in git here:
> >
> > https://cgit.freedesktop.org/~kwg/mesa/log/?h=bacondrm
> >
> > A couple of us have been talking about this in person and IRC for
> > a while, but I realize I haven't mentioned anything about it on the
> > mailing list yet, so this may come as a bit of a surprise.
> >
> > libdrm_intel is about 15 source files and almost 13,000 lines of code.
> > This series adds 3 files (one .c, two .h) and only 2,137 lines of code:
> >
> >     60 files changed, 2784 insertions(+), 647 deletions(-)
> >
> > The rest of the library is basically useless to us.  It contains a lot
> > of legacy cruft from the pre-GEM, DRI1, or 8xx/9xx era.  But even the
> > parts we do use are in bad shape.  BO offset tracking is non-threadsafe.
> > Relocation handling is way too complicated.  These things waste memory,
> > burn CPU time, and make it difficult for us to take advantage of new
> > kernel features like I915_EXEC_NO_RELOC which would reduce overhead
> > further.  The unsynchronized mapping API performs a synchronized mapping
> > on non-LLC platforms, which can massively hurt performance on Atoms.
> > Mesa is also using uncached GTT mappings for almost everything on Atoms,
> > rather than fast CPU or WC maps where possible.
> >
> > Evolving this code in libdrm is very painful, as we aren't allowed to
> > break the ABI.  All the legacy cruft and design mistakes (in hindsight)
> > make it difficult to follow what's going on.  We could keep piling new
> > layers on top, but that only makes it worse.  Furthermore, there's a
> > bunch of complexity that comes from defending against or supporting
> > broken or badly designed callers.
> >
> I believe I mentioned it a few days ago - there is no need to worry
> about API or ABI stability.
> 
> Need new API - add it. Things getting fragile or too many layers - sed
> /libdrm_intel$(N)/libdrm_intel$(N+1)/ and rework as needed.
> 
> I fear that Importing libdrm_intel will be detrimental to libva's
> intel-driver, Beignet and xf86-video-intel development.
> Those teams seem to be more resource contained than Mesa, thus they
> will trail behind even more.
> 
> As an example - the intel-driver is missing some trivial winsys
> optimisations that landed in Mesa 3+ years ago. That could have been
> avoided if the helpers were shared with the help of
> libdrm_intel/other.

That is kinda the longer-term goal with this. There's a lot more that
needs to be done besides Ken's series here, this is just the first step,
but in the end we'll probably move brw_batch back into libdrm_intel2 or
so, for consumption by beignet and libva.

But for rewriting the world and getting rid of 10+ years of compat
garbage, having a split between libdrm and mesa isn't great.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch