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

[Emil Velikov]

On 5 April 2017 at 18:55, Daniel Vetter <daniel at ffwll.ch> wrote:
> 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.
>
So the goal is to have the code in mesa as a form of incubator until
it reaches maturity.
This way one will have a more rapid development and greater
flexibility during that stage.

If I misunderstood you correctly and the above sounds right - then the
idea is amazing.
Silly me did not click while reading the summary email.

Thanks
Emil