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

[Alex Deucher]

On Wed, Apr 5, 2017 at 2:03 PM, Emil Velikov <emil.l.velikov at gmail.com> wrote:
> 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.

This is sort of indirectly what we did for radeon.  We basically
abandoned libdrm_radeon in mesa and wrote our own winsys, then that,
more or less, because the basis for libdrm_amdgpu.

Alex