=== userspace arithmetic proof ===
hash_long(key, 12) samples (drm map_hash / gem offset_hash):
  key=0x0000000000000000 -> bucket=0
  key=0x9e3779b97f4a7c15 -> bucket=0
  key=0x3c6ef372fe94f82a -> bucket=0
  key=0xdaa66d2c7ddf743f -> bucket=0
  key=0x78dde6e5fd29f054 -> bucket=0
  key=0x1715609f7c746c69 -> bucket=0
  key=0xb54cda58fbbee87e -> bucket=0
  key=0x538454127b096493 -> bucket=0
i386 hash_32(key,12) max bucket = 22 (table has 4096 buckets)
tested=6400000 zero64=6400000
RESULT: hash_long(key,bits)==0 for ALL 6400000 inputs with bits in [1,32] -- 64-bit drm hash tables collapse to bucket 0
=== kernel baseline (stock code) ===
HTCOLLAPSE: order=12 table=4096 buckets, insert 8192 sequential keys (alloc_unr style, drm_gem.c:576)
HTCOLLAPSE: per-1024-insert wall time (us): 596 2014 3578 5019 7141 8217 9599 11231
HTCOLLAPSE: nonempty_buckets=1/4096 max_chain=8192 total_items=8192 errors=0
HTCOLLAPSE: find(key=1)=0 (found)
HTCOLLAPSE: find(key=8192)=0 (found)
HTCOLLAPSE: find(key=0xdead0000)=-22 (absent (ok))
HTCOLLAPSE: dup-insert=-22 (rejected (ok))
HTCOLLAPSE: remove/reinsert=0 (0=ok)
HTCOLLAPSE: VERDICT errors=0 nonempty_buckets=1 max_chain=8192 => hash_long collapse CONFIRMED in-kernel
=== kernel fixed (fixinc/linux/hash.h) ===
HTCOLLAPSE: order=12 table=4096 buckets, insert 8192 sequential keys (alloc_unr style, drm_gem.c:576)
HTCOLLAPSE: per-1024-insert wall time (us): 28 18 17 18 18 18 18 18
HTCOLLAPSE: nonempty_buckets=4096/4096 max_chain=2 total_items=8192 errors=0
HTCOLLAPSE: find(key=1)=0 (found)
HTCOLLAPSE: find(key=8192)=0 (found)
HTCOLLAPSE: find(key=0xdead0000)=-22 (absent (ok))
HTCOLLAPSE: dup-insert=-22 (rejected (ok))
HTCOLLAPSE: remove/reinsert=0 (0=ok)
HTCOLLAPSE: VERDICT errors=0 nonempty_buckets=4096 max_chain=2 => hash distributed normally (fixed header in effect)
RUN-DONE