[RFC PATCH 0/3] Use a dma_resv wound-wait transaction API for drm_exec

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

The forward outlook of drm_exec is to be a genuine drm_exec helper
rather than a generic wound-wait transaction helper. Earlier attempts
to introduce exhaustive eviction within TTM using drm_exec have
been forced to extend drm_exec with modes that aren't of interest fo
a drm_exec helper, like eviction mode locking, snapshots, trylocks etc.

This series introduces dma_resv_txn, a subsystem-agnostic wound-wait
transaction layer for any objects that embed or reference a dma_resv.
Participant types register an ops vtable supplying callbacks to reach the
dma_resv and drop the object reference; the transaction layer handles all
lock ordering, wound-wait arbitration, and retry control.

drm_exec is then rebuilt as a thin wrapper on top, retaining its existing
API without changes to existing callers. The dma_resv code suggested
here also follows the previous drm_exec implementation closely with
the same design choices.

Possible use-cases may be ttm_object refcounting for eviction,
Additional eviction modes as described above, a dma_buf_map() functionality
passing a ww transaction locking context (drm_exec would've been a layering
violation), embedding resvs in GPU vm objects etc.

A live kunit benchmark for the drm_exec locking path is included.

It should be mentioned that this could be placed on an even lower level
(kernel/locking), but that would probably require potential other users
first and also a proper non-allocating trylock functionality.

Also tried an approach where object structures were allocated individually
using a kmem_cache and put on a linked list. However, the benchmark
showed a substantial performance hit compared to the pre-allocated
array approach that was originally used indrm_exec.

Thomas Hellström (3):
  dma-buf: Add generic dma_resv wound-wait transaction API
  drm/exec: Rebuild drm_exec on top of dma_resv_txn
  drm/xe/tests: Add drm_exec locking benchmark kunit test

 drivers/dma-buf/Makefile                     |   2 +-
 drivers/dma-buf/dma-resv-txn.c               | 300 +++++++++++++++++++
 drivers/gpu/drm/drm_exec.c                   | 203 ++-----------
 drivers/gpu/drm/xe/tests/Makefile            |   3 +-
 drivers/gpu/drm/xe/tests/xe_drm_exec_kunit.c | 130 ++++++++
 drivers/gpu/drm/xe/tests/xe_live_test_mod.c  |   2 +
 drivers/gpu/drm/xe/xe_bo.c                   |   3 +
 include/drm/drm_exec.h                       | 161 +++-------
 include/linux/dma-resv-txn.h                 | 223 ++++++++++++++
 9 files changed, 731 insertions(+), 296 deletions(-)
 create mode 100644 drivers/dma-buf/dma-resv-txn.c
 create mode 100644 drivers/gpu/drm/xe/tests/xe_drm_exec_kunit.c
 create mode 100644 include/linux/dma-resv-txn.h

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2.54.0