drm/xe: Document ULLS for migration jobs

Add a kernel-doc DOC section at the top of xe_migrate.c describing the
Ultra Low Latency Submission (ULLS) scheme used for migration jobs.

Cover the motivation (removing the H2G / GuC / context switch latency
from the page fault and SVM prefetch critical paths), the platform
requirements, the LRC PPHWSP semaphore layout and its relationship to
the migration queue job count, the fixed ULLS job size and why it is
needed, the ring preamble / postamble emitted by the ring ops including
the in-ring tail update, the semaphore-only submission fast path in the
GuC backend, and the enter / delayed exit flow along with the ULLS job
flags.

Hook the new section into Documentation/gpu/xe/xe_migrate.rst.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Assisted-by: Github-Copilot:Claude-opus-5
Reviewed-by: Maarten Lankhorst <dev@lankhorst.se>
Link: https://patch.msgid.link/20260925201526.1355279-25-matthew.brost@intel.com

0 files changed, 0 insertions, 0 deletions