[Intel-gfx] [PATCH 0/1] Ensure zero alignment on gens < 4

["Zbigniew Kempczyński" <zbigniew.kempczynski@intel.com>]

In short - we want to enforce alignment == 0 for gen4+ GEM object
settings.

Before we merge this we need to inspect all UMD we expect can use
this. My investigation was narrowed to UMD code:

1. IGT
2. Mesa
3. Media-Driver
4. NEO
5. libdrm
6. xf86-intel-video

I would like to ask subsystem developers / maintainers to confirm
my analysis.

1. IGT:
   We've already removed / fixed most of the code where alignment != 0.
   What left was few multi-card subtests I'm not able to rewrite due
   to lack of such hw (nv + intel on the board).

2. Mesa:
   gallium/drivers/iris/iris_batch.c,iris_bufmgr.c - it uses softpinning
   only with alignment handled by allocator, so drm_i915_gem_exec_object2
   alignment field == 0.

   drivers/dri/i965/brw_batch.c,brw_screen.c - it uses relocations but
   it is supported by allocator, there're no direct alignment settings
   to value != 0.

   vulcan/anv_batch_chain.c: drm_i915_gem_exec_object2 objects are
   initialized within anv_execbuf_add_bo() and .alignment field
   is set to 0 there. There's no other place where I've found vulcan
   driver touches it both for softpinning / relocations.

3. Media-Driver:
   It contains modified libdrm code and three functions which do
   allocations, all of them uses mos_gem_bo_alloc_internal():
   - mos_gem_bo_alloc() - internally uses alignment == 0, that's ok
   - mos_gem_bo_alloc_tiled() - same as mos_gem_bo_alloc()
   - mos_gem_bo_alloc_for_render() - this one passes alignment from
     the caller and it may be != 0. But I haven't found practical
     usage of this function externally (using mos_bo_alloc_for_render()
     wrapper).
   There's another userptr allocation function: mos_bo_alloc_userptr()
   but it doesn't use alignment.

4. NEO:
   Uses softpinning only with alignment == 0:
   source/os_interface/linux/drm_buffer_object.cpp: 
   void BufferObject::fillExecObject() has execObject.alignment = 0;

5. libdrm:
   Corresponding functions to Media-Driver:
   drm_intel_bo_alloc(), drm_intel_bo_alloc_for_render(), 
   drm_intel_bo_alloc_userptr() and drm_intel_bo_alloc_tiled().
   Alignment field is used in drm_intel_bo_alloc_for_render()
   so couple not rewritten IGTs may encounter issue here (alignment
   passed in IGTs which still uses libdrm == 4096).

6. xf86-intel-video:
   src/sna/kgem.c: _kgem_submit() - alignment is set to 0 so this
   shouldn't be a problem.


Cc: Petri Latvala <petri.latvala@intel.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
Cc: Michal Mrozek <michal.mrozek@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>

Zbigniew Kempczyński (1):
  i915/gem/i915_gem_execbuffer: Disallow passing alignment

 drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

-- 
2.26.0