[PATCH v4 0/4] Implement lazy unmapping for GEM VRAM buffers

[Thomas Zimmermann]

Generic fbdev emulation maps and unmaps the console BO for updating it's
content from the shadow buffer. If this involves an actual mapping
operation (instead of reusing an existing mapping), lots of debug messages
may be printed, such as

  x86/PAT: Overlap at 0xd0000000-0xd1000000
  x86/PAT: reserve_memtype added [mem 0xd0000000-0xd02fffff], track write-combining, req write-combining, ret write-combining
  x86/PAT: free_memtype request [mem 0xd0000000-0xd02fffff]

as reported at [1]. Drivers using VRAM helpers may also see reduced
performance as the mapping operations can create overhead.

In v3 and later of the patch set, this problem is being solved by lazily
unmapping the buffer as suggested by Gerd. Unmapping with drm_gem_vram_kunmap()
only changes a reference counter. VRAM helpers later perform the unmapping
operation when TTM evicts the buffer object from its current location. If
the buffer is never evicted, the existing mapping is reused by later calls
to drm_gem_vram_kmap().

v4:
	* lock kmap with ttm_bo_reserve()
	* acquire lock only once for vmap()
	* warn about stale mappings during buffer cleanup
v3:
      	* implement lazy unmapping
v2:
	* fixed comment typos

[1] https://lists.freedesktop.org/archives/dri-devel/2019-September/234308.html

Thomas Zimmermann (4):
  drm/vram: Add kmap ref-counting to GEM VRAM objects
  drm/vram: Acquire lock only once per call to vmap()/vunmap()
  drm/vram: Add infrastructure for move_notify()
  drm/vram: Implement lazy unmapping for GEM VRAM buffers

 drivers/gpu/drm/drm_gem_vram_helper.c | 229 ++++++++++++++++++--------
 drivers/gpu/drm/drm_vram_mm_helper.c  |  12 ++
 include/drm/drm_gem_vram_helper.h     |  18 ++
 include/drm/drm_vram_mm_helper.h      |   4 +
 4 files changed, 198 insertions(+), 65 deletions(-)

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2.23.0