Merge tag 'drm-next-2021-04-28' of git://anongit.freedesktop.org/drm/drm

Pull drm updates from Dave Airlie:
 "The usual lots of work all over the place.

  i915 has gotten some Alderlake work and prelim DG1 code, along with a
  major locking rework over the GEM code, and brings back the property
  of timing out long running jobs using a watchdog. amdgpu has some
  Alderbran support (new GPU), freesync HDMI support along with a lot
  other fixes.

  Outside of the drm, there is a new printf specifier added which should
  have all the correct acks/sobs:

   - printk fourcc modifier support added %p4cc

  Summary:

  core:
   - drm_crtc_commit_wait
   - atomic plane state helpers reworked for full state
   - dma-buf heaps API rework
   - edid: rework and improvements for displayid

  dp-mst:
   - better topology logging

  bridge:
   - Chipone ICN6211
   - Lontium LT8912B
   - anx7625 regulator support

  panel:
   - fix lt9611 4k panels handling

  simple-kms:
   - add plane state helpers

  ttm:
   - debugfs support
   - removal of unused sysfs
   - ignore signaled moved fences
   - ioremap buffer according to mem caching

  i915:
   - Alderlake S enablement
   - Conversion to dma_resv_locking
   - Bring back watchdog timeout support
   - legacy ioctl cleanups
   - add GEM TDDO and RFC process
   - DG1 LMEM preparation work
   - intel_display.c refactoring
   - Gen9/TGL PCH combination support
   - eDP MSO Support
   - multiple PSR instance support
   - Link training debug updates
   - Disable PSR2 support on JSL/EHL
   - DDR5/LPDDR5 support for bw calcs
   - LSPCON limited to gen9/10 platforms
   - HSW/BDW async flip/VTd corruption workaround
   - SAGV watermark fixes
   - SNB hard hang on ring resume fix
   - Limit imported dma-buf size
   - move to use new tasklet API
   - refactor KBL/TGL/ADL-S display/gt steppings
   - refactoring legacy DP/HDMI, FB plane code out

  amdgpu:
   - uapi: add ioctl to query video capabilities
   - Iniital AMD Freesync HDMI support
   - Initial Adebaran support
   - 10bpc dithering improvements
   - DCN secure display support
   - Drop legacy IO BAR requirements
   - PCIE/S0ix/RAS/Prime/Reset fixes
   - Display ASSR support
   - SMU gfx busy queues for RV/PCO
   - Initial LTTPR display work

  amdkfd:
   - MMU notifier fixes
   - APU fixes

  radeon:
   - debugfs cleanps
   - fw error handling ifix
   - Flexible array cleanups

  msm:
   - big DSI phy/pll cleanup
   - sc7280 initial support
   - commong bandwidth scaling path
   - shrinker locking contention fixes
   - unpin/swap support for GEM objcets

  ast:
   - cursor plane handling reworked

  tegra:
   - don't register DP AUX channels before connectors

  zynqmp:
   - fix OOB struct padding memset

  gma500:
   - drop ttm and medfield support

  exynos:
   - request_irq cleanup function

  mediatek:
   - fine tune line time for EOTp
   - MT8192 dpi support
   - atomic crtc config updates
   - don't support HDMI connector creation

  mxsdb:
   - imx8mm support

  panfrost:
   - MMU IRQ handling rework

  qxl:
   - locking fixes
   - resource deallocation changes

  sun4i:
   - add alpha properties to UI/VI layers

  vc4:
   - RPi4 CEC support

  vmwgfx:
   - doc cleanups

  arc:
   - moved to drm/tiny"

* tag 'drm-next-2021-04-28' of git://anongit.freedesktop.org/drm/drm: (1390 commits)
  drm/ttm: Don't count pages in SG BOs against pages_limit
  drm/ttm: fix return value check
  drm/bridge: lt8912b: fix incorrect handling of of_* return values
  drm: bridge: fix LONTIUM use of mipi_dsi_() functions
  drm: bridge: fix ANX7625 use of mipi_dsi_() functions
  drm/amdgpu: page retire over debugfs mechanism
  drm/radeon: Fix a missing check bug in radeon_dp_mst_detect()
  drm/amd/display: Fix the Wunused-function warning
  drm/radeon/r600: Fix variables that are not used after assignment
  drm/amdgpu/smu7: fix CAC setting on TOPAZ
  drm/amd/display: Update DCN302 SR Exit Latency
  drm/amdgpu: enable ras eeprom on aldebaran
  drm/amdgpu: RAS harvest on driver load
  drm/amdgpu: add ras aldebaran ras eeprom driver
  drm/amd/pm: increase time out value when sending msg to SMU
  drm/amdgpu: add DMUB outbox event IRQ source define/complete/debug flag
  drm/amd/pm: add the callback to get vbios bootup values for vangogh
  drm/radeon: Fix size overflow
  drm/amdgpu: Fix size overflow
  drm/amdgpu: move mmhub ras_func init to ip specific file
  ...

1 files changed, 76 insertions, 0 deletions

diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
index a2133d69872c..7f37ec30d9fd 100644
--- a/Documentation/driver-api/dma-buf.rst
+++ b/Documentation/driver-api/dma-buf.rst
@@ -257,3 +257,79 @@ fences in the kernel. This means:
   userspace is allowed to use userspace fencing or long running compute
   workloads. This also means no implicit fencing for shared buffers in these
   cases.
+
+Recoverable Hardware Page Faults Implications
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Modern hardware supports recoverable page faults, which has a lot of
+implications for DMA fences.
+
+First, a pending page fault obviously holds up the work that's running on the
+accelerator and a memory allocation is usually required to resolve the fault.
+But memory allocations are not allowed to gate completion of DMA fences, which
+means any workload using recoverable page faults cannot use DMA fences for
+synchronization. Synchronization fences controlled by userspace must be used
+instead.
+
+On GPUs this poses a problem, because current desktop compositor protocols on
+Linux rely on DMA fences, which means without an entirely new userspace stack
+built on top of userspace fences, they cannot benefit from recoverable page
+faults. Specifically this means implicit synchronization will not be possible.
+The exception is when page faults are only used as migration hints and never to
+on-demand fill a memory request. For now this means recoverable page
+faults on GPUs are limited to pure compute workloads.
+
+Furthermore GPUs usually have shared resources between the 3D rendering and
+compute side, like compute units or command submission engines. If both a 3D
+job with a DMA fence and a compute workload using recoverable page faults are
+pending they could deadlock:
+
+- The 3D workload might need to wait for the compute job to finish and release
+  hardware resources first.
+
+- The compute workload might be stuck in a page fault, because the memory
+  allocation is waiting for the DMA fence of the 3D workload to complete.
+
+There are a few options to prevent this problem, one of which drivers need to
+ensure:
+
+- Compute workloads can always be preempted, even when a page fault is pending
+  and not yet repaired. Not all hardware supports this.
+
+- DMA fence workloads and workloads which need page fault handling have
+  independent hardware resources to guarantee forward progress. This could be
+  achieved through e.g. through dedicated engines and minimal compute unit
+  reservations for DMA fence workloads.
+
+- The reservation approach could be further refined by only reserving the
+  hardware resources for DMA fence workloads when they are in-flight. This must
+  cover the time from when the DMA fence is visible to other threads up to
+  moment when fence is completed through dma_fence_signal().
+
+- As a last resort, if the hardware provides no useful reservation mechanics,
+  all workloads must be flushed from the GPU when switching between jobs
+  requiring DMA fences or jobs requiring page fault handling: This means all DMA
+  fences must complete before a compute job with page fault handling can be
+  inserted into the scheduler queue. And vice versa, before a DMA fence can be
+  made visible anywhere in the system, all compute workloads must be preempted
+  to guarantee all pending GPU page faults are flushed.
+
+- Only a fairly theoretical option would be to untangle these dependencies when
+  allocating memory to repair hardware page faults, either through separate
+  memory blocks or runtime tracking of the full dependency graph of all DMA
+  fences. This results very wide impact on the kernel, since resolving the page
+  on the CPU side can itself involve a page fault. It is much more feasible and
+  robust to limit the impact of handling hardware page faults to the specific
+  driver.
+
+Note that workloads that run on independent hardware like copy engines or other
+GPUs do not have any impact. This allows us to keep using DMA fences internally
+in the kernel even for resolving hardware page faults, e.g. by using copy
+engines to clear or copy memory needed to resolve the page fault.
+
+In some ways this page fault problem is a special case of the `Infinite DMA
+Fences` discussions: Infinite fences from compute workloads are allowed to
+depend on DMA fences, but not the other way around. And not even the page fault
+problem is new, because some other CPU thread in userspace might
+hit a page fault which holds up a userspace fence - supporting page faults on
+GPUs doesn't anything fundamentally new.