drm/doc/rfc: i915 DG1 uAPI

Add an entry for the new uAPI needed for DG1. Also add the overall
upstream plan, including some notes for the TTM conversion.

v2(Daniel):
  - include the overall upstreaming plan
  - add a note for mmap, there are differences here for TTM vs i915
  - bunch of other suggestions from Daniel
v3:
 (Daniel)
  - add a note for set/get caching stuff
  - add some more docs for existing query and extensions stuff
  - add an actual code example for regions query
  - bunch of other stuff
 (Jason)
  - uAPI change(!):
	- try a simpler design with the placements extension
	- rather than have a generic setparam which can cover multiple
	  use cases, have each extension be responsible for one thing
	  only
v4:
 (Daniel)
  - add some more notes for ttm conversion
  - bunch of other stuff
 (Jason)
  - uAPI change(!):
	- drop all the extra rsvd members for the region_query and
	  region_info, just keep the bare minimum needed for padding
v5:
(Jason)
  - for the upstream plan, add a requirement that we send the uAPI bits
    again for final sign off before turning it on for real
  - document how we intend to extend the rsvd bits for the region query
(Kenneth)
  - improve the comment for the smem+lmem mmap mode and caching

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@linux.intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Dave Airlie <airlied@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Cc: mesa-dev@lists.freedesktop.org
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Dave Airlie <airlied@redhat.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Jon Bloomfield <jon.bloomfield@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210429103056.407067-1-matthew.auld@intel.com

1 files changed, 131 insertions, 0 deletions

diff --git a/Documentation/gpu/rfc/i915_gem_lmem.rst b/Documentation/gpu/rfc/i915_gem_lmem.rst
new file mode 100644
index 000000000000..1d344c593018
--- /dev/null
+++ b/Documentation/gpu/rfc/i915_gem_lmem.rst
@@ -0,0 +1,131 @@
+=========================
+I915 DG1/LMEM RFC Section
+=========================
+
+Upstream plan
+=============
+For upstream the overall plan for landing all the DG1 stuff and turning it for
+real, with all the uAPI bits is:
+
+* Merge basic HW enabling of DG1(still without pciid)
+* Merge the uAPI bits behind special CONFIG_BROKEN(or so) flag
+        * At this point we can still make changes, but importantly this lets us
+          start running IGTs which can utilize local-memory in CI
+* Convert over to TTM, make sure it all keeps working. Some of the work items:
+        * TTM shrinker for discrete
+        * dma_resv_lockitem for full dma_resv_lock, i.e not just trylock
+        * Use TTM CPU pagefault handler
+        * Route shmem backend over to TTM SYSTEM for discrete
+        * TTM purgeable object support
+        * Move i915 buddy allocator over to TTM
+        * MMAP ioctl mode(see `I915 MMAP`_)
+        * SET/GET ioctl caching(see `I915 SET/GET CACHING`_)
+* Send RFC(with mesa-dev on cc) for final sign off on the uAPI
+* Add pciid for DG1 and turn on uAPI for real
+
+New object placement and region query uAPI
+==========================================
+Starting from DG1 we need to give userspace the ability to allocate buffers from
+device local-memory. Currently the driver supports gem_create, which can place
+buffers in system memory via shmem, and the usual assortment of other
+interfaces, like dumb buffers and userptr.
+
+To support this new capability, while also providing a uAPI which will work
+beyond just DG1, we propose to offer three new bits of uAPI:
+
+DRM_I915_QUERY_MEMORY_REGIONS
+-----------------------------
+New query ID which allows userspace to discover the list of supported memory
+regions(like system-memory and local-memory) for a given device. We identify
+each region with a class and instance pair, which should be unique. The class
+here would be DEVICE or SYSTEM, and the instance would be zero, on platforms
+like DG1.
+
+Side note: The class/instance design is borrowed from our existing engine uAPI,
+where we describe every physical engine in terms of its class, and the
+particular instance, since we can have more than one per class.
+
+In the future we also want to expose more information which can further
+describe the capabilities of a region.
+
+.. kernel-doc:: Documentation/gpu/rfc/i915_gem_lmem.h
+        :functions: drm_i915_gem_memory_class drm_i915_gem_memory_class_instance drm_i915_memory_region_info drm_i915_query_memory_regions
+
+GEM_CREATE_EXT
+--------------
+New ioctl which is basically just gem_create but now allows userspace to provide
+a chain of possible extensions. Note that if we don't provide any extensions and
+set flags=0 then we get the exact same behaviour as gem_create.
+
+Side note: We also need to support PXP[1] in the near future, which is also
+applicable to integrated platforms, and adds its own gem_create_ext extension,
+which basically lets userspace mark a buffer as "protected".
+
+.. kernel-doc:: Documentation/gpu/rfc/i915_gem_lmem.h
+        :functions: drm_i915_gem_create_ext
+
+I915_GEM_CREATE_EXT_MEMORY_REGIONS
+----------------------------------
+Implemented as an extension for gem_create_ext, we would now allow userspace to
+optionally provide an immutable list of preferred placements at creation time,
+in priority order, for a given buffer object.  For the placements we expect
+them each to use the class/instance encoding, as per the output of the regions
+query. Having the list in priority order will be useful in the future when
+placing an object, say during eviction.
+
+.. kernel-doc:: Documentation/gpu/rfc/i915_gem_lmem.h
+        :functions: drm_i915_gem_create_ext_memory_regions
+
+One fair criticism here is that this seems a little over-engineered[2]. If we
+just consider DG1 then yes, a simple gem_create.flags or something is totally
+all that's needed to tell the kernel to allocate the buffer in local-memory or
+whatever. However looking to the future we need uAPI which can also support
+upcoming Xe HP multi-tile architecture in a sane way, where there can be
+multiple local-memory instances for a given device, and so using both class and
+instance in our uAPI to describe regions is desirable, although specifically
+for DG1 it's uninteresting, since we only have a single local-memory instance.
+
+Existing uAPI issues
+====================
+Some potential issues we still need to resolve.
+
+I915 MMAP
+---------
+In i915 there are multiple ways to MMAP GEM object, including mapping the same
+object using different mapping types(WC vs WB), i.e multiple active mmaps per
+object. TTM expects one MMAP at most for the lifetime of the object. If it
+turns out that we have to backpedal here, there might be some potential
+userspace fallout.
+
+I915 SET/GET CACHING
+--------------------
+In i915 we have set/get_caching ioctl. TTM doesn't let us to change this, but
+DG1 doesn't support non-snooped pcie transactions, so we can just always
+allocate as WB for smem-only buffers.  If/when our hw gains support for
+non-snooped pcie transactions then we must fix this mode at allocation time as
+a new GEM extension.
+
+This is related to the mmap problem, because in general (meaning, when we're
+not running on intel cpus) the cpu mmap must not, ever, be inconsistent with
+allocation mode.
+
+Possible idea is to let the kernel picks the mmap mode for userspace from the
+following table:
+
+smem-only: WB. Userspace does not need to call clflush.
+
+smem+lmem: We only ever allow a single mode, so simply allocate this as uncached
+memory, and always give userspace a WC mapping. GPU still does snooped access
+here(assuming we can't turn it off like on DG1), which is a bit inefficient.
+
+lmem only: always WC
+
+This means on discrete you only get a single mmap mode, all others must be
+rejected. That's probably going to be a new default mode or something like
+that.
+
+Links
+=====
+[1] https://patchwork.freedesktop.org/series/86798/
+
+[2] https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5599#note_553791