[PATCH] drm/doc/rfc: Introduce the merge plan for the Xe driver.

Wed Apr 5 19:52:05 UTC 2023

Let’s establish a merge plan for Xe, by writing down clear pre-merge goals, in
order to avoid unnecessary delays.

This initial document starts with a TODO list containing items with clear and
measurable key results. Xe’s initial pull request should only be sent to
dri-devel after all the items are clearly resolved.

Since many of them involve some level of a community consensus, in many cases,
the consensus will be reached in follow-up patches to this document with more
details of the API or helpers that will be developed or modified.

Cc: Dave Airlie <airlied at redhat.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Oded Gabbay <ogabbay at kernel.org>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
Signed-off-by: Francois Dugast <francois.dugast at intel.com>
Signed-off-by: Luis Strano <luis.strano at intel.com>
Signed-off-by: Matthew Brost <matthew.brost at intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
---
 Documentation/gpu/rfc/index.rst |   4 +
 Documentation/gpu/rfc/xe.rst    | 216 ++++++++++++++++++++++++++++++++
 2 files changed, 220 insertions(+)
 create mode 100644 Documentation/gpu/rfc/xe.rst

diff --git a/Documentation/gpu/rfc/index.rst b/Documentation/gpu/rfc/index.rst
index 476719771eef..e4f7b005138d 100644
--- a/Documentation/gpu/rfc/index.rst
+++ b/Documentation/gpu/rfc/index.rst
@@ -31,3 +31,7 @@ host such documentation:
 .. toctree::
 
     i915_vm_bind.rst
+
+.. toctree::
+
+   xe.rst
diff --git a/Documentation/gpu/rfc/xe.rst b/Documentation/gpu/rfc/xe.rst
new file mode 100644
index 000000000000..1e3e7e9c67c3
--- /dev/null
+++ b/Documentation/gpu/rfc/xe.rst
@@ -0,0 +1,216 @@
+==========================
+Xe – Merge Acceptance Plan
+==========================
+Xe is a new driver for Intel GPUs that supports both integrated and
+discrete platforms starting with Tiger Lake (first Intel Xe Architecture).
+
+This document aims to establish a merge plan for the Xe, by writing down clear
+pre-merge goals, in order to avoid unnecessary delays.
+
+Xe – Overview
+=============
+The main motivation of Xe is to have a fresh base to work from that is
+unencumbered by older platforms, whilst also taking the opportunity to
+rearchitect our driver to increase sharing across the drm subsystem, both
+leveraging and allowing us to contribute more towards other shared components
+like TTM and drm/scheduler.
+
+This is also an opportunity to start from the beginning with a clean uAPI that is
+extensible by design and already aligned with the modern userspace needs. For
+this reason, the memory model is solely based on GPU Virtual Address space
+bind/unbind (‘VM_BIND’) of GEM buffer objects (BOs) and execution only supporting
+explicit synchronization. With persistent mapping across the execution, the
+userspace does not need to provide a list of all required mappings during each
+submission.
+
+The new driver leverages a lot from i915. As for display, the intent is to share
+the display code with the i915 driver so that there is maximum reuse there.
+
+As for the power management area, the goal is to have a much-simplified support
+for the system suspend states (S-states), PCI device suspend states (D-states),
+GPU/Render suspend states (R-states) and frequency management. It should leverage
+as much as possible all the existent PCI-subsystem infrastructure (pm and
+runtime_pm) and underlying firmware components such PCODE and GuC for the power
+states and frequency decisions.
+
+Repository:
+
+https://gitlab.freedesktop.org/drm/xe/kernel (branch drm-xe-next)
+
+Xe – Platforms
+==============
+Currently, Xe is already functional and has experimental support for multiple
+platforms starting from Tiger Lake, with initial support in userspace implemented
+in Mesa (for Iris and Anv, our OpenGL and Vulkan drivers), as well as in NEO
+(for OpenCL and Level0).
+
+During a transition period, platforms will be supported by both Xe and i915.
+However, the force_probe mechanism existent in both drivers will allow only one
+official and by-default probe at a given time.
+
+For instance, in order to probe a DG2 which PCI ID is 0x5690 by Xe instead of
+i915, the following set of parameters need to be used:
+
+```
+i915.force_probe=!5690 xe.force_probe=5690
+```
+
+In both drivers, the ‘.require_force_probe’ protection forces the user to use the
+force_probe parameter while the driver is under development. This protection is
+only removed when the support for the platform and the uAPI are stable. Stability
+which needs to be demonstrated by CI results.
+
+In order to avoid user space regressions, i915 will continue to support all the
+current platforms that are already out of this protection. Xe support will be
+forever experimental and dependent on the usage of force_probe for these
+platforms.
+
+When the time comes for Xe, the protection will be lifted on Xe and kept in i915.
+
+Xe driver will be protected with both STAGING Kconfig and force_probe. Changes in
+the uAPI are expected while the driver is behind these protections. STAGING will
+be removed when the driver uAPI gets to a mature state where we can guarantee the
+‘no regression’ rule. Then force_probe will be lifted only for future platforms
+that will be productized with Xe driver, but not with i915.
+
+Xe – Pre-Merge Goals
+====================
+
+Drm_scheduler
+-------------
+Xe primarily uses Firmware based scheduling (GuC FW). However, it will use
+drm_scheduler as the scheduler ‘frontend’ for userspace submission in order to
+resolve syncobj and dma-buf implicit sync dependencies. However, drm_scheduler is
+not yet prepared to handle the 1-to-1 relationship between drm_gpu_scheduler and
+drm_sched_entity.
+
+Deeper changes to drm_scheduler should *not* be required to get Xe accepted, but
+some consensus needs to be reached between Xe and other community drivers that
+could also benefit from this work, for coupling FW based/assisted submission such
+as the ARM’s new Mali GPU driver, and others.
+
+As a key measurable result, the patch series introducing Xe itself shall not
+depend on any other patch touching drm_scheduler itself that was not yet merged
+through drm-misc. This, by itself, already includes the reach of an agreement for
+uniform 1 to 1 relationship implementation / usage across drivers.
+
+GPU VA
+------
+Two main goals of Xe are meeting together here:
+
+1) Have an uAPI that aligns with modern UMD needs.
+
+2) Early upstream engagement.
+
+RedHat engineers working on Nouveau proposed a new DRM feature to handle keeping
+track of GPU virtual address mappings. This is still not merged upstream, but
+this aligns very well with our goals and with our VM_BIND. The engagement with
+upstream and the port of Xe towards GPUVA is already ongoing.
+
+As a key measurable result, Xe needs to be aligned with the GPU VA and working in
+our tree. Missing Nouveau patches should *not* block Xe and any needed GPUVA
+related patch should be independent and present on dri-devel or acked by
+maintainers to go along with the first Xe pull request towards drm-next.
+
+DRM_VM_BIND
+-----------
+Nouveau, and Xe are all implementing ‘VM_BIND’ and new ‘Exec’ uAPIs in order to
+fulfill the needs of the modern uAPI. Xe merge should *not* be blocked on the
+development of a common new drm_infrastructure. However, the Xe team needs to
+engage with the community to explore the options of a common API.
+
+As a key measurable result, the DRM_VM_BIND needs to be documented in this file
+below, or this entire block deleted if the consensus is for independent drivers
+vm_bind ioctls.
+
+Although having a common DRM level IOCTL for VM_BIND is not a requirement to get
+Xe merged, it is mandatory to enforce the overall locking scheme for all major
+structs and list (so vm and vma). So, a consensus is needed, and possibly some
+common helpers. If helpers are needed, they should be also documented in this
+document.
+
+ASYNC VM_BIND
+-------------
+Although having a common DRM level IOCTL for VM_BIND is not a requirement to get
+Xe merged, it is mandatory to have a cross-driver consensus and understanding how
+to handle async VM_BIND and interactions with userspace memory fences. Ideally
+with helper support so people don't get it wrong in all possible ways.
+
+As a key measurable result, the benefits of ASYNC VM_BIND and a discussion of
+various flavors, error handling and a sample API should be documented here or in
+a separate document pointed to by this document.
+
+Userptr integration and vm_bind
+-------------------------------
+Different drivers implement different ways of dealing with execution of userptr.
+With multiple drivers currently introducing support to VM_BIND, the goal is to
+aim for a DRM consensus on what’s the best way to have that support. To some
+extent this is already getting addressed itself with the GPUVA where likely the
+userptr will be a GPUVA with a NULL GEM call VM bind directly on the userptr.
+However, there are more aspects around the rules for that and the usage of
+mmu_notifiers, locking and other aspects.
+
+This task here has the goal of introducing a documentation of the basic rules.
+
+The documentation *needs* to first live in this document (API session below) and
+then moved to another more specific document or at Xe level or at DRM level.
+
+Documentation should include:
+
+ * The userptr part of the VM_BIND api.
+
+ * Locking, including the page-faulting case.
+
+ * O(1) complexity under VM_BIND.
+
+Some parts of userptr like mmu_notifiers should become GPUVA or DRM helpers when
+the second driver supporting VM_BIND+userptr appears. Details to be defined when
+the time comes.
+
+Long running compute: minimal data structure/scaffolding
+--------------------------------------------------------
+The generic scheduler code needs to include the handling of endless compute
+contexts, with the minimal scaffolding for preempt-ctx fences (probably on the
+drm_sched_entity) and making sure drm_scheduler can cope with the lack of job
+completion fence.
+
+The goal is to achieve a consensus ahead of Xe initial pull-request, ideally with
+this minimal drm/scheduler work, if needed, merged to drm-misc in a way that any
+drm driver, including Xe, could re-use and add their own individual needs on top
+in a next stage. However, this should not block the initial merge.
+
+As a key measurable result, the handling of the long running jobs and the minimal
+scaffolding should be documented here or in a separate document pointed to by
+this document.
+
+Display integration with i915
+-----------------------------
+In order to share the display code with the i915 driver so that there is maximum
+reuse, the i915/display/ code is built twice, once for i915.ko and then for
+xe.ko. Currently, the i915/display code in Xe tree is polluted with many 'ifdefs'
+depending on the build target. The goal is to refactor both Xe and i915/display
+code simultaneously in order to get a clean result before they land upstream, so
+that display can already be part of the initial pull request towards drm-next.
+
+However, display code should not gate the acceptance of Xe in upstream. Xe
+patches will be refactored in a way that display code can be removed, if needed,
+from the first pull request of Xe towards drm-next. The expectation is that when
+both drivers are part of the drm-tip, the introduction of cleaner patches will be
+easier and speed up.
+
+Drm_exec
+--------
+Helper to make dma_resv locking for a big number of buffers is getting removed in
+the drm_exec series proposed in https://patchwork.freedesktop.org/patch/524376/
+If that happens, Xe needs to change and incorporate the changes in the driver.
+The goal is to engage with the Community to understand if the best approach is to
+move that to the drivers that are using it or if we should keep the helpers in
+place waiting for Xe to get merged.
+
+As a key measurable result, we need to have a community consensus documented in
+this document and the Xe driver prepared for the changes, if necessary.
+
+Xe – uAPI high level overview
+=============================
+
+...Warning: To be done in follow up patches after/when/where the main consensus in various items are individually reached.
-- 
2.39.2

