Re: [PATCH v4 1/5] i915.rst: Narration overview on GEM + minor reorder to improve narration

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

kevin.rogovin@intel.com writes:

> From: Kevin Rogovin <kevin.rogovin@intel.com>
>
> Add a narration to i915.rst about Intel GEN GPU's: engines,
> driver context and relocation.
>
> Signed-off-by: Kevin Rogovin <kevin.rogovin@intel.com>

Few typos pointed on a previous mail with those fixed,
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

> ---
>  Documentation/gpu/i915.rst      | 116 ++++++++++++++++++++++++++++++++--------
>  drivers/gpu/drm/i915/i915_vma.h |  10 ++--
>  2 files changed, 100 insertions(+), 26 deletions(-)
>
> diff --git a/Documentation/gpu/i915.rst b/Documentation/gpu/i915.rst
> index 41dc881b00dc..00f897f67f85 100644
> --- a/Documentation/gpu/i915.rst
> +++ b/Documentation/gpu/i915.rst
> @@ -249,6 +249,99 @@ Memory Management and Command Submission
>  This sections covers all things related to the GEM implementation in the
>  i915 driver.
>  
> +Intel GPU Basics
> +----------------
> +
> +An Intel GPU has multiple engines. There are several engine types.
> +
> +- RCS engine is for rendering 3D and performing compute, this is named `I915_EXEC_RENDER` in user space.
> +- BCS is a blitting (copy) engine, this is named `I915_EXEC_BLT` in user space.
> +- VCS is a video encode and decode engine, this is named `I915_EXEC_BSD` in user space
> +- VECS is video enhancement engine, this is named `I915_EXEC_VEBOX` in user space.
> +- The enumeration `I915_EXEC_DEFAULT` does not refer to specific engine; instead it is to be used by user space to specify a default rendering engine (for 3D) that may or may not be the same as RCS.
> +
> +The Intel GPU family is a family of integrated GPU's using Unified
> +Memory Access. For having the GPU "do work", user space will feed the
> +GPU batch buffers via one of the ioctls `DRM_IOCTL_I915_GEM_EXECBUFFER2`
> +or `DRM_IOCTL_I915_GEM_EXECBUFFER2_WR`. Most such batchbuffers will
> +instruct the GPU to perform work (for example rendering) and that work
> +needs memory from which to read and memory to which to write. All memory
> +is encapsulated within GEM buffer objects (usually created with the ioctl
> +`DRM_IOCTL_I915_GEM_CREATE`). An ioctl providing a batchbuffer for the GPU
> +to create will also list all GEM buffer objects that the batchbuffer reads
> +and/or writes. For implementation details of memory management see
> +`GEM BO Management Implementation Details`_.
> +
> +The i915 driver allows user space to create a context via the ioctl
> +`DRM_IOCTL_I915_GEM_CONTEXT_CREATE` which is identified by a 32-bit
> +integer. Such a context should be veiwed by user-space as -loosely-
> +analogous to the idea of a CPU process of an operating system. The i915
> +driver guarantees that commands issued to a fixed context are to be
> +executed so that writes of a previously issued command are seen by
> +reads of following commands. Actions issued between different contexts
> +(even if from the same file descriptor) are NOT given that guarantee
> +and the only way to synchornize across contexts (even from the same
> +file descriptor) is through the use of fences. At least as far back as
> +Gen4, also have that a context carries with it a GPU HW context;
> +the HW context is essentially (most of atleast) the state of a GPU.
> +In addition to the ordering gaurantees, the kernel will restore GPU
> +state via HW context when commands are issued to a context, this saves
> +user space the need to restore (most of atleast) the GPU state at the
> +start of each batchbuffer. The ioctl `DRM_IOCTL_I915_GEM_CONTEXT_CREATE`
> +is used by user space to create a hardware context which is identified
> +by a 32-bit integer. The non-deprecated ioctls to submit batchbuffer
> +work can pass that ID (in the lower bits of drm_i915_gem_execbuffer2::rsvd1)
> +to identify what context to use with the command.
> +
> +The GPU has its own memory management and address space. The kernel
> +driver maintains the memory translation table for the GPU. For older
> +GPUs (i.e. those before Gen8), there is a single global such translation
> +table, a global Graphics Translation Table (GTT). For newer generation
> +GPUs each context has its own translation table, called Per-Process
> +Graphics Translation Table (PPGTT). Of important note, is that although
> +PPGTT is named per-process it is actually per context. When user space
> +submits a batchbuffer, the kernel walks the list of GEM buffer objects
> +used by the batchbuffer and guarantees that not only is the memory of
> +each such GEM buffer object resident but it is also present in the
> +(PP)GTT. If the GEM buffer object is not yet placed in the (PP)GTT,
> +then it is given an address. Two consequences of this are: the kernel
> +needs to edit the batchbuffer submitted to write the correct value of
> +the GPU address when a GEM BO is assigned a GPU address and the kernel
> +might evict a different GEM BO from the (PP)GTT to make address room
> +for another GEM BO. Consequently, the ioctls submitting a batchbuffer
> +for execution also include a list of all locations within buffers that
> +refer to GPU-addresses so that the kernel can edit the buffer correctly.
> +This process is dubbed relocation.
> +
> +GEM BO Management Implementation Details
> +----------------------------------------
> +
> +.. kernel-doc:: drivers/gpu/drm/i915/i915_vma.h
> +   :doc: Virtual Memory Address
> +
> +Buffer Object Eviction
> +----------------------
> +
> +This section documents the interface functions for evicting buffer
> +objects to make space available in the virtual gpu address spaces. Note
> +that this is mostly orthogonal to shrinking buffer objects caches, which
> +has the goal to make main memory (shared with the gpu through the
> +unified memory architecture) available.
> +
> +.. kernel-doc:: drivers/gpu/drm/i915/i915_gem_evict.c
> +   :internal:
> +
> +Buffer Object Memory Shrinking
> +------------------------------
> +
> +This section documents the interface function for shrinking memory usage
> +of buffer object caches. Shrinking is used to make main memory
> +available. Note that this is mostly orthogonal to evicting buffer
> +objects, which has the goal to make space in gpu virtual address spaces.
> +
> +.. kernel-doc:: drivers/gpu/drm/i915/i915_gem_shrinker.c
> +   :internal:
> +
>  Batchbuffer Parsing
>  -------------------
>  
> @@ -312,29 +405,6 @@ Object Tiling IOCTLs
>  .. kernel-doc:: drivers/gpu/drm/i915/i915_gem_tiling.c
>     :doc: buffer object tiling
>  
> -Buffer Object Eviction
> -----------------------
> -
> -This section documents the interface functions for evicting buffer
> -objects to make space available in the virtual gpu address spaces. Note
> -that this is mostly orthogonal to shrinking buffer objects caches, which
> -has the goal to make main memory (shared with the gpu through the
> -unified memory architecture) available.
> -
> -.. kernel-doc:: drivers/gpu/drm/i915/i915_gem_evict.c
> -   :internal:
> -
> -Buffer Object Memory Shrinking
> -------------------------------
> -
> -This section documents the interface function for shrinking memory usage
> -of buffer object caches. Shrinking is used to make main memory
> -available. Note that this is mostly orthogonal to evicting buffer
> -objects, which has the goal to make space in gpu virtual address spaces.
> -
> -.. kernel-doc:: drivers/gpu/drm/i915/i915_gem_shrinker.c
> -   :internal:
> -
>  GuC
>  ===
>  
> diff --git a/drivers/gpu/drm/i915/i915_vma.h b/drivers/gpu/drm/i915/i915_vma.h
> index 8c5022095418..0000f23a7266 100644
> --- a/drivers/gpu/drm/i915/i915_vma.h
> +++ b/drivers/gpu/drm/i915/i915_vma.h
> @@ -38,9 +38,13 @@
>  enum i915_cache_level;
>  
>  /**
> - * A VMA represents a GEM BO that is bound into an address space. Therefore, a
> - * VMA's presence cannot be guaranteed before binding, or after unbinding the
> - * object into/from the address space.
> + * DOC: Virtual Memory Address
> + *
> + * An `i915_vma` struct represents a GEM BO that is bound into an address
> + * space. Therefore, a VMA's presence cannot be guaranteed before binding, or
> + * after unbinding the object into/from the address space. The struct includes
> + * the bookkepping details needed for tracking it in all the lists with which
> + * it interacts.
>   *
>   * To make things as simple as possible (ie. no refcounting), a VMA's lifetime
>   * will always be <= an objects lifetime. So object refcounting should cover us.
> -- 
> 2.16.2
>
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