[PATCH] Documentation: add initial documenation for user queues

[Alex Deucher]

Add an initial documentation page for user mode queues.

Signed-off-by: Alex Deucher <alexander.deucher at amd.com>
---
 Documentation/gpu/amdgpu/index.rst |   1 +
 Documentation/gpu/amdgpu/userq.rst | 196 +++++++++++++++++++++++++++++
 2 files changed, 197 insertions(+)
 create mode 100644 Documentation/gpu/amdgpu/userq.rst

diff --git a/Documentation/gpu/amdgpu/index.rst b/Documentation/gpu/amdgpu/index.rst
index bb2894b5edaf2..45523e9860fc5 100644
--- a/Documentation/gpu/amdgpu/index.rst
+++ b/Documentation/gpu/amdgpu/index.rst
@@ -12,6 +12,7 @@ Next (GCN), Radeon DNA (RDNA), and Compute DNA (CDNA) architectures.
    module-parameters
    gc/index
    display/index
+   userq
    flashing
    xgmi
    ras
diff --git a/Documentation/gpu/amdgpu/userq.rst b/Documentation/gpu/amdgpu/userq.rst
new file mode 100644
index 0000000000000..53e6b053f652f
--- /dev/null
+++ b/Documentation/gpu/amdgpu/userq.rst
@@ -0,0 +1,196 @@
+==================
+ User Mode Queues
+==================
+
+Introduction
+============
+
+Similar to the KFD, GPU engine queues move into userspace.  The idea is to let
+user processes manage their submissions to the GPU engines directly, bypassing
+IOCTL calls to the driver to submit work.  This reduces overhead and also allows
+the GPU to submit work to itself.  Applications can set up work graphs of jobs
+across multiple GPU engines without needing trips through the CPU.
+
+UMDs directly interface with firmware via per application shared memory areas.
+The main vehicle for this is queue.  A queue is a ring buffer with a read
+pointer (rptr) and a write pointer (wptr).  The UMD writes IP specific packets
+into the queue and the firmware processes those packets, kicking off work on the
+GPU engines.  The CPU in the application (or another queue or device) updates
+the wptr to tell the firmware how far into the ring buffer to process packets
+and the rtpr provides feedback to the UMD on how far the firmware has progressed
+in executing those packets.  When the wptr and the rptr are equal, the queue is
+idle.
+
+Theory of Operation
+===================
+
+The various engines on modern AMD GPUs support multiple queues per engine with a
+scheduling firmware which handles dynamically scheduling user queues on the
+available hardware queue slots.  When the number of user queues outnumbers the
+available hardware queue slots, the scheduling firmware dynamically maps and
+unmaps queues based on priority and time quanta.  The state of each user queue
+is managed in the kernel driver in an MQD (Memory Queue Descriptor).  This is a
+buffer in GPU accessible memory that stores the state of a user queue.  The
+scheduling firmware uses the MQD to load the queue state into an HQD (Hardware
+Queue Descriptor) when a user queue is mapped.  Each user queue requires a
+number of additional buffers which represent the ring buffer and any metadata
+needed by the engine for runtime operation.  On most engines this consists of
+the ring buffer itself, a rptr buffer (where the firmware will shadow the rptr
+to userspace), a wrptr buffer (where the application will write the wptr for the
+firmware to fetch it), and a doorbell.  A doorbell is a piece of the device's
+MMIO BAR which can be mapped to specific user queues.  Writing to the doorbell
+wakes the firmware and causes it to fetch the wptr and start processing the
+packets in the queue. Each 4K page of the doorbell BAR supports specific offset
+ranges for specific engines.  The doorbell of a queue most be mapped into the
+aperture aligned to the IP used by the queue (e.g., GFX, VCN, SDMA, etc.).
+These doorbell apertures are set up via NBIO registers.  Doorbells are 32 bit or
+64 bit (depending on the engine) chunks of the doorbell BAR.  A 4K doorbell page
+provides 512 64-bit doorbells for up to 512 user queues.  A subset of each page
+is reserved for each IP type supported on the device.  The user can query the
+doorbell ranges for each IP via the INFO IOCTL.
+
+When an application wants to create a user queue, it allocates the the necessary
+buffers for the queue (ring buffer, wptr and rptr, context save areas, etc.).
+These can be separate buffers or all part of one larger buffer.  The application
+would map the buffer(s) into its GPUVM and use the GPU virtual addresses of for
+the areas of memory they want t use for the user queue.  They would also
+allocate a doorbell page for the doorbells used by the user queues.  The
+application would then populate the MQD in the USERQ IOCTL structure with the
+GPU virtual addresses and doorbell index they want to use.  The user can also
+specify the attributes for the user queue (priority, whether the queue is secure
+for protected content, etc.).  The application would then call the USERQ
+create IOCTL to create the queue from using the specified MQD.  The
+kernel driver then validates the MQD provided by the application and translates
+the MQD into the engine specific MQD format for the IP.  The IP specific MQD
+would be allocated and the queue would be added to the run list maintained by
+the scheduling firmware.  Once the queue has been created, the application can
+write packets directly into the queue, update the wptr, and write to the
+doorbell offset to kick off work in the user queue.
+
+When the application is done with the user queue, it would call the USERQ
+FREE IOCTL to destroy it.  The kernel driver would preempt the queue and
+remove it from the scheduling firmware's run list.  Then the IP specific MQD
+would be freed and the user queue state would be cleaned up.
+
+Some engines may require the aggregated doorbell to if the engine does not
+support doorbells from unmapped queues.  The aggregated doorbell is a special
+page of doorbell space which wakes the scheduler.  In cases where the engine may
+be oversubscribed, some queues may not be mapped.  If the doorbell is rung when
+the queue is not mapped, the engine firmware may miss the request.  Some
+scheduling firmware may work around this my polling wptr shadows when the
+hardware is oversubscribed, other engines may support doorbell updates from
+unmapped queues.  In the event that one of these options is not available, the
+kernel driver will map a page of aggregated doorbell space into each GPUVM
+space.  The UMD will then update the doorbell and wptr as normal and then write
+to the aggregated doorbell as well.
+
+Special Packets
+---------------
+
+In order to support legacy implicit synchronization, as well as mixed user and
+kernel queues, we need a synchronization mechanism that is secure.  Because
+kernel queues or memory management tasks depend on kernel fences, we need a way
+for user queues to update memory that the kernel can use for a fence, that can't
+be messed with by a bad actor.  To support this, we've added protected fence
+packet.  This packet works by writing the a monotonically increasing value to
+a memory location that is only the privileged clients have write access to.
+User queues only have read access.  When this packet is executed, the memory
+location is updated and other queues (kernel or user) can see the results.
+
+Memory Management
+=================
+
+It is assumed that all buffers mapped into the GPUVM space for the process are
+valid when engines on the GPU are running.  The kernel driver will only allow
+user queues to run when all buffers are mapped.  If there is a memory event that
+requires buffer migration, the kernel driver will preempt the user queues,
+migrate buffers to where they need to be, update the GPUVM page tables and
+invaldidate the TLB, and then resume the user queues.
+
+Interaction with Kernel Queues
+==============================
+
+Depending on the IP and the scheduling firmware, you can enable kernel queues
+and user queues at the same time,  However, you are limited by the HQD slots.
+Kernel queues are always mapped so any work the goes into kernel queues will
+take priority.  This limits the available HQD slots for user queues.
+
+Not all IPs will support user queues on all GPUs.  As such, UMDs will need to
+support both user queues and kernel queues depending on the IP.  For example, a
+GPU may support user queues for GFX, compute, and SDMA, but not for VCN, JPEG,
+and VPE.  UMDs need to support both.  The kernel driver provides a way to
+determine if user queues and kernel queues are supported on a per IP basis.
+UMDs can query this information via the INFO IOCTL and determine whether to use
+kernel queues or user queues for each IP.
+
+Queue Resets
+============
+
+For most engines, queues can be reset individually.  GFX, compute, and SDMA
+queues can be reset individually.  When a hung queue is detected, it can be
+reset either via the scheduling firmware or MMIO.  Since there are no kernel
+fences for most user queues, they will usually only be detected when some other
+event happens; e.g., a memory event which requires migration of buffers.  When
+the queues are preempted, if the queue is hung, the preemption will fail.
+Driver will them look up the queues that failed to preempt and reset them and
+record which queues are hung.
+
+
+On the UMD side, we will add an USERQ QUERY_STATUS IOCTL to query the queue
+status.  UMD will provide the queue id in the IOCTL and the kernel driver
+will check if it has already recorded the queue as hung (e.g., due to failed
+peemption) and report back the status.
+
+IOCTL Interfaces
+================
+
+GPU virtual addresses used for queues and related data (rptrs, wptrs, context
+save areas, etc.) should be validated by the kernel mode driver to prevent the
+user from specifying invalid GPU virtual addresses.  If the user provides
+invalid GPU virtual addresses or doorbell indicies, the IOCTL should return an
+error message.  These buffers should also be tracked in the kernel driver so
+that if the user attempts to unmap the buffer(s) from the GPUVM, the umap call
+would return an error.
+
+INFO
+----
+There are several new INFO queries related to user queues in order to query the
+size of user queue meta data needed for a user queue (e.g., context save areas
+or shadow buffers), and whether kernel or user queues or both are supported
+for each IP type.
+
+USERQ
+-----
+The USERQ IOCTL is used for creating, freeing, and querying the status of user
+queues.  It supports 3 opcodes:
+
+1. CREATE - Create a user queue.  The application provides a MQD-like structure
+   that devices the type of queue and associated metadata and flags for that
+   queue type.  Returns the queue id.
+2. FREE - Free a user queue.
+3. QUERY_STATRUS - Query that status of a queue.  Used to check if the queue is
+   healthy or not.  E.g., if the queue has been reset. (WIP)
+
+USERQ_SIGNAL
+------------
+The USERQ_SIGNAL IOCTL is used to provide a list of sync objects to be signaled.
+
+USERQ_WAIT
+----------
+The USERQ_WAIT IOCTL is used to provide a list of sync object to be waited on.
+
+Kernel and User Queues
+======================
+
+In order to properly validate and test performance, we have a driver option to
+select what type of queues are enabled (kernel queues, user queues or both).
+The user_queue driver parameter allows you to enable kernel queues only (0),
+user queues and kernel queues (1), and user queues only (2).  Enabling user
+queues only will free up static queue assignments that would otherwise be used
+by kernel queues for use by the scheduling firmware.  Some kernel queues are
+required for kernel driver operation and they will always be created.  When the
+kernel queues are not enabled, they are not registered with the drm scheduler
+and the CS IOCTL will reject any incoming command submissions which target those
+queue types.  Kernel queues only mirrors the behavior on all existing GPUs.
+Enabling both queues allows for backwards compatibility with old userspace while
+still supporting user queues.
-- 
2.49.0