Documentation: gpu: nova-core: Document devinit process

devinit is mentioned in the code. This patch explains it so it is clear
what it does. devinit is not only essential at boot-time, but also at
runtime due to suspend-resume and things like re-clocking.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250708-nova-docs-v4-6-9d188772c4c7@nvidia.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Documentation/gpu/nova/core/devinit.rst

@@ -0,0 +1,61 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================================
+Device Initialization (devinit)
+==================================
+The devinit process is complex and subject to change. This document provides a high-level
+overview using the Ampere GPU family as an example. The goal is to provide a conceptual
+overview of the process to aid in understanding the corresponding kernel code.
+
+Device initialization (devinit) is a crucial sequence of register read/write operations
+that occur after a GPU reset. The devinit sequence is essential for properly configuring
+the GPU hardware before it can be used.
+
+The devinit engine is an interpreter program that typically runs on the PMU (Power Management
+Unit) microcontroller of the GPU. This interpreter executes a "script" of initialization
+commands. The devinit engine itself is part of the VBIOS ROM in the same ROM image as the
+FWSEC (Firmware Security) image (see fwsec.rst and vbios.rst) and it runs before the
+nova-core driver is even loaded. On an Ampere GPU, the devinit ucode is separate from the
+FWSEC ucode. It is launched by FWSEC, which runs on the GSP in 'heavy-secure' mode, while
+devinit runs on the PMU in 'light-secure' mode.
+
+Key Functions of devinit
+------------------------
+devinit performs several critical tasks:
+
+1. Programming VRAM memory controller timings
+2. Power sequencing
+3. Clock and PLL (Phase-Locked Loop) configuration
+4. Thermal management
+
+Low-level Firmware Initialization Flow
+--------------------------------------
+Upon reset, several microcontrollers on the GPU (such as PMU, SEC2, GSP, etc.) run GPU
+firmware (gfw) code to set up the GPU and its core parameters. Most of the GPU is
+considered unusable until this initialization process completes.
+
+These low-level GPU firmware components are typically:
+
+1. Located in the VBIOS ROM in the same ROM partition (see vbios.rst and fwsec.rst).
+2. Executed in sequence on different microcontrollers:
+
+  - The devinit engine typically but not necessarily runs on the PMU.
+  - On an Ampere GPU, the FWSEC typically runs on the GSP (GPU System Processor) in
+    heavy-secure mode.
+
+Before the driver can proceed with further initialization, it must wait for a signal
+indicating that core initialization is complete (known as GFW_BOOT). This signal is
+asserted by the FWSEC running on the GSP in heavy-secure mode.
+
+Runtime Considerations
+----------------------
+It's important to note that the devinit sequence also needs to run during suspend/resume
+operations at runtime, not just during initial boot, as it is critical to power management.
+
+Security and Access Control
+---------------------------
+The initialization process involves careful privilege management. For example, before
+accessing certain completion status registers, the driver must check privilege level
+masks. Some registers are only accessible after secure firmware (FWSEC) lowers the
+privilege level to allow CPU (LS/low-secure) access. This is the case, for example,
+when receiving the GFW_BOOT signal.

Documentation/gpu/nova/index.rst

@@ -29,3 +29,4 @@ vGPU manager VFIO driver and the nova-drm driver.
    core/guidelines
    core/todo
    core/vbios
+   core/devinit