Re: [PATCH RFC 01/18] accel/qda: Add Qualcomm QDA DSP accelerator driver docs

[Dmitry Baryshkov]

On Tue, Feb 24, 2026 at 12:38:55AM +0530, Ekansh Gupta wrote:
> Add initial documentation for the Qualcomm DSP Accelerator (QDA) driver
> integrated in the DRM accel subsystem.
> 
> The new docs introduce QDA as a DRM/accel-based implementation of
> Hexagon DSP offload that is intended as a modern alternative to the
> legacy FastRPC driver in drivers/misc. The text describes the driver
> motivation, high-level architecture and interaction with IOMMU context
> banks, GEM-based buffer management and the RPMsg transport.
> 
> The user-space facing section documents the main QDA IOCTLs used to
> establish DSP sessions, manage GEM buffer objects and invoke remote
> procedures using the FastRPC protocol, along with a typical lifecycle
> example for applications.
> 
> Finally, the driver is wired into the Compute Accelerators
> documentation index under Documentation/accel, and a brief debugging
> section shows how to enable dynamic debug for the QDA implementation.
> 
> Signed-off-by: Ekansh Gupta <ekansh.gupta@xxxxxxxxxxxxxxxx>
> ---
>  Documentation/accel/index.rst     |   1 +
>  Documentation/accel/qda/index.rst |  14 +++++
>  Documentation/accel/qda/qda.rst   | 129 ++++++++++++++++++++++++++++++++++++++
>  3 files changed, 144 insertions(+)
> 
> diff --git a/Documentation/accel/index.rst b/Documentation/accel/index.rst
> index cbc7d4c3876a..5901ea7f784c 100644
> --- a/Documentation/accel/index.rst
> +++ b/Documentation/accel/index.rst
> @@ -10,4 +10,5 @@ Compute Accelerators
>     introduction
>     amdxdna/index
>     qaic/index
> +   qda/index
>     rocket/index
> diff --git a/Documentation/accel/qda/index.rst b/Documentation/accel/qda/index.rst
> new file mode 100644
> index 000000000000..bce188f21117
> --- /dev/null
> +++ b/Documentation/accel/qda/index.rst
> @@ -0,0 +1,14 @@
> +.. SPDX-License-Identifier: GPL-2.0-only
> +
> +==============================
> + accel/qda Qualcomm DSP Driver
> +==============================
> +
> +The **accel/qda** driver provides support for Qualcomm Hexagon DSPs (Digital
> +Signal Processors) within the DRM accelerator framework. It serves as a modern
> +replacement for the legacy FastRPC driver, offering improved resource management
> +and standard subsystem integration.
> +
> +.. toctree::
> +
> +   qda
> diff --git a/Documentation/accel/qda/qda.rst b/Documentation/accel/qda/qda.rst
> new file mode 100644
> index 000000000000..742159841b95
> --- /dev/null
> +++ b/Documentation/accel/qda/qda.rst
> @@ -0,0 +1,129 @@
> +.. SPDX-License-Identifier: GPL-2.0-only
> +
> +==================================
> +Qualcomm Hexagon DSP (QDA) Driver
> +==================================
> +
> +Introduction
> +============
> +
> +The **QDA** (Qualcomm DSP Accelerator) driver is a new DRM-based
> +accelerator driver for Qualcomm's Hexagon DSPs. It provides a standardized
> +interface for user-space applications to offload computational tasks ranging
> +from audio processing and sensor offload to computer vision and AI
> +inference to the Hexagon DSPs found on Qualcomm SoCs.
> +
> +This driver is designed to align with the Linux kernel's modern **Compute
> +Accelerators** subsystem (`drivers/accel/`), providing a robust and modular
> +alternative to the legacy FastRPC driver in `drivers/misc/`, offering
> +improved resource management and better integration with standard kernel
> +subsystems.
> +
> +Motivation
> +==========
> +
> +The existing FastRPC implementation in the kernel utilizes a custom character
> +device and lacks integration with modern kernel memory management frameworks.
> +The QDA driver addresses these limitations by:
> +
> +1.  **Adopting the DRM accel Framework**: Leveraging standard uAPIs for device
> +    management, job submission, and synchronization.
> +2.  **Utilizing GEM for Memory**: Providing proper buffer object management,
> +    including DMA-BUF import/export capabilities.
> +3.  **Improving Isolation**: Using IOMMU context banks to enforce memory
> +    isolation between different DSP user sessions.
> +
> +Key Features
> +============
> +
> +*   **Standard Accelerator Interface**: Exposes a standard character device
> +    node (e.g., `/dev/accel/accel0`) via the DRM subsystem.
> +*   **Unified Offload Support**: Supports all DSP domains (ADSP, CDSP, SDSP,
> +    GDSP) via a single driver architecture.
> +*   **FastRPC Protocol**: Implements the reliable Remote Procedure Call
> +    (FastRPC) protocol for communication between the application processor
> +    and DSP.
> +*   **DMA-BUF Interop**: Seamless sharing of memory buffers between the DSP
> +    and other multimedia subsystems (GPU, Camera, Video) via standard DMA-BUFs.
> +*   **Modular Design**: Clean separation between the core DRM logic, the memory
> +    manager, and the RPMsg-based transport layer.
> +
> +Architecture
> +============
> +
> +The QDA driver is composed of several modular components:
> +
> +1.  **Core Driver (`qda_drv`)**: Manages device registration, file operations,
> +    and bridges the driver with the DRM accelerator subsystem.
> +2.  **Memory Manager (`qda_memory_manager`)**: A flexible memory management
> +    layer that handles IOMMU context banks. It supports pluggable backends
> +    (such as DMA-coherent) to adapt to different SoC memory architectures.
> +3.  **GEM Subsystem**: Implements the DRM GEM interface for buffer management:
> +
> +    * **`qda_gem`**: Core GEM object management, including allocation, mmap
> +      operations, and buffer lifecycle management.
> +    * **`qda_prime`**: PRIME import functionality for DMA-BUF interoperability,
> +      enabling seamless buffer sharing with other kernel subsystems.
> +
> +4.  **Transport Layer (`qda_rpmsg`)**: Abstraction over the RPMsg framework
> +    to handle low-level message passing with the DSP firmware.
> +5.  **Compute Bus (`qda_compute_bus`)**: A custom virtual bus used to
> +    enumerate and manage the specific compute context banks defined in the
> +    device tree.

I'm really not sure if it's a bonus or not. I'm waiting for iommu-map
improvements to land to send patches reworking FastRPC CB from using
probe into being created by the main driver: it would remove some of the
possible race conditions between main driver finishing probe and the CB
devices probing in the background.

What's the actual benefit of the CB bus?

> +6.  **FastRPC Core (`qda_fastrpc`)**: Implements the protocol logic for
> +    marshalling arguments and handling remote invocations.
> +
> +User-Space API
> +==============
> +
> +The driver exposes a set of DRM-compliant IOCTLs. Note that these are designed
> +to be familiar to existing FastRPC users while adhering to DRM standards.
> +
> +*   `DRM_IOCTL_QDA_QUERY`: Query DSP type (e.g., "cdsp", "adsp")
> +    and capabilities.
> +*   `DRM_IOCTL_QDA_INIT_ATTACH`: Attach a user session to the DSP's protection
> +    domain.
> +*   `DRM_IOCTL_QDA_INIT_CREATE`: Initialize a new process context on the DSP.

You need to explain the difference between these two.

> +*   `DRM_IOCTL_QDA_INVOKE`: Submit a remote method invocation (the primary
> +    execution unit).
> +*   `DRM_IOCTL_QDA_GEM_CREATE`: Allocate a GEM buffer object for DSP usage.
> +*   `DRM_IOCTL_QDA_GEM_MMAP_OFFSET`: Retrieve mmap offsets for memory mapping.
> +*   `DRM_IOCTL_QDA_MAP` / `DRM_IOCTL_QDA_MUNMAP`: Map or unmap buffers into the
> +    DSP's virtual address space.

Do we need to make this separate? Can we map/unmap buffers on their
usage? Or when they are created? I'm thinking about that the
virtualization. An alternative approach would be to merge
GET_MMAP_OFFSET with _MAP: once you map it to the DSP memory, you will
get the offset. 

> +
> +Usage Example
> +=============
> +
> +A typical lifecycle for a user-space application:
> +
> +1.  **Discovery**: Open `/dev/accel/accel*` and check
> +    `DRM_IOCTL_QDA_QUERY` to find the desired DSP (e.g., CDSP for
> +    compute workloads).
> +2.  **Initialization**: Call `DRM_IOCTL_QDA_INIT_ATTACH` and
> +    `DRM_IOCTL_QDA_INIT_CREATE` to establish a session.
> +3.  **Memory**: Allocate buffers via `DRM_IOCTL_QDA_GEM_CREATE` or import
> +    DMA-BUFs (PRIME fd) from other drivers using `DRM_IOCTL_PRIME_FD_TO_HANDLE`.
> +4.  **Execution**: Use `DRM_IOCTL_QDA_INVOKE` to pass arguments and execute
> +    functions on the DSP.
> +5.  **Cleanup**: Close file descriptors to automatically release resources and
> +    detach the session.
> +
> +Internal Implementation
> +=======================
> +
> +Memory Management
> +-----------------
> +The driver's memory manager creates virtual "IOMMU devices" that map to
> +hardware context banks. This allows the driver to manage multiple isolated
> +address spaces. The implementation currently uses a **DMA-coherent backend**
> +to ensure data consistency between the CPU and DSP without manual cache
> +maintenance in most cases.
> +
> +Debugging
> +=========
> +The driver includes extensive dynamic debug support. Enable it via the
> +kernel's dynamic debug control:
> +
> +.. code-block:: bash
> +
> +    echo "file drivers/accel/qda/* +p" > /sys/kernel/debug/dynamic_debug/control

Please add documentation on how to build the test apps and how to load
them to the DSP.

> 
> -- 
> 2.34.1
> 

-- 
With best wishes
Dmitry