On Tue, May 19, 2026 at 11:45:52AM +0530, Ekansh Gupta via B4 Relay wrote: > From: Ekansh Gupta <[email protected]> > > Add documentation for the Qualcomm DSP Accelerator (QDA) driver under > Documentation/accel/qda/. The documentation covers the driver > architecture, GEM-based buffer management, IOMMU context bank > isolation, and the RPMsg transport layer. > > The user-space API section describes the DRM IOCTLs for session > management, GEM buffer allocation, and remote procedure invocation via > the FastRPC protocol, along with a typical application lifecycle > example. Sections for dynamic debug and basic testing are also > included. > > Wire the new documentation into the Compute Accelerators index at > Documentation/accel/index.rst. > > Assisted-by: Claude:claude-4-6-sonnet > Signed-off-by: Ekansh Gupta <[email protected]> > --- > Documentation/accel/index.rst | 1 + > Documentation/accel/qda/index.rst | 13 ++++ > Documentation/accel/qda/qda.rst | 146 > ++++++++++++++++++++++++++++++++++++++ > 3 files changed, 160 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..013400cf9c25 > --- /dev/null > +++ b/Documentation/accel/qda/index.rst > @@ -0,0 +1,13 @@ > +.. SPDX-License-Identifier: GPL-2.0-only > + > +================================== > +accel/qda Qualcomm DSP Accelerator > +================================== > + > +The QDA driver provides a DRM accel based interface for Qualcomm DSP offload. > +It uses the FastRPC protocol and integrates with DRM and GEM infrastructure > +for device and buffer management. > + > +.. toctree:: > + > + qda > diff --git a/Documentation/accel/qda/qda.rst b/Documentation/accel/qda/qda.rst > new file mode 100644 > index 000000000000..9f49af6e6acc > --- /dev/null > +++ b/Documentation/accel/qda/qda.rst > @@ -0,0 +1,146 @@ > +.. SPDX-License-Identifier: GPL-2.0-only > + > +===================================== > +Qualcomm DSP Accelerator (QDA) Driver > +===================================== > + > +Introduction > +============ > + > +The QDA driver is a DRM accel driver for Qualcomm's DSPs. It provides a > +DRM accel based interface for Qualcomm DSP offload, supporting workloads > +such as AI inference, computer vision, audio processing, and sensor offload > +on Qualcomm SoCs. It uses the FastRPC protocol and integrates with DRM and > +GEM infrastructure for device and buffer management. > + > +Key Features > +============ > + > +* **DRM accel Interface**: Exposes a standard character device node > + (e.g., ``/dev/accel/accel0``) via the DRM accel subsystem. > +* **FastRPC Protocol**: Implements the FastRPC protocol for communication > + between the application processor and the DSP. > +* **GEM Buffer Management**: Uses the DRM GEM interface for buffer > + allocation, lifecycle management, and DMA-BUF import/export. > +* **IOMMU Isolation**: Uses IOMMU context banks to enforce memory isolation > + between different DSP user sessions. > +* **Modular Design**: Clean separation between the core DRM logic, the > + memory manager, and the RPMsg-based transport layer. > + > +Architecture > +============ > + > +The QDA driver consists of several functional blocks: > + > +1. **Core Driver (``qda_drv``)**: Manages device registration, file > operations, > + and DRM accel integration. > +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 > + 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. The bus was introduced because IOMMU context banks (CBs) are > + synthetic constructs — not real platform devices — making a platform > driver > + an incorrect abstraction for them. The earlier platform-driver approach > also > + had a race condition: device nodes were created before the RPMsg channel > + resources were fully initialized, and because ``probe`` runs > asynchronously, > + applications could open a CB device and attempt to start a session before > + the underlying transport was ready. The compute bus makes CB lifetime > + explicitly subordinate to the parent QDA device, closing that window. > +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: > + > +* ``DRM_IOCTL_QDA_QUERY``: Query DSP type (e.g., "cdsp", "adsp") > + and capabilities. > +* ``DRM_IOCTL_QDA_REMOTE_SESSION_CREATE``: Initialize a new process context > + on the DSP. > +* ``DRM_IOCTL_QDA_REMOTE_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_REMOTE_MAP`` / ``DRM_IOCTL_QDA_REMOTE_MUNMAP``: Map or > unmap > + buffers into the DSP's virtual address space. Each accepts a ``request`` > + field selecting between a legacy operation (``QDA_MAP_REQUEST_LEGACY`` / > + ``QDA_MUNMAP_REQUEST_LEGACY``) and an attribute-based operation > + (``QDA_MAP_REQUEST_ATTR`` / ``QDA_MUNMAP_REQUEST_ATTR``).
Explain, what happens in the users don't map the buffers into the DSP space. Will DRM_IOCTL_QDA_REMOTE_INVOKE handle the mapping or not? What is the difference between those two modes? Would the driver benefit from using GPUVM? > + > +Usage Example > +============= > + > +A typical lifecycle for a user-space application: > + > +1. **Discovery**: Open ``/dev/accel/accel*`` and use > + ``DRM_IOCTL_QDA_QUERY`` to identify the DSP domain served by that > + device node. > +2. **Initialization**: Call ``DRM_IOCTL_QDA_REMOTE_SESSION_CREATE`` to > + establish a session and create a process context on the DSP. > +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_REMOTE_INVOKE`` to pass arguments and > + execute functions on the DSP. > +5. **Cleanup**: Close file descriptors to automatically release resources > and > + detach the session. I'd have expected the description of the actual example. I.e. clone the app from https://the.addr, prepare clang >= NN.MM, QAIC (https://foo), run make, run the app, check the results. I'd remind that DRM Accel has a very specific requirement of having the working toolhain in the open-source. > + > +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 uses a DMA-coherent backend to ensure > data consistency > +between the CPU and DSP without manual cache maintenance in most cases. GEM usage? > + > +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 > + > +Testing > +======= > +The QDA driver can be exercised using the ``fastrpc_test`` utility from the > +FastRPC userspace library. Run the test application: pointer > + > +.. code-block:: bash > + > + fastrpc_test -d 3 -U 1 -t linux -a v68 > + > +**Options** > + > +``-d domain`` > + Select the DSP domain to run on: > + > + * ``0`` — ADSP > + * ``1`` — MDSP > + * ``2`` — SDSP > + * ``3`` — CDSP *(default on targets with CDSP)* > + > +``-U unsigned_PD`` > + Select signed or unsigned protection domain: > + > + * ``0`` — signed PD > + * ``1`` — unsigned PD *(default)* > + > +``-t target`` > + Target platform: ``android`` or ``linux`` *(default: linux)* > + > +``-a arch_version`` > + DSP architecture version, e.g. ``v68``, ``v75`` *(default: v68)* > > -- > 2.34.1 > > -- With best wishes Dmitry
