On Tue, Feb 24, 2026 at 12:38:54AM +0530, Ekansh Gupta wrote: > This patch series introduces the Qualcomm DSP Accelerator (QDA) driver, > a modern DRM-based accelerator implementation for Qualcomm Hexagon DSPs. > The driver provides a standardized interface for offloading computational > tasks to DSPs found on Qualcomm SoCs, supporting all DSP domains (ADSP, > CDSP, SDSP, GDSP). > > The QDA driver is designed as an alternative for the FastRPC driver > in drivers/misc/, offering improved resource management, better integration > with standard kernel subsystems, and alignment with the Linux kernel's > Compute Accelerators framework. >
If I understand correctly, this is just the same FastRPC protocol but in the accel framework, and hence with a new userspace ABI? I don't fancy the name "QDA" as an acronym for "FastRPC Accel". I would much prefer to see this living in drivers/accel/fastrpc and be named some variation of "fastrpc" (e.g. fastrpc_accel). (Driver name can be "fastrpc" as the other one apparently is named "qcom,fastrpc"). > User-space staging branch > ============ > https://github.com/qualcomm/fastrpc/tree/accel/staging > > Key Features > ============ > > * Standard DRM accelerator interface via /dev/accel/accelN > * GEM-based buffer management with DMA-BUF import/export support > * IOMMU-based memory isolation using per-process context banks > * FastRPC protocol implementation for DSP communication > * RPMsg transport layer for reliable message passing > * Support for all DSP domains (ADSP, CDSP, SDSP, GDSP) > * Comprehensive IOCTL interface for DSP operations > > High-Level Architecture Differences with Existing FastRPC Driver > ================================================================= > > The QDA driver represents a significant architectural departure from the > existing FastRPC driver (drivers/misc/fastrpc.c), addressing several key > limitations while maintaining protocol compatibility: > > 1. DRM Accelerator Framework Integration > - FastRPC: Custom character device (/dev/fastrpc-*) > - QDA: Standard DRM accel device (/dev/accel/accelN) > - Benefit: Leverages established DRM infrastructure for device > management. > > 2. Memory Management > - FastRPC: Custom memory allocator with ION/DMA-BUF integration > - QDA: Native GEM objects with full PRIME support > - Benefit: Seamless buffer sharing using standard DRM mechanisms > > 3. IOMMU Context Bank Management > - FastRPC: Direct IOMMU domain manipulation, limited isolation > - QDA: Custom compute bus (qda_cb_bus_type) with proper device model > - Benefit: Each CB device is a proper struct device with IOMMU group > support, enabling better isolation and resource tracking. > - > https://lore.kernel.org/all/[email protected]/ > > 4. Memory Manager Architecture > - FastRPC: Monolithic allocator > - QDA: Pluggable memory manager with backend abstraction > - Benefit: Currently uses DMA-coherent backend, easily extensible for > future memory types (e.g., carveout, CMA) > > 5. Transport Layer > - FastRPC: Direct RPMsg integration in core driver > - QDA: Abstracted transport layer (qda_rpmsg.c) > - Benefit: Clean separation of concerns, easier to add alternative > transports if needed > > 8. Code Organization > - FastRPC: ~3000 lines in single file > - QDA: Modular design across multiple files (~4600 lines total) "Now 50% more LOC and you need 6 tabs open in your IDE!" Might be better, but in itself it provides no immediate value. > * qda_drv.c: Core driver and DRM integration > * qda_gem.c: GEM object management > * qda_memory_manager.c: Memory and IOMMU management > * qda_fastrpc.c: FastRPC protocol implementation > * qda_rpmsg.c: Transport layer > * qda_cb.c: Context bank device management > - Benefit: Better maintainability, clearer separation of concerns > > 9. UAPI Design > - FastRPC: Custom IOCTL interface > - QDA: DRM-style IOCTLs with proper versioning support > - Benefit: Follows DRM conventions, easier userspace integration > > 10. Documentation > - FastRPC: Minimal in-tree documentation > - QDA: Comprehensive documentation in Documentation/accel/qda/ > - Benefit: Better developer experience, clearer API contracts > > 11. Buffer Reference Mechanism > - FastRPC: Uses buffer file descriptors (FDs) for all book-keeping > in both kernel and DSP > - QDA: Uses GEM handles for kernel-side management, providing better > integration with DRM subsystem > - Benefit: Leverages DRM GEM infrastructure for reference counting, > lifetime management, and integration with other DRM components > This is all good, but what is the plan regarding /dev/fastrpc-*? The idea here clearly is to provide an alternative implementation, and they seem to bind to the same toplevel compatible - so you can only compile one into your kernel at any point in time. So if I understand correctly, at some point in time we need to say CONFIG_DRM_ACCEL_QDA=m and CONFIG_QCOM_FASTRPC=n, which will break all existing user space applications? That's not acceptable. Would it be possible to have a final driver that is implemented as a accel, but provides wrappers for the legacy misc and ioctl interface to the applications? Regards, Bjorn > Key Technical Improvements > =========================== > > * Proper device model: CB devices are real struct device instances on a > custom bus, enabling proper IOMMU group management and power management > integration > > * Reference-counted IOMMU devices: Multiple file descriptors from the same > process share a single IOMMU device, reducing overhead > > * GEM-based buffer lifecycle: Automatic cleanup via DRM GEM reference > counting, eliminating many resource leak scenarios > > * Modular memory backends: The memory manager supports pluggable backends, > currently implementing DMA-coherent allocations with SID-prefixed > addresses for DSP firmware > > * Context-based invocation tracking: XArray-based context management with > proper synchronization and cleanup > > Patch Series Organization > ========================== > > Patches 1-2: Driver skeleton and documentation > Patches 3-6: RPMsg transport and IOMMU/CB infrastructure > Patches 7-9: DRM device registration and basic IOCTL > Patches 10-12: GEM buffer management and PRIME support > Patches 13-17: FastRPC protocol implementation (attach, invoke, create, > map/unmap) > Patch 18: MAINTAINERS entry > > Open Items > =========== > > The following items are identified as open items: > > 1. Privilege Level Management > - Currently, daemon processes and user processes have the same access > level as both use the same accel device node. This needs to be > addressed as daemons attach to privileged DSP PDs and require > higher privilege levels for system-level operations > - Seeking guidance on the best approach: separate device nodes, > capability-based checks, or DRM master/authentication mechanisms > > 2. UAPI Compatibility Layer > - Add UAPI compat layer to facilitate migration of client applications > from existing FastRPC UAPI to the new QDA accel driver UAPI, > ensuring smooth transition for existing userspace code > - Seeking guidance on implementation approach: in-kernel translation > layer, userspace wrapper library, or hybrid solution > > 3. Documentation Improvements > - Add detailed IOCTL usage examples > - Document DSP firmware interface requirements > - Create migration guide from existing FastRPC > > 4. Per-Domain Memory Allocation > - Develop new userspace API to support memory allocation on a per > domain basis, enabling domain-specific memory management and > optimization > > 5. Audio and Sensors PD Support > - The current patch series does not handle Audio PD and Sensors PD > functionalities. These specialized protection domains require > additional support for real-time constraints and power management > > Interface Compatibility > ======================== > > The QDA driver maintains compatibility with existing FastRPC infrastructure: > > * Device Tree Bindings: The driver uses the same device tree bindings as > the existing FastRPC driver, ensuring no changes are required to device > tree sources. The "qcom,fastrpc" compatible string and child node > structure remain unchanged. > > * Userspace Interface: While the driver provides a new DRM-based UAPI, > the underlying FastRPC protocol and DSP firmware interface remain > compatible. This ensures that DSP firmware and libraries continue to > work without modification. > > * Migration Path: The modular design allows for gradual migration, where > both drivers can coexist during the transition period. Applications can > be migrated incrementally to the new UAPI with the help of the planned > compatibility layer. > > References > ========== > > Previous discussions on this migration: > - https://lkml.org/lkml/2024/6/24/479 > - https://lkml.org/lkml/2024/6/21/1252 > > Testing > ======= > > The driver has been tested on Qualcomm platforms with: > - Basic FastRPC attach/release operations > - DSP process creation and initialization > - Memory mapping/unmapping operations > - Dynamic invocation with various buffer types > - GEM buffer allocation and mmap > - PRIME buffer import from other subsystems > > Signed-off-by: Ekansh Gupta <[email protected]> > --- > Ekansh Gupta (18): > accel/qda: Add Qualcomm QDA DSP accelerator driver docs > accel/qda: Add Qualcomm DSP accelerator driver skeleton > accel/qda: Add RPMsg transport for Qualcomm DSP accelerator > accel/qda: Add built-in compute CB bus for QDA and integrate with IOMMU > accel/qda: Create compute CB devices on QDA compute bus > accel/qda: Add memory manager for CB devices > accel/qda: Add DRM accel device registration for QDA driver > accel/qda: Add per-file DRM context and open/close handling > accel/qda: Add QUERY IOCTL and basic QDA UAPI header > accel/qda: Add DMA-backed GEM objects and memory manager integration > accel/qda: Add GEM_CREATE and GEM_MMAP_OFFSET IOCTLs > accel/qda: Add PRIME dma-buf import support > accel/qda: Add initial FastRPC attach and release support > accel/qda: Add FastRPC dynamic invocation support > accel/qda: Add FastRPC DSP process creation support > accel/qda: Add FastRPC-based DSP memory mapping support > accel/qda: Add FastRPC-based DSP memory unmapping support > MAINTAINERS: Add MAINTAINERS entry for QDA driver > > Documentation/accel/index.rst | 1 + > Documentation/accel/qda/index.rst | 14 + > Documentation/accel/qda/qda.rst | 129 ++++ > MAINTAINERS | 9 + > arch/arm64/configs/defconfig | 2 + > drivers/accel/Kconfig | 1 + > drivers/accel/Makefile | 2 + > drivers/accel/qda/Kconfig | 35 ++ > drivers/accel/qda/Makefile | 19 + > drivers/accel/qda/qda_cb.c | 182 ++++++ > drivers/accel/qda/qda_cb.h | 26 + > drivers/accel/qda/qda_compute_bus.c | 23 + > drivers/accel/qda/qda_drv.c | 375 ++++++++++++ > drivers/accel/qda/qda_drv.h | 171 ++++++ > drivers/accel/qda/qda_fastrpc.c | 1002 > ++++++++++++++++++++++++++++++++ > drivers/accel/qda/qda_fastrpc.h | 433 ++++++++++++++ > drivers/accel/qda/qda_gem.c | 211 +++++++ > drivers/accel/qda/qda_gem.h | 103 ++++ > drivers/accel/qda/qda_ioctl.c | 271 +++++++++ > drivers/accel/qda/qda_ioctl.h | 118 ++++ > drivers/accel/qda/qda_memory_dma.c | 91 +++ > drivers/accel/qda/qda_memory_dma.h | 46 ++ > drivers/accel/qda/qda_memory_manager.c | 382 ++++++++++++ > drivers/accel/qda/qda_memory_manager.h | 148 +++++ > drivers/accel/qda/qda_prime.c | 194 +++++++ > drivers/accel/qda/qda_prime.h | 43 ++ > drivers/accel/qda/qda_rpmsg.c | 327 +++++++++++ > drivers/accel/qda/qda_rpmsg.h | 57 ++ > drivers/iommu/iommu.c | 4 + > include/linux/qda_compute_bus.h | 22 + > include/uapi/drm/qda_accel.h | 224 +++++++ > 31 files changed, 4665 insertions(+) > --- > base-commit: d4906ae14a5f136ceb671bb14cedbf13fa560da6 > change-id: 20260223-qda-firstpost-4ab05249e2cc > > Best regards, > -- > Ekansh Gupta <[email protected]> > >
