Re: [PATCH v2 00/15] accel/qda: Qualcomm DSP Accelerator driver

[Krzysztof Kozlowski <krzk@kernel.org>]

On 18/08/2026 21:13, Krzysztof Kozlowski wrote:
> On 17/08/2026 06:47, Ekansh Gupta wrote:
>> This patch series introduces the Qualcomm DSP Accelerator (QDA) driver,
>> a DRM-based accelerator driver for Qualcomm DSPs. The driver provides a
>> standardized interface for offloading computational tasks to DSPs found
>> on Qualcomm SoCs, supporting all DSP domains.
>>
>> The QDA driver implements the FastRPC protocol over the DRM accel
>> subsystem. It uses the same device-tree node structure as the existing
>> fastrpc driver in drivers/misc/. The approach for binding the QDA driver
>> to device-tree nodes while coexisting with the fastrpc driver is an open
>> item described below.
> 
> No. Grow/replace/improve existing driver instead of coming with a duplicate.
> 
> That's a standard upstream requirement, basically given on every
> upstreaming guide.
> 
> Please watch old talk from Greg - "I Don’t Want Your Code!".
> 
>>
>> v1: https://lore.kernel.org/all/20260519-qda-series-v1-0-b2d984c297f8@oss.qualcomm.com/
>> RFC: https://lore.kernel.org/dri-devel/20260224-qda-firstpost-v1-0-fe46a9c1a046@oss.qualcomm.com/T/
>>
>> Changes since v1
>> ================
>>
>> The v1 review raised two architectural objections and one correctness
>> issue; all three are resolved in v2:
>>
>> * Christian König (dma-buf maintainer) pointed out that the imported-
>>   buffer path silently assumed the IOMMU maps every buffer as a single
>>   contiguous range, which is not guaranteed. v2 walks the scatterlist
>>   and cleanly rejects non-contiguous imports; contiguous imports (e.g.
>>   CMA DMA-buf heap) are accepted. (patch 11)
>>
>> * Dmitry Baryshkov objected to three different buffer-passing formats
>>   in the invoke IOCTL (DMA-BUF fd, direct/inline, DMA handle). v2
>>   passes only GEM handles; userspace imports any fd to a GEM handle
>>   with DRM_IOCTL_PRIME_FD_TO_HANDLE before invoking. Packing and
>>   overlap handling are left to userspace. (patch 12)
>>
>> * The memory manager (patch 07) used a fixed 16-entry array without
>>   justification and leaked the device descriptor on teardown. v2
>>   allocates the array from the DT context-bank count (as Dmitry
>>   suggested) and frees it correctly.
>>
>> 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 (PRIME)
>> * 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)
>> * DRM IOCTL interface for DSP session management, buffer allocation,
>>   and remote procedure invocation
>>
>> Architecture
>> ============
>>
>> 1. DRM Accelerator Framework Integration
>>    The driver registers as a DRM accel device, exposing a standard
>>    /dev/accel/accelN character device node. This provides established
>>    DRM infrastructure for device management, file operations, and
>>    IOCTL dispatch.
>>
>> 2. Memory Management
>>    Buffers are managed as GEM objects with PRIME support for DMA-BUF
>>    import. This enables buffer sharing with other DRM drivers (GPU,
>>    camera, video) using standard kernel mechanisms. Only contiguous
>>    imports are accepted; the driver verifies contiguity at import time
>>    rather than assuming it.
>>
>> 3. IOMMU Context Bank Management
>>    IOMMU context banks (CBs) are represented as proper struct device
>>    instances on a custom virtual bus (qda-compute-cb). Each CB device
>>    is registered with the IOMMU subsystem and receives its own IOMMU
>>    domain, enabling per-session address space isolation. The custom
>>    bus was introduced because IOMMU context banks are synthetic
>>    constructs — not real platform devices — and to ensure CB device
>>    lifetime is strictly subordinate to the parent QDA device.
>>    See also: https://lore.kernel.org/all/245d602f-3037-4ae3-9af9-d98f37258aae@oss.qualcomm.com/
>>
>> 4. Memory Manager Architecture
>>    The memory manager maintains a registry of IOMMU devices in an
>>    array sized to the number of context banks described in the device
>>    tree, and coordinates per-process device assignment with reference-
>>    counted lifetime management. The DMA-coherent backend allocates
>>    buffers with SID-prefixed DMA addresses for DSP firmware
>>    compatibility.
>>
>> 5. Transport Layer
>>    RPMsg communication is handled in a dedicated transport layer
>>    (qda_rpmsg.c), separate from the core DRM driver logic.
>>
>> 6. Code Organization
>>    The driver is organized across multiple files (~4800 lines total):
>>    * qda_drv.c:            Core driver and DRM integration
>>    * qda_rpmsg.c:          RPMsg transport layer
>>    * qda_cb.c:             Context bank device management
>>    * qda_compute_bus.c:    Custom virtual bus for CB devices
>>    * qda_gem.c:            GEM object management
>>    * qda_prime.c:          DMA-BUF import (PRIME)
>>    * qda_memory_manager.c: IOMMU device registry and allocation
>>    * qda_memory_dma.c:     DMA-coherent allocation backend
>>    * qda_fastrpc.c:        FastRPC protocol implementation
>>    * qda_ioctl.c:          IOCTL dispatch
>>
>> 7. UAPI Design
>>    The driver exposes DRM-style IOCTLs defined in
>>    include/uapi/drm/qda_accel.h, following DRM UAPI conventions
>>    (__u32/__u64 types, C++ guard, GPL-2.0-only WITH Linux-syscall-note).
>>    Buffer arguments are identified by GEM handles; the driver never
>>    accepts DMA-BUF fds directly in any IOCTL.
>>
>> Patch Series Organization
>> ==========================
>>
>> Patch 01:      MAINTAINERS entry
>> Patch 02:      Driver documentation (Documentation/accel/qda/)
>> Patches 03-04: Core driver skeleton and compute bus
>> Patch 05:      iommu: Register qda-compute-cb bus with IOMMU subsystem
>> Patches 06-07: CB device enumeration and memory manager
>> Patch 08:      QUERY IOCTL and UAPI header
>> Patches 09-11: GEM buffer management and PRIME import
>> Patches 12-15: FastRPC protocol (invoke, session create/release,
>>                map/unmap)
>>
>> Open Items
>> ===========
>>
>> 1. Device-Tree Compatible String
>>    The QDA driver uses the same device-tree node structure and
>>    properties as the existing fastrpc driver in drivers/misc/. A
>>    mechanism is needed to allow the QDA driver to bind to its device
>>    node independently of the fastrpc driver.
>>
>>    The intended coexistence model is: platforms that require the
>>    complete fastrpc feature set continue to use "qcom,fastrpc"; new
>>    platforms where QDA's feature set is sufficient use a QDA-specific
>>    compatible string. New feature development is directed toward QDA.
>>
>>    The options under consideration are:
>>
>>    a) Add a new "qcom,qda" compatible string to the existing
>>       qcom,fastrpc.yaml binding, since the DT node structure and
>>       properties are identical.
> No
> 
>>
>>    b) Introduce a separate qcom,qda.yaml binding that references or
>>       inherits the fastrpc binding properties.
> 
> No
> 
>>
>>    Seeking guidance from DT binding maintainers on the preferred
>>    approach.
> 
> Grow existing driver. You do not get new driver, you do not get new
> bindings.
> 

And this was already questioned at v1 (the true v1, not v1+1) but you
ignored the comment.

Great, so here goes away trust.

NAK

Best regards,
Krzysztof