On Tue, May 19, 2026 at 11:45:54AM +0530, Ekansh Gupta via B4 Relay wrote:
> From: Ekansh Gupta <[email protected]>
>
> Introduce a custom virtual bus (qda-compute-cb) for managing IOMMU
> context bank (CB) devices used by the QDA driver.
>
> IOMMU context banks are synthetic constructs — they are not real
> platform devices and do not appear as children of a platform bus node
> in the device tree. Using a platform driver to represent them was
> therefore incorrect and introduced a probe-ordering race: device nodes
> were created before the RPMsg channel resources were fully initialized,
> and because probe runs asynchronously, user-space could open a CB
> device and attempt to start a session before the underlying transport
> was ready.
>
> The qda-compute-cb bus solves this by allowing the main QDA driver to
> create CB devices explicitly and under its own control, making their
> lifetime strictly subordinate to the parent qda_dev. The bus provides
> a dma_configure callback that calls of_dma_configure() so that each CB
> device gets its own IOMMU domain derived from its device-tree node,
> enabling per-session memory isolation.
>
> The bus type and the CB device constructor (create_qda_cb_device) are
> exported for use by the QDA memory manager.
>
> A hidden Kconfig symbol (DRM_ACCEL_QDA_COMPUTE_BUS) is introduced and
> automatically selected by DRM_ACCEL_QDA so that the bus initialisation
> runs via postcore_initcall before any QDA device probes.
>
> Assisted-by: Claude:claude-4-6-sonnet
> Signed-off-by: Ekansh Gupta <[email protected]>
> ---
> drivers/accel/Makefile | 1 +
> drivers/accel/qda/Kconfig | 4 +++
> drivers/accel/qda/Makefile | 2 ++
> drivers/accel/qda/qda_compute_bus.c | 68
> +++++++++++++++++++++++++++++++++++++
> include/linux/qda_compute_bus.h | 32 +++++++++++++++++
> 5 files changed, 107 insertions(+)
>
> diff --git a/drivers/accel/Makefile b/drivers/accel/Makefile
> index 58c08dd5f389..9ed843cd293f 100644
> --- a/drivers/accel/Makefile
> +++ b/drivers/accel/Makefile
> @@ -6,4 +6,5 @@ obj-$(CONFIG_DRM_ACCEL_HABANALABS) += habanalabs/
> obj-$(CONFIG_DRM_ACCEL_IVPU) += ivpu/
> obj-$(CONFIG_DRM_ACCEL_QAIC) += qaic/
> obj-$(CONFIG_DRM_ACCEL_QDA) += qda/
> +obj-$(CONFIG_DRM_ACCEL_QDA_COMPUTE_BUS) += qda/
Ugh. The previous line should be enough (but don't trust me).
> obj-$(CONFIG_DRM_ACCEL_ROCKET) += rocket/
> \ No newline at end of file
> diff --git a/drivers/accel/qda/Kconfig b/drivers/accel/qda/Kconfig
> index 484d21ff1b55..2a61a4dda054 100644
> --- a/drivers/accel/qda/Kconfig
> +++ b/drivers/accel/qda/Kconfig
> @@ -3,11 +3,15 @@
> # Qualcomm DSP accelerator driver
> #
>
> +config DRM_ACCEL_QDA_COMPUTE_BUS
> + bool
> +
> config DRM_ACCEL_QDA
> tristate "Qualcomm DSP accelerator"
> depends on DRM_ACCEL
> depends on ARCH_QCOM || COMPILE_TEST
> depends on RPMSG
> + select DRM_ACCEL_QDA_COMPUTE_BUS
> help
> Enables the DRM-based accelerator driver for Qualcomm's Hexagon DSPs.
> This driver provides a standardized interface for offloading
> computational
> diff --git a/drivers/accel/qda/Makefile b/drivers/accel/qda/Makefile
> index dbe809067a8b..424176f652a5 100644
> --- a/drivers/accel/qda/Makefile
> +++ b/drivers/accel/qda/Makefile
> @@ -8,3 +8,5 @@ obj-$(CONFIG_DRM_ACCEL_QDA) := qda.o
> qda-y := \
> qda_drv.o \
> qda_rpmsg.o
> +
> +obj-$(CONFIG_DRM_ACCEL_QDA_COMPUTE_BUS) += qda_compute_bus.o
> diff --git a/drivers/accel/qda/qda_compute_bus.c
> b/drivers/accel/qda/qda_compute_bus.c
> new file mode 100644
> index 000000000000..c59d977e924d
> --- /dev/null
> +++ b/drivers/accel/qda/qda_compute_bus.c
> @@ -0,0 +1,68 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +// Copyright (c) Qualcomm Technologies, Inc. and/or its subsidiaries.
> +#include <linux/device.h>
> +#include <linux/init.h>
> +#include <linux/of.h>
> +#include <linux/of_device.h>
> +#include <linux/qda_compute_bus.h>
> +#include <linux/slab.h>
> +
> +static int qda_cb_bus_dma_configure(struct device *dev)
> +{
> + return of_dma_configure(dev, dev->of_node, true);
> +}
> +
> +const struct bus_type qda_cb_bus_type = {
> + .name = "qda-compute-cb",
> + .dma_configure = qda_cb_bus_dma_configure,
> +};
> +EXPORT_SYMBOL_GPL(qda_cb_bus_type);
> +
> +static void release_qda_cb_device(struct device *dev)
> +{
> + of_node_put(dev->of_node);
> + kfree(dev);
> +}
> +
> +struct device *create_qda_cb_device(struct device *parent_device, const char
> *name,
> + u64 dma_mask, struct device_node *of_node)
> +{
> + struct device *dev;
> + int ret;
> +
> + dev = kzalloc_obj(*dev);
> + if (!dev)
> + return ERR_PTR(-ENOMEM);
> +
> + dev->release = release_qda_cb_device;
> + dev->bus = &qda_cb_bus_type;
> + dev->parent = parent_device;
> + dev->coherent_dma_mask = dma_mask;
> + dev->dma_mask = &dev->coherent_dma_mask;
> + dev->of_node = of_node_get(of_node);
> +
> + dev_set_name(dev, "%s", name);
> +
> + ret = device_register(dev);
> + if (ret) {
> + put_device(dev);
> + return ERR_PTR(ret);
> + }
> +
> + return dev;
> +}
> +EXPORT_SYMBOL_GPL(create_qda_cb_device);
> +
> +static int __init qda_cb_bus_init(void)
> +{
> + int err;
> +
> + err = bus_register(&qda_cb_bus_type);
> + if (err < 0) {
> + pr_err("qda-compute-cb bus registration failed: %d\n", err);
> + return err;
> + }
> + return 0;
> +}
> +
> +postcore_initcall(qda_cb_bus_init);
> diff --git a/include/linux/qda_compute_bus.h b/include/linux/qda_compute_bus.h
> new file mode 100644
> index 000000000000..90bf248c7285
> --- /dev/null
> +++ b/include/linux/qda_compute_bus.h
> @@ -0,0 +1,32 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (c) Qualcomm Technologies, Inc. and/or its subsidiaries.
> + */
> +
> +#ifndef __QDA_COMPUTE_BUS_H__
> +#define __QDA_COMPUTE_BUS_H__
> +
> +#include <linux/device.h>
> +
> +/*
> + * Custom bus type for QDA compute context bank (CB) devices
> + *
> + * This bus type is used for manually created CB devices that represent
> + * IOMMU context banks. The custom bus allows proper IOMMU configuration
> + * and device management for these virtual devices.
> + */
> +#ifdef CONFIG_DRM_ACCEL_QDA_COMPUTE_BUS
> +extern const struct bus_type qda_cb_bus_type;
> +
> +struct device *create_qda_cb_device(struct device *parent_device, const char
> *name,
> + u64 dma_mask, struct device_node *of_node);
> +#else
> +static inline struct device *create_qda_cb_device(struct device
> *parent_device,
> + const char *name, u64
> dma_mask,
> + struct device_node *of_node)
> +{
> + return ERR_PTR(-ENODEV);
> +}
> +#endif
> +
> +#endif /* __QDA_COMPUTE_BUS_H__ */
>
> --
> 2.34.1
>
>
--
With best wishes
Dmitry
