On 20-05-2026 19:49, Dmitry Baryshkov wrote:
> 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-$(CONFIG_DRM_ACCEL_QDA_COMPUTE_BUS) += qda/
>
> Ugh. The previous line should be enough (but don't trust me).
I was seeing build failures if I don't add this. Took it as a reference
from host1x driver and recent iris patch.>
>> obj-$(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
>>
>>
>
