Re: [PATCH 04/15] accel/qda: Add compute bus for QDA context banks

[Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>]

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 <ekansh.gupta@oss.qualcomm.com>
>>
>> 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 <ekansh.gupta@oss.qualcomm.com>
>> ---
>>  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
>>
>>
>