Re: [PATCH RFC 04/18] accel/qda: Add built-in compute CB bus for QDA and integrate with IOMMU

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

On 2/24/2026 4:14 AM, Dmitry Baryshkov wrote:
> On Tue, Feb 24, 2026 at 12:38:58AM +0530, Ekansh Gupta wrote:
>> Introduce a built-in compute context-bank (CB) bus used by the Qualcomm
>> DSP accelerator (QDA) driver to represent DSP CB devices that require
>> IOMMU configuration. This separates the CB bus from the QDA driver and
>> allows QDA to remain a loadable module while the bus is always built-in.
> Why? What is the actual problem that you are trying to solve?
Bus needs to be built-in as it is being used by iommu driver. I'll add more details here.
>
>> A new bool Kconfig symbol DRM_ACCEL_QDA_COMPUTE_BUS is added and is
> Don't describe the patch contents. Please.
Ack.
>
>> selected by the main DRM_ACCEL_QDA driver. The parent accel Makefile is
>> updated to descend into the QDA directory for both built-in and module
>> builds so that the CB bus is compiled into vmlinux while the driver
>> remains modular.
>>
>> The CB bus is registered at postcore_initcall() time and is exposed to
>> the IOMMU core through iommu_buses[] in the same way as the Tegra
>> host1x context-bus. This enables later patches to create CB devices on
>> this bus and obtain IOMMU domains for them.
> Note, there is nothing QDA-specific in this patch. Please explain, why
> the bus is QDA-specific? Can we generalize it?
I needed a custom bus here to use for the compute cb devices for iommu
configurations, I don't see any reason to keep it QDA-specific. The only requirement
is that this should be enabled built in whenever QDA is enabled.

But if I keep it generic, where should this be placed? Should it be accel(or drm?) specific?

>
>> Signed-off-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>
>> ---
>>  drivers/accel/Makefile              |  1 +
>>  drivers/accel/qda/Kconfig           |  5 +++++
>>  drivers/accel/qda/Makefile          |  2 ++
>>  drivers/accel/qda/qda_compute_bus.c | 23 +++++++++++++++++++++++
>>  drivers/iommu/iommu.c               |  4 ++++
>>  include/linux/qda_compute_bus.h     | 22 ++++++++++++++++++++++
>>  6 files changed, 57 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-$(CONFIG_DRM_ACCEL_QDA_COMPUTE_BUS) += qda/
>>  obj-$(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..ef1fa384efbe 100644
>> --- a/drivers/accel/qda/Kconfig
>> +++ b/drivers/accel/qda/Kconfig
>> @@ -3,11 +3,16 @@
>>  # 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 e7f23182589b..242684ef1af7 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..1d9c39948fb5
>> --- /dev/null
>> +++ b/drivers/accel/qda/qda_compute_bus.c
>> @@ -0,0 +1,23 @@
>> +// SPDX-License-Identifier: GPL-2.0-only
>> +// Copyright (c) Qualcomm Technologies, Inc. and/or its subsidiaries.
>> +#include <linux/device.h>
>> +#include <linux/init.h>
>> +
>> +struct bus_type qda_cb_bus_type = {
>> +	.name = "qda-compute-cb",
>> +};
>> +EXPORT_SYMBOL_GPL(qda_cb_bus_type);
>> +
>> +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/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>> index 4926a43118e6..5dee912686ee 100644
>> --- a/drivers/iommu/iommu.c
>> +++ b/drivers/iommu/iommu.c
>> @@ -33,6 +33,7 @@
>>  #include <trace/events/iommu.h>
>>  #include <linux/sched/mm.h>
>>  #include <linux/msi.h>
>> +#include <linux/qda_compute_bus.h>
>>  #include <uapi/linux/iommufd.h>
>>  
>>  #include "dma-iommu.h"
>> @@ -178,6 +179,9 @@ static const struct bus_type * const iommu_buses[] = {
>>  #ifdef CONFIG_CDX_BUS
>>  	&cdx_bus_type,
>>  #endif
>> +#ifdef CONFIG_DRM_ACCEL_QDA_COMPUTE_BUS
>> +	&qda_cb_bus_type,
>> +#endif
>>  };
>>  
>>  /*
>> diff --git a/include/linux/qda_compute_bus.h b/include/linux/qda_compute_bus.h
>> new file mode 100644
>> index 000000000000..807122d84e3f
>> --- /dev/null
>> +++ b/include/linux/qda_compute_bus.h
>> @@ -0,0 +1,22 @@
>> +/* 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 struct bus_type qda_cb_bus_type;
>> +#endif
>> +
>> +#endif /* __QDA_COMPUTE_BUS_H__ */
>>
>> -- 
>> 2.34.1
>>