Re: [PATCH v2 02/13] drivers: base: Add generic dma context bus

[Vishnu Reddy <busanna.reddy@oss.qualcomm.com>]

On 4/23/2026 7:07 PM, Greg Kroah-Hartman wrote:
> On Thu, Apr 23, 2026 at 06:59:31PM +0530, Vishnu Reddy wrote:
>> From: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>
>>
>> When a driver needs to create virtual device at runtime and map it to
>> an IOMMU context for memory isolation, there is no common bus available
>> for this purpose. Each driver ends up implementing its own bus type,
>> leading to duplicated logic across multiple drivers.
>>
>> host1x driver implemented its own bus type to attach an IOMMU context to
>> a dynamically created device. The Iris VPU driver now has the same
>> requirement. Rather than duplicating the same bus logic again, a shared
>> bus type is introduced under drivers/base that multiple drivers can use
>> directly.
>>
>> The bus takes care of creating a device and attaching the IOMMU context
>> to it based on the client inputs.
>>
>> Suggested-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
>> Signed-off-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>
>> Signed-off-by: Vikash Garodia <vikash.garodia@oss.qualcomm.com>
>> Signed-off-by: Vishnu Reddy <busanna.reddy@oss.qualcomm.com>
>> ---
>>  drivers/base/Kconfig            |  3 ++
>>  drivers/base/Makefile           |  1 +
>>  drivers/base/dma_context_bus.c  | 77 +++++++++++++++++++++++++++++++++++++++++
>>  include/linux/dma_context_bus.h | 26 ++++++++++++++
>>  4 files changed, 107 insertions(+)
> as you can not have a device on multiple busses at the same time, this
> makes no sense to me at all.  "dma context" is a bus-specific thing, so
> please add it to the bus that you are wanting it for.  It can't be a
> generic bus as that just doesn't work.
>
> Or what am I missing here?
>
> And why is DMA somehow "special" here from any other hardware attribute?

Let me give brief information which was discussed, in the initial series,
the iris VPU used platform bus for dynamically created devices and we got
the comment/suggestion from Robin to implement a proper bus_type with a
.dma_configure callback.

https://lore.kernel.org/all/02b3d0f5-f94c-43cd-93af-97cfcf7751b1@arm.com/

based on the discussion, implemented the dma_context_bus and used for iris
VPU devices instead of platform bus.

Here, the device have only one bus (dma_context_bus), not multiple buses.

Regarding the "DMA" naming, the core operation of this bus is its
.dma_configure callback, which calls of_dma_configure_id() to map the device
to a corresponding IOMMU stream ID. The name "dma_context" reflects this
purpose.

I am open to suggestions from you or Robin or anyone else, if there is a
better or preferred way to achieve this, I am happy to consider it and
rework the implementation accordingly.

Thanks,
Vishnu Reddy.

> thanks,
>
> greg k-h