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

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

On 4/24/2026 4:43 PM, Greg Kroah-Hartman wrote:
> On Fri, Apr 24, 2026 at 04:01:13PM +0530, Vishnu Reddy wrote:
>> 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.
> Why not make a irus_vpu_bus where you can do what you want?
Initially iris_vpu_bus was introduced, and it was made generic based on the
discussion,

https://lore.kernel.org/all/20260227-kaanapali-iris-v2-3-850043ac3933@oss.qualcomm.com/
>> 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.
> As there is only one user, just make this your own bus please and do all
> of the needed bus operations for your devices there (i.e. don't hang an
> "empty" device off of it.)
The reasoning behind to make it generic was to have more users - host1x,
Iris VPU, QDA on the generic context bus, instead of each of them having
their own. Let me know if you suggest to have the iris_vpu_bus.

QDA -
https://lore.kernel.org/all/20260224-qda-firstpost-v1-4-fe46a9c1a046@oss.qualcomm.com/
Iris VPU -
https://lore.kernel.org/all/20260227-kaanapali-iris-v2-3-850043ac3933@oss.qualcomm.com/
> thanks,
>
> greg k-h