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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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?

thanks,

greg k-h