Re: [PATCH v3] dt-bindings: dma: qcom,bam-dma: Allow memory-region property

[Vishnu Santhosh <vishnu.santhosh@oss.qualcomm.com>]

On 26-10-2026 06:35 pm, Rob Herring wrote:
> On Fri, Oct 23, 2026 at 12:05:50PM +0530, Vishnu Santhosh wrote:
>> On 22-10-2026 07:31 pm, Rob Herring wrote:
>>> On Wed, Oct 21, 2026 at 11:22:35AM +0530, Vishnu Santhosh wrote:
>>>> Some Qualcomm SoCs, such as Shikra, run the modem in a separate security
>>>> domain, preventing it from accessing BAM descriptor FIFOs allocated from
>>>> arbitrary host memory.
>>>>
>>>> Add an optional memory-region property to constrain DMA allocations made
>>>> by the BAM DMA controller to a restricted DMA pool. Access permissions
>>>> for the remote execution environment are managed separately.
>>> dma-ranges is normally how address restrictions for a device are
>>> defined. Why can't that be used here?
>>>
>>> Rob
>> The constraint here is access permission, not addressing. The modem can
>> only access a carve-out that is assigned to it through SCM.
> The reason for the constraint doesn't really matter.

Ok, understood.

>
>> The soc bus on Shikra already has an identity dma-ranges covering
>> 64GB [1], so there is no addressing limitation to describe. dma-ranges
>> does not reserve memory, so allocations would still come from the
>> general page allocator, outside the assigned carve-out, and modem
>> accesses to them trigger an XPU violation. It also applies to every
>> device on the bus.
> It's a bit awkward that dma-ranges needs to be in a parent node. So you
> would need to put the device in its own bus node. But if dma-ranges is
> in place, DMA allocations will only come from within that range.

Thanks for the correction.

>
>
>> The A2 BAM on Shikra is not behind an IOMMU. restricted-dma-pool
>> provides a dedicated, per-device region, which is the setup it was
>> introduced for no IOMMU, with firmware restricting device accesses to
>> specific regions [2]. Stephan suggested this approach on v1 of the
>> BAM-DMUX series [3]. The BAM DMA controller and BAM-DMUX reference the
>> same pool, so a single assignment covers both the descriptor FIFOs and
>> the data buffers.
> I suppose that since the memory region is dynamic, 'memory-region' makes
> sense here. If firmware determined the address of the region, I'd
> probably feel differently.

Yes, the region is dynamic. Thanks for the review.

Vishnu