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

[Vishnu Santhosh]

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 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.

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.

[1] https://elixir.bootlin.com/linux/v7.3-rc7/source/arch/arm64/boot/dts/qcom/shikra.dtsi#L508
[2] https://lwn.net/Articles/841916/
[3] https://lore.kernel.org/all/an27PVxkCpD09xSR@xxxxxxxxxx/

Vishnu