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

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

[Vishnu Santhosh]

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.

Co-developed-by: Deepak Kumar Singh <deepak.singh@oss.qualcomm.com>
Signed-off-by: Deepak Kumar Singh <deepak.singh@oss.qualcomm.com>
Signed-off-by: Vishnu Santhosh <vishnu.santhosh@oss.qualcomm.com>
---
Changes in v3:
- Drop the example changes (Krzysztof)
- Link to v2: https://lore.kernel.org/r/20260920-qcom-bam-dma-vmid-ext-v2-1-4dbbff0e57c8@oss.qualcomm.com

Changes in v2:
- Replace the qcom,vmid property and BAM DMA driver SCM assignment with
  the standard memory-region restricted DMA pool mechanism.
- Rely on generic DMA handling to constrain descriptor FIFO allocations
  to the restricted pool.
- Drop the Shikra-specific BAM compatible and all BAM DMA driver changes.
- Drop Rob's Reviewed-by, as the binding was substantially redesigned.
- Link to v1: https://lore.kernel.org/r/20260714-qcom-bam-dma-vmid-ext-v1-0-cef87c57b7dc@oss.qualcomm.com
---
 Documentation/devicetree/bindings/dma/qcom,bam-dma.yaml | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/devicetree/bindings/dma/qcom,bam-dma.yaml b/Documentation/devicetree/bindings/dma/qcom,bam-dma.yaml
index e72adc172af102d8f400044ea0dc32db6b2365be..b7b348bf6ae2feba32ecaf17ca8696817a1ada4a 100644
--- a/Documentation/devicetree/bindings/dma/qcom,bam-dma.yaml
+++ b/Documentation/devicetree/bindings/dma/qcom,bam-dma.yaml
@@ -50,6 +50,13 @@ properties:
     minItems: 1
     maxItems: 7
 
+  memory-region:
+    maxItems: 1
+    description:
+      Phandle to a reserved-memory node compatible with restricted-dma-pool.
+      The BAM DMA controller uses the pool for DMA allocations, including
+      channel descriptor FIFOs.
+
   num-channels:
     $ref: /schemas/types.yaml#/definitions/uint32
     description:

---
base-commit: 2c455ddca7827ad8fa3a64fdbf5a60efb928b46a
change-id: 20260714-qcom-bam-dma-vmid-ext-8a3bd3c5c00e

Best regards,
-- 
Vishnu Santhosh <vishnu.santhosh@oss.qualcomm.com>

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

[Rob Herring]

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

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@linaro.org/

Vishnu