Re: [PATCH 2/2] net: wwan: qcom_bam_dmux: Alloc RX buffers as a single coherent block

[Vishnu Santhosh]

On 14-07-2026 01:05 pm, Stephan Gerhold wrote:
> On Tue, Jul 14, 2026 at 11:02:32AM +0530, Vishnu Santhosh wrote:
> > On Qualcomm SoCs where the modem (e.g. the mDSP on Shikra, VMID 43 /
> > NAV) is the AXI master for BAM-DMUX RX transfers and the XPU enforces
> > per-region access control, each individually DMA-mapped RX buffer
> > requires its own XPU resource group (RG). With ~16 RGs available, the
> > 32 per-buffer dma_map_single() calls exhaust the table and the first
> > inbound transfer faults with an XPU violation.
> >
> > BAM-DMUX is a singleton (exactly one instance per SoC), so the
> > destination VMID does not need to be a DT property; it is looked up
> > from the compatible string's match data instead. Add struct
> > bam_dmux_data with a single vmid field, and a shikra_data instance
> > hardcoding QCOM_SCM_VMID_NAV for qcom,shikra-bam-dmux.
> >
> > When match data is present, allocate all BAM_DMUX_NUM_SKB RX buffers as
> > a single contiguous dma_alloc_coherent() block and SCM-assign that
> > block to HLOS plus the VMID once at probe. This reduces RG consumption
> > from 32 to 1. The block is never reclaimed across a modem power cycle
> > (bam_dmux_power_off() does not touch it), so the probe-time assignment
> > covers every subsequent restart without re-assigning or reclaiming. It
> > is reclaimed to HLOS only once, at remove or on a probe error, and if
> > that reclaim fails it is leaked rather than returned to the page
> > allocator.
> >
> > Each rx_skbs[] slot is pre-assigned its virtual and DMA address from
> > the block, so no per-buffer mapping is needed at power-on. Because the
> > coherent block is not page-backed, received payload is copied into a
> > regular netdev skb before handoff to the network stack; this is an
> > unavoidable extra copy on the XPU-enforced RX path.
> >
> > Platforms without match data are unaffected: rx_virt stays NULL, no
> > coherent memory is allocated, and the per-buffer dma_map_single() path
> > is unchanged.
> >
> > Co-developed-by: Deepak Kumar Singh <deepak.singh@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Deepak Kumar Singh <deepak.singh@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Vishnu Santhosh <vishnu.santhosh@xxxxxxxxxxxxxxxx>
> So how do you handle TX buffers? Right now, they are just passed on from
> the net subsystem. There can be up to 32 TX buffers in progress as well.
>
> Overall, I have mixed feelings about this patch. It looks reasonably
> simple, but fundamentally I don't understand why we need to go back to
> the old days of implementing protection using a highly limited MPU (in
> your case: the xPU).
>
> Why does the setup of BAM-DMUX differ e.g. from the setup for the crypto
> engine? Crypto is also using bam-dma, but it avoids this inflexibility
> by making use of the &apps_smmu. Is BAM-DMUX not covered by the SMMU? Or
> did you just decide to bypass the SMMU in this case? (If so: Why?)
>
> If you had BAM-DMUX mapped using the SMMU you would get all of this for
> free. No changes would be needed in the BAM-DMUX driver ...
>
> Thanks,
> Stephan

Thanks for pointing out. We were seeing XPU violations on descriptor
FIFO accesses in the RX path. This series resolves the RX-side faults,
which is why we posted it. It appears that the TX path requires equivalent
handling, and we'll include the necessary TX changes in v2 of this series.

Currently, the SMMU does not cover this A2 BAM instance on Shikra, which
is why we opted for the SCM-assign approach. I'll check with the hardware
team to better understand why BAM-DMUX is not behind the SMMU, gather the
relevant details, and get back.

Thanks,
Vishnu