Re: [PATCH net-next v5 2/2] net: wwan: qcom_bam_dmux: Assign restricted DMA pool to remote processor

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

On 05-11-2026 07:51 pm, Loic Poulain wrote:
> On Thu, Nov 5, 2026 at 11:42 AM Vishnu Santhosh
> <vishnu.santhosh@oss.qualcomm.com> wrote:
>> Some Qualcomm SoCs, such as Shikra, run the modem in a separate security
>> domain with restricted access to system memory. BAM-DMUX DMA mappings
>> and BAM descriptor FIFOs must therefore be constrained to a designated
>> memory region that can be shared with the modem. Accesses outside that
>> region can trigger an XPU violation.
>>
>> When qcom,vmid is present, require a restricted DMA pool and assign the
>> entire pool to both the local and remote execution environments before
>> requesting the DMA channels.
>>
>> This ensures that BAM-DMUX mappings are within the assigned region. When
>> the BAM DMA controller references the same pool, its descriptor FIFOs
>> are covered by the assignment as well.
>>
>> Track the assigned execution environments explicitly and reclaim the
>> pool for the local environment after DMA channels and mappings are
>> released during remove or probe error cleanup. Skip the reclaim and
>> leave the pool assigned if the remote side still holds its power vote,
>> since it may still access the pool.
>>
>> Fail probing when restricted DMA pool support is unavailable, when the
>> VMID property is malformed, when memory-region is missing, or when the
>> restricted DMA pool is not attached to the device.
>>
>> 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>
>> ---
>>   drivers/net/wwan/Kconfig         |   1 +
>>   drivers/net/wwan/qcom_bam_dmux.c | 108 ++++++++++++++++++++++++++++++++++++++-
>>   2 files changed, 107 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/net/wwan/Kconfig b/drivers/net/wwan/Kconfig
>> index 958dbc7347fa84ee869439bf8b503037faab8bef..1b133c56231615269698140187ca3141dfe48dbf 100644
>> --- a/drivers/net/wwan/Kconfig
>> +++ b/drivers/net/wwan/Kconfig
>> @@ -65,6 +65,7 @@ config MHI_WWAN_MBIM
>>   config QCOM_BAM_DMUX
>>          tristate "Qualcomm BAM-DMUX WWAN network driver"
>>          depends on (DMA_ENGINE && PM && QCOM_SMEM_STATE) || COMPILE_TEST
>> +       select QCOM_SCM
>>          help
>>            The BAM Data Multiplexer provides access to the network data channels
>>            of modems integrated into many older Qualcomm SoCs, e.g. Qualcomm
>> diff --git a/drivers/net/wwan/qcom_bam_dmux.c b/drivers/net/wwan/qcom_bam_dmux.c
>> index cc6ace8d64371eb8d00c638a39b234ee540b83c9..0e7ad62f8b4e059264e89837f296ee13c0c31c9b 100644
>> --- a/drivers/net/wwan/qcom_bam_dmux.c
>> +++ b/drivers/net/wwan/qcom_bam_dmux.c
>> @@ -9,14 +9,17 @@
>>   #include <linux/completion.h>
>>   #include <linux/dma-mapping.h>
>>   #include <linux/dmaengine.h>
>> +#include <linux/firmware/qcom/qcom_scm.h>
>>   #include <linux/if_arp.h>
>>   #include <linux/interrupt.h>
>>   #include <linux/module.h>
>>   #include <linux/netdevice.h>
>> +#include <linux/of_reserved_mem.h>
>>   #include <linux/platform_device.h>
>>   #include <linux/pm_runtime.h>
>>   #include <linux/soc/qcom/smem_state.h>
>>   #include <linux/spinlock.h>
>> +#include <linux/swiotlb.h>
>>   #include <linux/wait.h>
>>   #include <linux/workqueue.h>
>>   #include <net/pkt_sched.h>
>> @@ -75,6 +78,9 @@ struct bam_dmux {
>>          struct completion pc_ack_completion;
>>
>>          struct dma_chan *rx, *tx;
>> +       phys_addr_t dma_pool_base;
>> +       size_t dma_pool_size;
>> +       u64 dma_pool_perms;
>>          struct bam_dmux_skb_dma rx_skbs[BAM_DMUX_NUM_SKB];
>>          struct bam_dmux_skb_dma tx_skbs[BAM_DMUX_NUM_SKB];
>>          spinlock_t tx_lock; /* Protect tx_skbs, tx_next_skb */
>> @@ -762,6 +768,90 @@ static int __maybe_unused bam_dmux_runtime_resume(struct device *dev)
>>          return 0;
>>   }
>>
>> +static int bam_dmux_assign_dma_pool(struct bam_dmux *dmux)
>> +{
>> +       struct device *dev = dmux->dev;
>> +       struct device_node *rmem_np;
>> +       struct reserved_mem *rmem;
>> +       struct qcom_scm_vmperm dst[2];
>> +       u64 src = BIT_ULL(QCOM_SCM_VMID_HLOS);
>> +       u32 vmid;
>> +       int ret;
>> +
>> +       if (!of_property_present(dev->of_node, "qcom,vmid"))
>> +               return 0;
>> +
>> +       ret = of_property_read_u32(dev->of_node, "qcom,vmid", &vmid);
>> +       if (ret)
>> +               return dev_err_probe(dev, ret, "Failed to read qcom,vmid\n");
>> +       if (vmid == QCOM_SCM_VMID_HLOS || vmid >= BITS_PER_TYPE(u64))
>> +               return dev_err_probe(dev, -EINVAL, "Invalid qcom,vmid %u\n", vmid);
>> +
>> +       if (!IS_ENABLED(CONFIG_DMA_RESTRICTED_POOL))
>> +               return dev_err_probe(dev, -EOPNOTSUPP,
>> +                                    "qcom,vmid requires DMA_RESTRICTED_POOL\n");
>> +
>> +       rmem_np = of_parse_phandle(dev->of_node, "memory-region", 0);
>> +       if (!rmem_np)
>> +               return dev_err_probe(dev, -EINVAL,
>> +                                            "qcom,vmid requires memory-region\n");
>> +
>> +       rmem = of_reserved_mem_lookup(rmem_np);
>> +       of_node_put(rmem_np);
>> +       if (!rmem)
>> +               return dev_err_probe(dev, -EINVAL,
>> +                                            "Failed to look up restricted DMA pool\n");
>> +
>> +       if (!is_swiotlb_for_alloc(dev))
>> +               return dev_err_probe(dev, -EINVAL,
>> +                                    "Restricted DMA pool not attached\n");
>> +
>> +       if (!qcom_scm_is_available())
>> +               return -EPROBE_DEFER;
>> +
>> +       dst[0].vmid = QCOM_SCM_VMID_HLOS;
>> +       dst[0].perm = QCOM_SCM_PERM_RW;
>> +       dst[1].vmid = vmid;
>> +       dst[1].perm = QCOM_SCM_PERM_RW;
>> +
>> +       ret = qcom_scm_assign_mem(rmem->base, rmem->size, &src, dst,
>> +                                 ARRAY_SIZE(dst));
>> +       if (ret)
>> +               return dev_err_probe(dev, ret,
>> +                                    "SCM assign restricted DMA pool failed\n");
>> +
>> +       dmux->dma_pool_base = rmem->base;
>> +       dmux->dma_pool_size = rmem->size;
>> +       /* Track the destination VMIDs explicitly for the reclaim operation. */
>> +       dmux->dma_pool_perms = BIT_ULL(QCOM_SCM_VMID_HLOS) | BIT_ULL(vmid);
>> +
>> +       return 0;
>> +}
>> +
>> +static void bam_dmux_reclaim_dma_pool(struct bam_dmux *dmux)
>> +{
>> +       struct qcom_scm_vmperm hlos = {
>> +               .vmid = QCOM_SCM_VMID_HLOS,
>> +               .perm = QCOM_SCM_PERM_RW,
>> +       };
>> +       u64 src = dmux->dma_pool_perms;
>> +       int ret;
>> +
>> +       if (!dmux->dma_pool_perms)
>> +               return;
>> +
>> +       ret = qcom_scm_assign_mem(dmux->dma_pool_base, dmux->dma_pool_size, &src,
>> +                                 &hlos, 1);
>> +       if (ret) {
>> +               dev_err(dmux->dev, "SCM reclaim restricted DMA pool failed: %d\n", ret);
>> +               return;
>> +       }
>> +
>> +       dmux->dma_pool_perms = 0;
>> +}
>> +
>> +static void bam_dmux_remove(struct platform_device *pdev);
>> +
>>   static int bam_dmux_probe(struct platform_device *pdev)
>>   {
>>          struct device *dev = &pdev->dev;
>> @@ -796,6 +886,10 @@ static int bam_dmux_probe(struct platform_device *pdev)
>>                                       "Failed to get pc-ack state\n");
>>          dmux->pc_ack_mask = BIT(bit);
>>
>> +       ret = bam_dmux_assign_dma_pool(dmux);
>> +       if (ret)
>> +               return ret;
>> +
>>          init_waitqueue_head(&dmux->pc_wait);
>>          init_completion(&dmux->pc_ack_completion);
>>          complete_all(&dmux->pc_ack_completion);
>> @@ -829,8 +923,11 @@ static int bam_dmux_probe(struct platform_device *pdev)
>>
>>          ret = irq_get_irqchip_state(dmux->pc_irq, IRQCHIP_STATE_LINE_LEVEL,
>>                                      &dmux->pc_state);
>> -       if (ret)
>> -               goto err_disable_pm;
>> +       if (ret) {
>> +               /* The PC IRQ may already have powered up the RX path */
>> +               bam_dmux_remove(pdev);
> it's still unclear in the comment and commit message why we can't go
> with err_disable_pm?

Agreed. I plan to update the comment in the next revision to:

/*
  * The PC IRQ is already enabled and may have powered up the RX
  * path. Use the full remove() teardown so that RX DMA is stopped
  * before the pool is reclaimed.
  */

and add to the commit message:

If probe fails after the PC IRQ has been requested, clean up through
bam_dmux_remove(), since the IRQ may already have powered up the RX
path, which must be torn down before the pool is reclaimed.

Does this look fine to you?


Vishnu

>
>> +               return ret;
>> +       }
>>
>>          /* Check if remote finished initialization before us */
>>          if (dmux->pc_state) {
>> @@ -845,6 +942,7 @@ static int bam_dmux_probe(struct platform_device *pdev)
>>   err_disable_pm:
>>          pm_runtime_disable(dev);
>>          pm_runtime_dont_use_autosuspend(dev);
>> +       bam_dmux_reclaim_dma_pool(dmux);
>>          return ret;
>>   }
>>
>> @@ -879,6 +977,12 @@ static void bam_dmux_remove(struct platform_device *pdev)
>>          disable_irq(dmux->pc_irq);
>>          bam_dmux_power_off(dmux);
>>          bam_dmux_free_skbs(dmux->tx_skbs, DMA_TO_DEVICE);
>> +
>> +       /* The remote side may still access the pool if it holds its power vote */
>> +       if (!dmux->pc_state)
>> +               bam_dmux_reclaim_dma_pool(dmux);
>> +       else if (dmux->dma_pool_perms)
>> +               dev_warn(dev, "Leaving restricted DMA pool assigned to remote\n");
>>   }
>>
>>   static const struct dev_pm_ops bam_dmux_pm_ops = {
>>
>> --
>> 2.34.1
>>