Re: [PATCH net-next v4 2/2] net: wwan: qcom_bam_dmux: Assign restricted DMA pool to remote processor

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

Hi Loic,

On 03-11-2026 09:34 pm, Loic Poulain wrote:
> Hi Vishnu,
>
> On Tue, Nov 3, 2026 at 3:19 PM 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 does not reference a
>> restricted DMA pool, or when the 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 | 114 ++++++++++++++++++++++++++++++++++++++-
>>   2 files changed, 113 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..c9639474e58acc16ce0d04f181b2e999cff42bd1 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 +768,96 @@ 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");
>> +
>> +       if (!of_device_is_compatible(rmem_np, "restricted-dma-pool")) {
>> +               of_node_put(rmem_np);
>> +               return dev_err_probe(dev, -EINVAL,
>> +                                    "memory-region must reference a restricted DMA pool\n");
>> +       }
> You later test with is_swiotlb_for_alloc(), so isn't the above test redundant?

Right. is_swiotlb_for_alloc() is only true when the DMA core attached a
restricted-dma-pool from memory-region, so this check is redundant. I will drop it.

>
>
>> +
>> +       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 +892,10 @@ 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 +929,11 @@ 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);
> Calling remove from probe is a bit unusual, have you checked that
> everything is correctly balanced?

This was changed in v3 following the Sashiko review of v2 [1]. The PC
IRQ is already requested at this point and may have powered up the RX
path, so in v2 the error path reclaimed the pool while the DMA channels
and RX buffers could still be active.

Yes, it is balanced. At this point probe has assigned the pool (if,
qcom,vmid is set), initialized the works, wait queue and runtime PM, and
requested both IRQs, which is everything remove() undoes. The steps that
may not have happened yet (netdev registration, RX power-up, the PC vote)
are handled the same way as in a regular remove: unregistering an empty
netdev list, power_off() and free_skbs() are no-ops, dropping the vote
only clears the SMSM bit, and the wait returns immediately if RX was never
powered up. The PC IRQ is disabled before power_off(), and the IRQs themselves
are released by devm afterwards.

Do you see any specific concerns here?

[1] https://lore.kernel.org/all/178996250389.2160803.17277339384338088713@kernel.org/

Vishnu

>
>> +               return ret;
>> +       }
>>
>>          /* Check if remote finished initialization before us */
>>          if (dmux->pc_state) {
>> @@ -845,6 +948,7 @@ 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 +983,12 @@ 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
>>