Re: [PATCH RFC v4 2/4] iommu/arm-smmu-v3: Support shared SIDs in insert/remove_master

[Nicolin Chen <nicolinc@nvidia.com>]

On Thu, Oct 01, 2026 at 08:49:59PM +0800, Peng Fan (OSS) wrote:
> -static int arm_smmu_streams_cmp_node(struct rb_node *lhs,
> -				     const struct rb_node *rhs)
> +static int arm_smmu_streams_cmp_node(struct rb_node *lhs, const struct rb_node *rhs)

Why is this changed?

> @@ -4136,23 +4151,33 @@ static int arm_smmu_insert_master(struct arm_smmu_device *smmu,
>  		existing = rb_find_add(&new_stream->node, &smmu->streams,
>  				       arm_smmu_streams_cmp_node);
>  		if (existing) {
> -			struct arm_smmu_master *existing_master =
> -				rb_entry(existing, struct arm_smmu_stream, node)
> -					->master;
> +			struct arm_smmu_stream *existing_stream =
> +				rb_entry(existing, struct arm_smmu_stream, node);
>  
>  			/* Bridged PCI devices may end up with duplicated IDs */
> -			if (existing_master == master)
> +			if (existing_stream->master == master)
>  				continue;
>  
> -			dev_warn(master->dev,
> -				 "Aliasing StreamID 0x%x (from %s) unsupported, expect DMA to be broken\n",
> -				 sid, dev_name(existing_master->dev));
> -			ret = -ENODEV;
> -			break;
> +			if (master->num_streams != 1 ||
> +			    existing_stream->master->num_streams != 1) {
> +				dev_warn(master->dev,
> +					 "Shared StreamID 0x%x not supported for multi-SID masters\n",
> +					 sid);
> +				ret = -ENODEV;
> +				break;
> +			}
> +
> +			if (list_empty(&existing_stream->shared_masters))
> +				list_add_tail(&existing_stream->master->shared_masters_elm,
> +					      &existing_stream->shared_masters);
> +			list_add_tail(&master->shared_masters_elm,
> +				      &existing_stream->shared_masters);

Masters still hold duplicated streams for the shared SID. And their
ste_installed flags are out of sync..

    master_a->stream[0]->a's stream0 {ste_installed=true}
    master_b->stream[0]->b's stream0 {ste_installed=false}

What I have been suggesting is to have a shared stream:
    master_a->stream[0]-->shared stream0 {ste_installed=true}
    master_b->stream[0]-|

So, the driver would need a rework first changing master:
-       struct arm_smmu_stream          *streams;
+       struct arm_smmu_stream          **streams;

Also, arm_smmu_insert_master() needs to reverse its allocation:
	for (i = 0; i < fwspec->num_ids; i++) {
		stream = arm_smmu_find_stream(smmu, sid);
		if (stream) {
			// link to the existing stream
		} else {
			// allocate new stream
		}
		master->streams[i] = stream;
	}
	if (!ret)
		sort();

Nicolin