Re: [PATCH RFC v2 2/4] iommu/arm-smmu-v3: Support shared SIDs in insert/remove_master

[Nicolin Chen <nicolinc@nvidia.com>]

On Mon, Sep 21, 2026 at 08:38:46PM +0800, Peng Fan (OSS) wrote:
> @@ -976,6 +976,24 @@ struct arm_smmu_device {
>  struct arm_smmu_stream {
>  	u32				id;
>  	struct arm_smmu_master		*master;
> +	/*
> +	 * ref_count > 1 means multiple masters share this SID.  Protected by
> +	 * smmu->streams_mutex.
> +	 */
> +	unsigned int			ref_count;
> +	/*
> +	 * When ref_count > 1 the STE has already been written by the first
> +	 * master; subsequent masters must skip the write.
> +	 */
> +	bool				ste_installed;
> +	/*
> +	 * Links all arm_smmu_stream objects that share the same SID across
> +	 * different masters.  The canonical (XArray-stored) entry is the list
> +	 * head; non-owning sharers are linked into it.  Used to transfer
> +	 * XArray ownership when the current owner is removed.
> +	 * Protected by smmu->streams_mutex.
> +	 */
> +	struct list_head		shared_link;
>  };

SID-sharing masters still allocate their own streams. Though the
SIDs are the same number, they own duplicated streams linked via
that "shared_link" list.

  master_a->stream[0] --> stream (SID=X) --|
                                      shared_link
  master_b->stream[0] --> stream (SID=X) --|

But maybe they could share the stream structure directly? I.e.

  master_a->stream[0] --|
                        |--> shared stream (SID=X)
  master_b->stream[0] --|


The stream structure would still need a list for shared_masters:

struct arm_smmu_stream {
[...]
	/* Every sharing master, ->master included. Empty unless shared */
	struct list_head shared_masters;
};

ref_count could be simply !list_empty(&stream->shared_masters).


Does i.MX need to support SID-sharing with multi-SD device? If not,
the master could have only a simple shared_masters_elm:

struct arm_smmu_master {
[....]
	/* Supports num_streams == 1 only. Empty if SID is not shared */
	struct list_head shared_masters_elm;
};

> @@ -1024,6 +1042,11 @@ struct arm_smmu_master {
>  	bool				ste_ats_enabled : 1;
>  	bool				stall_enabled;
>  	bool				ats_always_on;
> +	/*
> +	 * True when at least one of this master's SIDs is shared with another
> +	 * master.  SVA, stall and IOPF are disabled for such masters.
> +	 */
> +	bool				shared_sid;

Then, this could be !list_empty(&master->shared_masters_elm).

Nicolin