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

[Peng Fan <peng.fan@oss.nxp.com>]

On Mon, Sep 21, 2026 at 04:15:18PM -0700, Nicolin Chen wrote:
>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).

Agree.

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

For i.MX, there is no need to support SID sharing for multi-SID device for now.
Not sure, in future there are cases that needs SID sharing for multi-SID device.

I could update in V3 following your suggestion.

Thanks,
Peng

>
>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
>
>