Re: [PATCH RFC v2 1/4] iommu/arm-smmu-v3: Convert streams from RB tree to XArray

[Nicolin Chen]

On Mon, Sep 21, 2026 at 07:35:00PM +0800, Peng Fan (OSS) wrote:
> From: Peng Fan <peng.fan@xxxxxxx>
> 
> Replace the smmu->streams RB tree with an XArray for SID ->
> arm_smmu_stream lookups. The existing streams_mutex serialises all
> accesses (both xa_store/xa_erase and xa_load), protecting the lifetime
> of returned pointers against concurrent arm_smmu_remove_master()
> without requiring RCU grace periods. A mutex (rather than xa_lock) is
> needed because several paths sleep while the lock is held:
> dmam_alloc_coherent(GFP_KERNEL) in arm_smmu_init_sid_strtab(), and
> down_read() inside iommu_report_device_fault().

The commit message reads very unconvincing.

The first paragraph starts with an action "replace" without giving
a clear reason for the action.

> This removes the RB tree comparators, the rb_node from

Then the second paragraph repeats the action...

> arm_smmu_stream, and simplifies duplicate-SID handling for bridged PCI
> devices.

... and slightly mentions "simplifies". Maybe it should write about
why this is necessary and how xarray would simplify vs RB tree?

Nicolin