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X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Jan 27, 2026 at 01:50:54PM +0000, Will Deacon wrote: > Argh, I had conflated a transaction using SSID 0 vs a transaction > without a substream at all. So I think this makes sense now... Yeah, it is bit subtle, but as a SW choice the iommu subsystem reserves PASID 0/SSID 0 as the "untagged" translation. Several HW's force this in their implementation (ie AMD) ARM however includes a "Substream Valid" in the input bus. Linux doesn't use the combination "Substream Valid, SSID=0", that should never occur. If it wrongly does happen then IDENTITY will generate a fault, either C_BAD_CD (due to it being non-valid) or C_BAD_SUBSTREAMID (due to S1CDMax disabling substreams). While PAGING will either fault with C_BAD_SUBSTREAMID (S2 paging domain) or success when S1DSS=b10. Jason