[RFC PATCH] iommu/arm-smmu-v3: Allow stream ID sharing for devices behind PCIe-to-PCI bridges

[Jason Gunthorpe]

On Thu, Dec 11, 2025 at 05:39:33AM -0800, Nirmoy Das wrote:
> ASPEED BMC controllers have VGA and USB functions behind a PCIe-to-PCI
> bridge that causes them to share the same stream ID:
> 
>   \-[0005:e0]---00.0-[e1-e2]----00.0-[e2]--+-00.0  ASPEED Graphics Family
>                                            \-02.0  ASPEED USB Controller
> 
> Both devices get stream ID(0x5e200) due to bridge aliasing. Currently this
> USB controller gets rejected with "Aliasing StreamID unsupported", causing
> USB timeout errors. Allow stream ID sharing when both devices are on
> the same bus behind a PCIe-to-PCI bridge to fix this issue.

I don't think this is right, if you actually need stream ID sharing
then I seem to recall the driver needs more changes. We don't want
this in general.

IMHO ASPEED has wrong PCI config space and there isn't any actual PCI
bus here that would force RID aliasing to worry about. I suggest going
to ASPEED to find out and confirm that they don't actually share the
RIDs and then let's quirk the PCI devices to disable alias generation
in the first place.

Also, complain to them to fix their config space to report a PCIe bridge
type in future devices. :)

Jason