linux-kernel - Re: [PATCH] PCI: Add quirk for ASPEED AST1150 bridge to prevent false RID aliasing

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Message-ID: <22df4586-00da-408b-99c2-4592dade4d90@nvidia.com>
Date: Wed, 17 Dec 2025 16:37:46 +0100
From: Nirmoy Das <nirmoyd@...dia.com>
To: Robin Murphy <robin.murphy@....com>, Bjorn Helgaas <bhelgaas@...gle.com>
CC: <linux-pci@...r.kernel.org>, <linux-kernel@...r.kernel.org>, "Jason
 Gunthorpe" <jgg@...dia.com>, Will Deacon <will@...nel.org>, Joerg Roedel
	<joro@...tes.org>, <iommu@...ts.linux.dev>, <jammy_huang@...eedtech.com>
Subject: Re: [PATCH] PCI: Add quirk for ASPEED AST1150 bridge to prevent false
 RID aliasing


On 17.12.25 14:48, Robin Murphy wrote:
> On 17/12/2025 1:32 pm, 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:
>>
>>    [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, causing the
>> USB controller to be rejected with 'Aliasing StreamID unsupported'.
>>
>> Per ASPEED, the AST1150 operates in PCI mode where downstream devices
>> generate their own distinct Requester IDs. Set
>> PCI_DEV_FLAGS_BRIDGE_XLATE_ROOT to stop false alias detection.
>
> I don't think that's really the right quirk - that one effectively 
> means "RID aliasing upstream of this bridge is irrelevant", whereas 
> what you want here is "RID aliasing *at* this bridge doesn't happen 
> even though it looks like it could". With BRIDGE_XLATE_ROOT, I think 
> if you then put this behind a contrived upstream chain of additional 
> PCIe->PCI->PCIE bridges that *could* legitimately introduce further 
> aliasing, it would give the wrong result.

Thanks Robin. You're right - I'll send a v2 using the a new 
PCI_DEV_FLAGS_PCI_BRIDGE_NO_ALIASES quirk Jason suggested, which 
correctly handles only aliasing at this bridge.


>
> Thanks,
> Robin.
>
>> Signed-off-by: Nirmoy Das <nirmoyd@...dia.com>
>> ---
>>   drivers/pci/quirks.c | 14 ++++++++++++++
>>   1 file changed, 14 insertions(+)
>>
>> diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
>> index b9c252aa6fe0..a7a1bf4c4354 100644
>> --- a/drivers/pci/quirks.c
>> +++ b/drivers/pci/quirks.c
>> @@ -4453,6 +4453,20 @@ 
>> DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_BROADCOM, 0x9000,
>>   DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_BROADCOM, 0x9084,
>>                   quirk_bridge_cavm_thrx2_pcie_root);
>>   +/*
>> + * ASPEED AST1150 is a PCIe-to-PCI bridge that operates in PCI mode 
>> (not PCI-X).
>> + * Although it reports as a PCIe-to-PCI bridge, the downstream PCI 
>> bus does not
>> + * perform conventional Requester ID aliasing - each device behind 
>> the bridge
>> + * generates its own distinct Requester ID. This quirk stops false 
>> alias
>> + * detection at the bridge, fixing IOMMU StreamID conflicts that 
>> break DMA for
>> + * devices like the USB controller behind this bridge.
>> + */
>> +static void quirk_aspeed_ast1150_bridge(struct pci_dev *pdev)
>> +{
>> +    pdev->dev_flags |= PCI_DEV_FLAGS_BRIDGE_XLATE_ROOT;
>> +}
>> +DECLARE_PCI_FIXUP_HEADER(0x1a03, 0x1150, quirk_aspeed_ast1150_bridge);
>> +
>>   /*
>>    * Intersil/Techwell TW686[4589]-based video capture cards have an 
>> empty (zero)
>>    * class code.  Fix it.
>