On Wed, 10 Sept 2025 at 23:09, Manivannan Sadhasivam via B4 Relay
<devnull+manivannan.sadhasivam.oss.qualcomm.com@xxxxxxxxxx> wrote:
>
> Hi,
>
> This series fixes the long standing issue with ACS in DT platforms. There are
> two fixes in this series, both fixing independent issues on their own, but both
> are needed to properly enable ACS on DT platforms (well, patch 1 is only needed
> for Juno board, but that was a blocker for patch 2, more below...).
>
> Issue(s) background
> ===================
>
> Back in 2024, Xingang Wang first noted a failure in attaching the HiSilicon SEC
> device to QEMU ARM64 pci-root-port device [1]. He then tracked down the issue to
> ACS not being enabled for the QEMU Root Port device and he proposed a patch to
> fix it [2].
>
> Once the patch got applied, people reported PCIe issues with linux-next on the
> ARM Juno Development boards, where they saw failure in enumerating the endpoint
> devices [3][4]. So soon, the patch got dropped, but the actual issue with the
> ARM Juno boards was left behind.
>
> Fast forward to 2024, Pavan resubmitted the same fix [5] for his own usecase,
> hoping that someone in the community would fix the issue with ARM Juno boards.
> But the patch was rightly rejected, as a patch that was known to cause issues
> should not be merged to the kernel. But again, no one investigated the Juno
> issue and it was left behind again.
>
> Now it ended up in my plate and I managed to track down the issue with the help
> of Naresh who got access to the Juno boards in LKFT. The Juno issue is with the
> PCIe switch from Microsemi/IDT, which triggers ACS Source Validation error on
> Completions received for the Configuration Read Request from a device connected
> to the downstream port that has not yet captured the PCIe bus number. As per the
> PCIe spec r6.0 sec 2.2.6.2, "Functions must capture the Bus and Device Numbers
> supplied with all Type 0 Configuration Write Requests completed by the Function
> and supply these numbers in the Bus and Device Number fields of the Requester ID
> for all Requests". So during the first Configuration Read Request issued by the
> switch downstream port during enumeration (for reading Vendor ID), Bus and
> Device numbers will be unknown to the device. So it responds to the Read Request
> with Completion having Bus and Device number as 0. The switch interprets the
> Completion as an ACS Source Validation error and drops the completion, leading
> to the failure in detecting the endpoint device. Though the PCIe spec r6.0, sec
> 6.12.1.1, states that "Completions are never affected by ACS Source Validation".
> This behavior is in violation of the spec.
>
> This issue was already found and addressed with a quirk for a different device
> from Microsemi with 'commit, aa667c6408d2 ("PCI: Workaround IDT switch ACS
> Source Validation erratum")'. Apparently, this issue seems to be documented in
> the erratum #36 of IDT 89H32H8G3-YC, which is not publicly available.
>
> Solution for Juno issue
> =======================
>
> To fix this issue, I've extended the quirk to the Device ID of the switch
> found in Juno R2 boards. I believe the same switch is also present in Juno R1
> board as well.
>
> With Patch 1, the Juno R2 boards can now detect the endpoints even with ACS
> enabled for the Switch downstream ports. Finally, I added patch 2 that properly
> enables ACS for all the PCI devices on DT platforms.
>
> It should be noted that even without patch 2 which enables ACS for the Root
> Port, the Juno boards were failing since 'commit, bcb81ac6ae3c ("iommu: Get
> DT/ACPI parsing into the proper probe path")' as reported in LKFT [6]. I
> believe, this commit made sure pci_request_acs() gets called before the
> enumeration of the switch downstream ports. The LKFT team ended up disabling
> ACS using cmdline param 'pci=config_acs=000000@pci:0:0'. So I added the above
> mentioned commit as a Fixes tag for patch 1.
>
> Also, to mitigate this issue, one could enumerate all the PCIe devices in
> bootloader without enabling ACS (as also noted by Robin in the LKFT thread).
> This will make sure that the endpoint device has a valid bus number when it
> responds to the first Configuration Read Request from the switch downstream
> port. So the ACS Source Validation error doesn't get triggered.
>
> Solution for ACS issue
> ======================
>
> To fix this issue, I've kept the patch from Xingang as is (with rewording of the
> patch subject/description). This patch moves the pci_request_acs() call to
> devm_of_pci_bridge_init(), which gets called during the host bridge
> registration. This makes sure that the 'pci_acs_enable' flag set by
> pci_request_acs() is getting set before the enumeration of the Root Port device.
> So now, ACS will be enabled for all ACS capable devices of DT platforms.
I have applied this patch series on top of Linux next-20250910 and
next-20250911 tags and tested.
>
> [1] https://lore.kernel.org/all/038397a6-57e2-b6fc-6e1c-7c03b7be9d96@xxxxxxxxxx
> [2] https://lore.kernel.org/all/1621566204-37456-1-git-send-email-wangxingang5@xxxxxxxxxx
> [3] https://lore.kernel.org/all/01314d70-41e6-70f9-e496-84091948701a@xxxxxxxxxxx
> [4] https://lore.kernel.org/all/CADYN=9JWU3CMLzMEcD5MSQGnaLyDRSKc5SofBFHUax6YuTRaJA@xxxxxxxxxxxxxx
> [5] https://lore.kernel.org/linux-pci/20241107-pci_acs_fix-v1-1-185a2462a571@xxxxxxxxxxx
> [6] https://lists.linaro.org/archives/list/lkft-triage@xxxxxxxxxxxxxxxx/message/CBYO7V3C5TGYPKCMWEMNFFMRYALCUDTK
>
> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@xxxxxxxxxxxxxxxx>
Tested-by: Linux Kernel Functional Testing <lkft@xxxxxxxxxx>
Tested-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
> ---
> Manivannan Sadhasivam (1):
> PCI: Extend pci_idt_bus_quirk() for IDT switch with Device ID 0x8090
>
> Xingang Wang (1):
> iommu/of: Call pci_request_acs() before enumerating the Root Port device
>
> drivers/iommu/of_iommu.c | 1 -
> drivers/pci/of.c | 8 +++++++-
> drivers/pci/probe.c | 2 +-
> 3 files changed, 8 insertions(+), 3 deletions(-)
> ---
> base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
> change-id: 20250910-pci-acs-cb4fa3983a2c
>
> Best regards,
> --
> Manivannan Sadhasivam <manivannan.sadhasivam@xxxxxxxxxxxxxxxx>
>
>
--
Linaro LKFT
