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.

[1] https://lore.kernel.org/all/038397a6-57e2-b6fc-6e1c-7c03b7be9d96@huawei.com [2] https://lore.kernel.org/all/1621566204-37456-1-git-send-email-wangxingang5@h... [3] https://lore.kernel.org/all/01314d70-41e6-70f9-e496-84091948701a@samsung.com [4] https://lore.kernel.org/all/CADYN=9JWU3CMLzMEcD5MSQGnaLyDRSKc5SofBFHUax6YuTR... [5] https://lore.kernel.org/linux-pci/20241107-pci_acs_fix-v1-1-185a2462a571@qui... [6] https://lists.linaro.org/archives/list/lkft-triage@lists.linaro.org/message/...

Signed-off-by: Manivannan Sadhasivam manivannan.sadhasivam@oss.qualcomm.com --- 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,