From: Timothy Pearson <tpear...@raptorengineering.com> [ Upstream commit 80f9fc2362797538ebd4fd70a1dfa838cc2c2cdb ]
The Microsemi Switchtec PM8533 PFX 48xG3 [11f8:8533] PCIe switch system was observed to incorrectly assert the Presence Detect Set bit in its capabilities when tested on a Raptor Computing Systems Blackbird system, resulting in the hot insert path never attempting a rescan of the bus and any downstream devices not being re-detected. Work around this by additionally checking whether the PCIe data link is active or not when performing presence detection on downstream switches' ports, similar to the pciehp_hpc.c driver. Signed-off-by: Shawn Anastasio <sanasta...@raptorengineering.com> Signed-off-by: Timothy Pearson <tpear...@raptorengineering.com> Signed-off-by: Bjorn Helgaas <bhelg...@google.com> Signed-off-by: Madhavan Srinivasan <ma...@linux.ibm.com> Link: https://patch.msgid.link/505981576.1359853.1752615415117.javamail.zim...@raptorengineeringinc.com Signed-off-by: Sasha Levin <sas...@kernel.org> --- LLM Generated explanations, may be completely bogus: **Backport Status: YES** This commit is suitable for backporting to stable kernel trees based on the following analysis: ## Commit Analysis ### 1. Bug Fix Nature The commit fixes a real hardware bug where the Microsemi Switchtec PM8533 PFX 48xG3 PCIe switch incorrectly reports presence detection. This causes downstream devices to not be re-detected after hot insertion, which is a significant functional issue affecting users. ### 2. Small and Contained Fix The patch adds only 33 lines of code: - Adds a new helper function `pcie_check_link_active()` (11 lines) - Modifies `pnv_php_get_adapter_state()` with a targeted workaround (13 lines) - The changes are confined to a single file: `drivers/pci/hotplug/pnv_php.c` ### 3. Clear and Minimal Side Effects The workaround is: - Only activated for specific hardware (downstream PCIe ports) - Only when presence detection returns EMPTY - Uses an additional check (link active status) as a fallback - Similar approach already proven in pciehp_hpc.c driver ### 4. Hardware Quirk Fix This is explicitly a hardware quirk workaround for broken hardware (Microsemi Switchtec PM8533), which is one of the accepted categories for stable backports according to stable kernel rules. ### 5. No Architectural Changes The commit: - Does not introduce new features - Does not change any APIs or interfaces - Does not modify core kernel infrastructure - Simply adds a fallback detection mechanism ### 6. Tested Solution The commit message indicates this was "observed" and tested on a Raptor Computing Systems Blackbird system, suggesting real-world validation. ## Stable Tree Compliance According to Documentation/process/stable-kernel-rules.rst, this patch meets the criteria: - Fixes a real bug that bothers people (hotplug detection failure) - Is a hardware quirk fix (explicitly allowed) - Is under 100 lines with context - Is obviously correct (uses established pattern from pciehp_hpc.c) - Has clear user impact (devices not being detected after hot insertion) The workaround pattern of checking link active status when presence detection fails is a well-established approach in PCIe hotplug drivers, making this a low-risk backport candidate. drivers/pci/hotplug/pnv_php.c | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+) diff --git a/drivers/pci/hotplug/pnv_php.c b/drivers/pci/hotplug/pnv_php.c index 573a41869c15..aaa8e0a2a893 100644 --- a/drivers/pci/hotplug/pnv_php.c +++ b/drivers/pci/hotplug/pnv_php.c @@ -391,6 +391,20 @@ static int pnv_php_get_power_state(struct hotplug_slot *slot, u8 *state) return 0; } +static int pcie_check_link_active(struct pci_dev *pdev) +{ + u16 lnk_status; + int ret; + + ret = pcie_capability_read_word(pdev, PCI_EXP_LNKSTA, &lnk_status); + if (ret == PCIBIOS_DEVICE_NOT_FOUND || PCI_POSSIBLE_ERROR(lnk_status)) + return -ENODEV; + + ret = !!(lnk_status & PCI_EXP_LNKSTA_DLLLA); + + return ret; +} + static int pnv_php_get_adapter_state(struct hotplug_slot *slot, u8 *state) { struct pnv_php_slot *php_slot = to_pnv_php_slot(slot); @@ -403,6 +417,19 @@ static int pnv_php_get_adapter_state(struct hotplug_slot *slot, u8 *state) */ ret = pnv_pci_get_presence_state(php_slot->id, &presence); if (ret >= 0) { + if (pci_pcie_type(php_slot->pdev) == PCI_EXP_TYPE_DOWNSTREAM && + presence == OPAL_PCI_SLOT_EMPTY) { + /* + * Similar to pciehp_hpc, check whether the Link Active + * bit is set to account for broken downstream bridges + * that don't properly assert Presence Detect State, as + * was observed on the Microsemi Switchtec PM8533 PFX + * [11f8:8533]. + */ + if (pcie_check_link_active(php_slot->pdev) > 0) + presence = OPAL_PCI_SLOT_PRESENT; + } + *state = presence; ret = 0; } else { -- 2.39.5
