linux-kernel - [PATCH 2/5] PCI: dwc: Add new APIs to remove standard and extended Capability

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Message-Id: <20251109-remove_cap-v1-2-2208f46f4dc2@oss.qualcomm.com>
Date: Sun, 09 Nov 2025 22:59:41 -0800
From: Qiang Yu <qiang.yu@....qualcomm.com>
To: Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kwilczynski@...nel.org>,
        Manivannan Sadhasivam <mani@...nel.org>, Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>, Jingoo Han <jingoohan1@...il.com>
Cc: linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, Qiang Yu <qiang.yu@....qualcomm.com>,
        Wenbin Yao <wenbin.yao@....qualcomm.com>
Subject: [PATCH 2/5] PCI: dwc: Add new APIs to remove standard and extended
 Capability

On some platforms, certain PCIe Capabilities may be present in hardware
but are not fully implemented as defined in PCIe spec. These incomplete
capabilities should be hidden from the PCI framework to prevent unexpected
behavior.

Introduce two APIs to remove a specific PCIe Capability and Extended
Capability by updating the previous capability's next offset field to skip
over the unwanted capability. These APIs allow RC drivers to easily hide
unsupported or partially implemented capabilities from software.

Co-developed-by: Wenbin Yao <wenbin.yao@....qualcomm.com>
Signed-off-by: Wenbin Yao <wenbin.yao@....qualcomm.com>
Signed-off-by: Qiang Yu <qiang.yu@....qualcomm.com>
---
 drivers/pci/controller/dwc/pcie-designware.c | 53 ++++++++++++++++++++++++++++
 drivers/pci/controller/dwc/pcie-designware.h |  2 ++
 2 files changed, 55 insertions(+)

diff --git a/drivers/pci/controller/dwc/pcie-designware.c b/drivers/pci/controller/dwc/pcie-designware.c
index 5585d3ed74316bd218572484f6320019db8d6a10..24f8e9959cb81ca41e91d27057cc115d32e8d523 100644
--- a/drivers/pci/controller/dwc/pcie-designware.c
+++ b/drivers/pci/controller/dwc/pcie-designware.c
@@ -234,6 +234,59 @@ u16 dw_pcie_find_ext_capability(struct dw_pcie *pci, u8 cap)
 }
 EXPORT_SYMBOL_GPL(dw_pcie_find_ext_capability);
 
+void dw_pcie_remove_capability(struct dw_pcie *pci, u8 cap)
+{
+	u8 cap_pos, pre_pos, next_pos;
+	u16 reg;
+
+	cap_pos = PCI_FIND_NEXT_CAP(dw_pcie_read_cfg, PCI_CAPABILITY_LIST, cap,
+				 &pre_pos, pci);
+	if (!cap_pos)
+		return;
+
+	reg = dw_pcie_readw_dbi(pci, cap_pos);
+	next_pos = (reg & 0xff00) >> 8;
+
+	dw_pcie_dbi_ro_wr_en(pci);
+	if (pre_pos == PCI_CAPABILITY_LIST)
+		dw_pcie_writeb_dbi(pci, PCI_CAPABILITY_LIST, next_pos);
+	else
+		dw_pcie_writeb_dbi(pci, pre_pos + 1, next_pos);
+	dw_pcie_dbi_ro_wr_dis(pci);
+}
+EXPORT_SYMBOL_GPL(dw_pcie_remove_capability);
+
+void dw_pcie_remove_ext_capability(struct dw_pcie *pci, u8 cap)
+{
+	int cap_pos, next_pos, pre_pos;
+	u32 pre_header, header;
+
+	cap_pos = PCI_FIND_NEXT_EXT_CAP(dw_pcie_read_cfg, 0, cap, &pre_pos, pci);
+	if (!cap_pos)
+		return;
+
+	header = dw_pcie_readl_dbi(pci, cap_pos);
+	/*
+	 * If the first cap at offset PCI_CFG_SPACE_SIZE is removed,
+	 * only set it's capid to zero as it cannot be skipped.
+	 */
+	if (cap_pos == PCI_CFG_SPACE_SIZE) {
+		dw_pcie_dbi_ro_wr_en(pci);
+		dw_pcie_writel_dbi(pci, cap_pos, header & 0xffff0000);
+		dw_pcie_dbi_ro_wr_dis(pci);
+		return;
+	}
+
+	pre_header = dw_pcie_readl_dbi(pci, pre_pos);
+	next_pos = PCI_EXT_CAP_NEXT(header);
+
+	dw_pcie_dbi_ro_wr_en(pci);
+	dw_pcie_writel_dbi(pci, pre_pos,
+			  (pre_header & 0xfffff) | (next_pos << 20));
+	dw_pcie_dbi_ro_wr_dis(pci);
+}
+EXPORT_SYMBOL_GPL(dw_pcie_remove_ext_capability);
+
 static u16 __dw_pcie_find_vsec_capability(struct dw_pcie *pci, u16 vendor_id,
 					  u16 vsec_id)
 {
diff --git a/drivers/pci/controller/dwc/pcie-designware.h b/drivers/pci/controller/dwc/pcie-designware.h
index e995f692a1ecd10130d3be3358827f801811387f..b68dbc528001b63448db8b1a93bf56a5e53bd33e 100644
--- a/drivers/pci/controller/dwc/pcie-designware.h
+++ b/drivers/pci/controller/dwc/pcie-designware.h
@@ -552,6 +552,8 @@ void dw_pcie_version_detect(struct dw_pcie *pci);
 
 u8 dw_pcie_find_capability(struct dw_pcie *pci, u8 cap);
 u16 dw_pcie_find_ext_capability(struct dw_pcie *pci, u8 cap);
+void dw_pcie_remove_capability(struct dw_pcie *pci, u8 cap);
+void dw_pcie_remove_ext_capability(struct dw_pcie *pci, u8 cap);
 u16 dw_pcie_find_rasdes_capability(struct dw_pcie *pci);
 u16 dw_pcie_find_ptm_capability(struct dw_pcie *pci);
 

-- 
2.34.1