linux-kernel - [PATCH v2 3/6] PCI: brcmstb: Add "refusal mode" to preclude PCIe-induced CPU aborts

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Message-Id: <20220716222454.29914-4-jim2101024@gmail.com>
Date:   Sat, 16 Jul 2022 18:24:50 -0400
From:   Jim Quinlan <jim2101024@...il.com>
To:     linux-pci@...r.kernel.org,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Cyril Brulebois <kibi@...ian.org>,
        bcm-kernel-feedback-list@...adcom.com, jim2101024@...il.com,
        james.quinlan@...adcom.com
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        linux-rpi-kernel@...ts.infradead.org (moderated list:BROADCOM
        BCM2711/BCM2835 ARM ARCHITECTURE),
        linux-arm-kernel@...ts.infradead.org (moderated list:BROADCOM
        BCM2711/BCM2835 ARM ARCHITECTURE),
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH v2 3/6] PCI: brcmstb: Add "refusal mode" to preclude PCIe-induced CPU aborts

Our PCIe RC HW has an atypical behavior: if it does not have PCIe link
established between itself and downstream, any subsequent config space
access causes a CPU abort.  This commit sets a "refusal mode" if the PCIe
link-up fails, and this has our pci_ops map_bus function returning a NULL
address, which in turn precludes the access from happening.

Right now, "refusal mode" is window dressing.  It will become relevant
in a future commit when brcm_pcie_start_link() is invoked during
enumeration instead of before it.

Signed-off-by: Jim Quinlan <jim2101024@...il.com>
---
 drivers/pci/controller/pcie-brcmstb.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/drivers/pci/controller/pcie-brcmstb.c b/drivers/pci/controller/pcie-brcmstb.c
index c026446d5830..72219a4f3964 100644
--- a/drivers/pci/controller/pcie-brcmstb.c
+++ b/drivers/pci/controller/pcie-brcmstb.c
@@ -255,6 +255,7 @@ struct brcm_pcie {
 	u32			hw_rev;
 	void			(*perst_set)(struct brcm_pcie *pcie, u32 val);
 	void			(*bridge_sw_init_set)(struct brcm_pcie *pcie, u32 val);
+	bool			refusal_mode;
 };
 
 static inline bool is_bmips(const struct brcm_pcie *pcie)
@@ -687,6 +688,19 @@ static void __iomem *brcm_pcie_map_conf(struct pci_bus *bus, unsigned int devfn,
 	if (pci_is_root_bus(bus))
 		return PCI_SLOT(devfn) ? NULL : base + where;
 
+	if (pcie->refusal_mode) {
+		/*
+		 * At this point we do not have PCIe link-up.  If there is
+		 * a config read or write access besides those targeting
+		 * the host bridge, our PCIe HW throws a CPU abort.  To
+		 * prevent this we return the NULL address.  The calling
+		 * functions -- pci_generic_config_*() -- will notice this
+		 * and not perform the access, and if it is a read access,
+		 * 0xffffffff is returned.
+		 */
+		return NULL;
+	}
+
 	/* For devices, write to the config space index register */
 	idx = PCIE_ECAM_OFFSET(bus->number, devfn, 0);
 	writel(idx, pcie->base + PCIE_EXT_CFG_INDEX);
@@ -704,6 +718,11 @@ static void __iomem *brcm_pcie_map_conf32(struct pci_bus *bus, unsigned int devf
 	if (pci_is_root_bus(bus))
 		return PCI_SLOT(devfn) ? NULL : base + (where & ~0x3);
 
+	if (pcie->refusal_mode) {
+		/* See note above in brcm_pcie_map_conf() */
+		return NULL;
+	}
+
 	/* For devices, write to the config space index register */
 	idx = PCIE_ECAM_OFFSET(bus->number, devfn, (where & ~3));
 	writel(idx, base + IDX_ADDR(pcie));
@@ -989,6 +1008,7 @@ static int brcm_pcie_start_link(struct brcm_pcie *pcie)
 		dev_err(dev, "link down\n");
 		return -ENODEV;
 	}
+	pcie->refusal_mode = false;
 
 	if (!brcm_pcie_rc_mode(pcie)) {
 		dev_err(dev, "PCIe misconfigured; is in EP mode\n");
@@ -1134,6 +1154,8 @@ static void brcm_pcie_turn_off(struct brcm_pcie *pcie)
 	void __iomem *base = pcie->base;
 	int tmp;
 
+	pcie->refusal_mode = true;
+
 	if (brcm_pcie_link_up(pcie))
 		brcm_pcie_enter_l23(pcie);
 	/* Assert fundamental reset */
@@ -1185,6 +1207,7 @@ static int brcm_pcie_resume(struct device *dev)
 	u32 tmp;
 	int ret;
 
+	pcie->refusal_mode = true;
 	base = pcie->base;
 	ret = clk_prepare_enable(pcie->clk);
 	if (ret)
@@ -1361,6 +1384,7 @@ static int brcm_pcie_probe(struct platform_device *pdev)
 	pcie->type = data->type;
 	pcie->perst_set = data->perst_set;
 	pcie->bridge_sw_init_set = data->bridge_sw_init_set;
+	pcie->refusal_mode = true;
 
 	pcie->base = devm_platform_ioremap_resource(pdev, 0);
 	if (IS_ERR(pcie->base))
-- 
2.17.1