[PATCH v7 2/3] PCI: brcmstb: Configure HW CLKREQ# mode appropriate for downstream device

[Jim Quinlan]

The Broadcom STB/CM PCIe HW core, which is also used in RPi SOCs, must be
deliberately set by the PCIe RC HW into one of three mutually exclusive
modes:

"safe" -- No CLKREQ# expected or required, refclk is always provided.  This
    mode should work for all devices but is not be capable of any refclk
    power savings.

"no-l1ss" -- CLKREQ# is expected to be driven by the downstream device for
    CPM and ASPM L0s and L1.  Provides Clock Power Management, L0s, and L1,
    but cannot provide L1 substate (L1SS) power savings. If the downstream
    device connected to the RC is L1SS capable AND the OS enables L1SS, all
    PCIe traffic may abruptly halt, potentially hanging the system.

"default" -- Bidirectional CLKREQ# between the RC and downstream device.
    Provides ASPM L0s, L1, and L1SS, but not compliant to provide Clock
    Power Management; specifically, may not be able to meet the Tclron max
    timing of 400ns as specified in "Dynamic Clock Control", section
    3.2.5.2.2 of the PCIe spec.  This situation is atypical and should
    happen only with older devices.

Previously, this driver always set the mode to "no-l1ss", as almost all
STB/CM boards operate in this mode.  But now there is interest in
activating L1SS power savings from STB/CM customers, which requires "aspm"
mode.  In addition, a bug was filed for RPi4 CM platform because most
devices did not work in "no-l1ss" mode.

Note that the mode is specified by the DT property "brcm,clkreq-mode".  If
this property is omitted, then "default" mode is chosen.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217276

Signed-off-by: Jim Quinlan <james.quinlan@xxxxxxxxxxxx>
---
 drivers/pci/controller/pcie-brcmstb.c | 65 ++++++++++++++++++++++-----
 1 file changed, 55 insertions(+), 10 deletions(-)

diff --git a/drivers/pci/controller/pcie-brcmstb.c b/drivers/pci/controller/pcie-brcmstb.c
index f9dd6622fe10..f45c5d0168d3 100644
--- a/drivers/pci/controller/pcie-brcmstb.c
+++ b/drivers/pci/controller/pcie-brcmstb.c
@@ -121,9 +121,12 @@
 
 #define PCIE_MISC_HARD_PCIE_HARD_DEBUG					0x4204
 #define  PCIE_MISC_HARD_PCIE_HARD_DEBUG_CLKREQ_DEBUG_ENABLE_MASK	0x2
+#define  PCIE_MISC_HARD_PCIE_HARD_DEBUG_L1SS_ENABLE_MASK		0x200000
 #define  PCIE_MISC_HARD_PCIE_HARD_DEBUG_SERDES_IDDQ_MASK		0x08000000
 #define  PCIE_BMIPS_MISC_HARD_PCIE_HARD_DEBUG_SERDES_IDDQ_MASK		0x00800000
-
+#define  PCIE_CLKREQ_MASK \
+	  (PCIE_MISC_HARD_PCIE_HARD_DEBUG_CLKREQ_DEBUG_ENABLE_MASK | \
+	   PCIE_MISC_HARD_PCIE_HARD_DEBUG_L1SS_ENABLE_MASK)
 
 #define PCIE_INTR2_CPU_BASE		0x4300
 #define PCIE_MSI_INTR2_BASE		0x4500
@@ -1028,13 +1031,61 @@ static int brcm_pcie_setup(struct brcm_pcie *pcie)
 	return 0;
 }
 
+static void brcm_config_clkreq(struct brcm_pcie *pcie)
+{
+	static const char err_msg[] = "invalid 'brcm,clkreq-mode' DT string\n";
+	const char *mode = "default";
+	u32 clkreq_cntl;
+	int ret;
+
+	ret = of_property_read_string(pcie->np, "brcm,clkreq-mode", &mode);
+	if (ret && ret != -EINVAL) {
+		dev_err(pcie->dev, err_msg);
+		mode = "safe";
+	}
+
+	/* Start out assuming safe mode (both mode bits cleared) */
+	clkreq_cntl = readl(pcie->base + PCIE_MISC_HARD_PCIE_HARD_DEBUG);
+	clkreq_cntl &= ~PCIE_CLKREQ_MASK;
+
+	if (strcmp(mode, "no-l1ss") == 0) {
+		/*
+		 * "no-l1ss" -- Provides Clock Power Management, L0s, and
+		 * L1, but cannot provide L1 substate (L1SS) power
+		 * savings. If the downstream device connected to the RC is
+		 * L1SS capable AND the OS enables L1SS, all PCIe traffic
+		 * may abruptly halt, potentially hanging the system.
+		 */
+		clkreq_cntl |= PCIE_MISC_HARD_PCIE_HARD_DEBUG_CLKREQ_DEBUG_ENABLE_MASK;
+	} else if (strcmp(mode, "default") == 0) {
+		/*
+		 * "default" -- Provides L0s, L1, and L1SS, but not
+		 * compliant to provide Clock Power Management;
+		 * specifically, may not be able to meet the Tclron max
+		 * timing of 400ns as specified in "Dynamic Clock Control",
+		 * section 3.2.5.2.2 of the PCIe spec.  This situation is
+		 * atypical and should happen only with older devices.
+		 */
+		clkreq_cntl |= PCIE_MISC_HARD_PCIE_HARD_DEBUG_L1SS_ENABLE_MASK;
+	} else {
+		/*
+		 * "safe" -- No power savings; refclk is driven by RC
+		 * unconditionally.
+		 */
+		if (strcmp(mode, "safe") != 0)
+			dev_err(pcie->dev, err_msg);
+		mode = "safe";
+	}
+	writel(clkreq_cntl, pcie->base + PCIE_MISC_HARD_PCIE_HARD_DEBUG);
+	dev_info(pcie->dev, "clkreq-mode set to %s\n", mode);
+}
+
 static int brcm_pcie_start_link(struct brcm_pcie *pcie)
 {
 	struct device *dev = pcie->dev;
 	void __iomem *base = pcie->base;
 	u16 nlw, cls, lnksta;
 	bool ssc_good = false;
-	u32 tmp;
 	int ret, i;
 
 	/* Unassert the fundamental reset */
@@ -1059,6 +1110,8 @@ static int brcm_pcie_start_link(struct brcm_pcie *pcie)
 		return -ENODEV;
 	}
 
+	brcm_config_clkreq(pcie);
+
 	if (pcie->gen)
 		brcm_pcie_set_gen(pcie, pcie->gen);
 
@@ -1077,14 +1130,6 @@ static int brcm_pcie_start_link(struct brcm_pcie *pcie)
 		 pci_speed_string(pcie_link_speed[cls]), nlw,
 		 ssc_good ? "(SSC)" : "(!SSC)");
 
-	/*
-	 * Refclk from RC should be gated with CLKREQ# input when ASPM L0s,L1
-	 * is enabled => setting the CLKREQ_DEBUG_ENABLE field to 1.
-	 */
-	tmp = readl(base + PCIE_MISC_HARD_PCIE_HARD_DEBUG);
-	tmp |= PCIE_MISC_HARD_PCIE_HARD_DEBUG_CLKREQ_DEBUG_ENABLE_MASK;
-	writel(tmp, base + PCIE_MISC_HARD_PCIE_HARD_DEBUG);
-
 	return 0;
 }
 
-- 
2.17.1

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