PCI: pcie-brcmstb: optionally extend Tperst_clk time

Some endpoints need longer than the minimum Tperst_clk time of 100us
that the PCIe specification allows for, as they may need to sequence
internal resets off the stable output of internal PLLs prior to removal
of fundamental reset. PCIe switches are an especially bad case, in some
cases requiring up to 100 milliseconds for stable downstream link
behaviour.

Parse the DT property brcm,tperst-clk-ms and use this to hold PERST# low
during brcm_pcie_start_link().

The BRCM RC typically outputs 200us of stable refclk before deasserting
PERST#. By masking/forcing the output signal while deasserting the
internal reset, the effect is to extend the length of time that the
refclk is active and stable before PERST# is released.

The TX lanes will enter the Polling state before PERST# is released, but
this appears to be harmless.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>

Author: P33M

drivers/pci/controller/pcie-brcmstb.c

@@ -134,6 +134,7 @@
 		PCIE_MISC_CPU_2_PCIE_MEM_WIN0_LIMIT_HI + ((win) * 8)
 
 #define  PCIE_MISC_HARD_PCIE_HARD_DEBUG_CLKREQ_DEBUG_ENABLE_MASK	0x2
+#define  PCIE_MISC_HARD_PCIE_HARD_DEBUG_PERST_ASSERT_MASK		0x8
 #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
@@ -360,6 +361,7 @@ struct brcm_pcie {
 	struct subdev_regulators *sr;
 	bool			ep_wakeup_capable;
 	const struct pcie_cfg_data	*cfg;
+	u32			tperst_clk_ms;
 };
 
 static inline bool is_bmips(const struct brcm_pcie *pcie)
@@ -1487,13 +1489,32 @@ static int brcm_pcie_start_link(struct brcm_pcie *pcie)
 	u16 nlw, cls, lnksta, tmp16;
 	bool ssc_good = false;
 	int ret, i;
+	u32 tmp;
 
 	/* Limit the generation if specified */
 	if (pcie->gen)
 		brcm_pcie_set_gen(pcie, pcie->gen);
 
 	/* Unassert the fundamental reset */
-	ret = pcie->cfg->perst_set(pcie, 0);
+	if (pcie->tperst_clk_ms) {
+		/*
+		 * Increase Tperst_clk time by forcing PERST# output low while
+		 * the internal reset is released, so the PLL generates stable
+		 * refclk output further in advance of PERST# deassertion.
+		 */
+		tmp = readl(pcie->base + HARD_DEBUG(pcie));
+		u32p_replace_bits(&tmp, 1, PCIE_MISC_HARD_PCIE_HARD_DEBUG_PERST_ASSERT_MASK);
+		writel(tmp, pcie->base + HARD_DEBUG(pcie));
+
+		ret = pcie->cfg->perst_set(pcie, 0);
+		fsleep(pcie->tperst_clk_ms * USEC_PER_MSEC);
+
+		tmp = readl(pcie->base + HARD_DEBUG(pcie));
+		u32p_replace_bits(&tmp, 0, PCIE_MISC_HARD_PCIE_HARD_DEBUG_PERST_ASSERT_MASK);
+		writel(tmp, pcie->base + HARD_DEBUG(pcie));
+	} else {
+		ret = pcie->cfg->perst_set(pcie, 0);
+	}
 	if (ret)
 		return ret;
 
@@ -2058,6 +2079,8 @@ static int brcm_pcie_probe(struct platform_device *pdev)
 	pcie->ssc = !(pcie->cfg->quirks & CFG_QUIRK_NO_SSC) &&
 		    of_property_read_bool(np, "brcm,enable-ssc");
 
+	of_property_read_u32(np, "brcm,tperst-clk-ms", &pcie->tperst_clk_ms);
+
 	pcie->rescal = devm_reset_control_get_optional_shared(&pdev->dev, "rescal");
 	if (IS_ERR(pcie->rescal))
 		return PTR_ERR(pcie->rescal);