[RFC PATCH 1/2] clk: sunxi-ng: a64: disable dividers in PLL-CPUX

[Icenowy Zheng <icenowy@aosc.io>]

According to the user manual, PLL-CPUX have two dividers, in which P is
only allowed when the desired rate is less than 240MHz. As the CCU
framework have no such feature yet and the clock rate that allows P is
much lower than where we normally operate, disallow the usage of P
factor now.

M is not restricted in the user manual, however according to the BSP PLL
setup table (see [1]), it's not used at all. To follow what the BSP
does, disable this factor too.

Disabling the dividers will make it possible to remove the need to
switch to osc24M when doing frequency scaling on PLL-CPUX.

In order to prevent boot-time usage of dividers (current known mainline
U-Boot implementation use m = 2), tweaking of the factors are done when
probing CCU driver.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
---
 drivers/clk/sunxi-ng/ccu-sun50i-a64.c | 79 ++++++++++++++++++++++++++-
 1 file changed, 77 insertions(+), 2 deletions(-)

diff --git a/drivers/clk/sunxi-ng/ccu-sun50i-a64.c b/drivers/clk/sunxi-ng/ccu-sun50i-a64.c
index 5f66bf879772..6108d150a0e3 100644
--- a/drivers/clk/sunxi-ng/ccu-sun50i-a64.c
+++ b/drivers/clk/sunxi-ng/ccu-sun50i-a64.c
@@ -4,6 +4,7 @@
  */
 
 #include <linux/clk-provider.h>
+#include <linux/delay.h>
 #include <linux/io.h>
 #include <linux/of_address.h>
 #include <linux/platform_device.h>
@@ -23,13 +24,14 @@
 
 #include "ccu-sun50i-a64.h"
 
+#define SUN50I_A64_PLL_CPUX_REG		0x000
 static struct ccu_nkmp pll_cpux_clk = {
 	.enable		= BIT(31),
 	.lock		= BIT(28),
 	.n		= _SUNXI_CCU_MULT(8, 5),
 	.k		= _SUNXI_CCU_MULT(4, 2),
-	.m		= _SUNXI_CCU_DIV(0, 2),
-	.p		= _SUNXI_CCU_DIV_MAX(16, 2, 4),
+	.m		= _SUNXI_CCU_DIV_MAX(16, 2, 1),
+	.p		= _SUNXI_CCU_DIV_MAX(0, 2, 1),
 	.common		= {
 		.reg		= 0x000,
 		.hw.init	= CLK_HW_INIT("pll-cpux",
@@ -215,6 +217,7 @@ static SUNXI_CCU_NM_WITH_GATE_LOCK(pll_ddr1_clk, "pll-ddr1",
 				   BIT(28),	/* lock */
 				   CLK_SET_RATE_UNGATE);
 
+#define SUN50I_A64_CPUX_AXI_REG		0x050
 static const char * const cpux_parents[] = { "osc32k", "osc24M",
 					     "pll-cpux", "pll-cpux" };
 static SUNXI_CCU_MUX(cpux_clk, "cpux", cpux_parents,
@@ -954,6 +957,78 @@ static int sun50i_a64_ccu_probe(struct platform_device *pdev)
 
 	writel(0x515, reg + SUN50I_A64_PLL_MIPI_REG);
 
+	/* Disable any possible dividers on PLL-CPUX */
+	val = readl(reg + SUN50I_A64_PLL_CPUX_REG);
+	if (val & (GENMASK(17, 16) | GENMASK(1, 0))) {
+		unsigned int n, k, m, p;
+
+		n = ((val & GENMASK(12, 8)) >> 8) + 1;
+		k = ((val & GENMASK(5, 4)) >> 4) + 1;
+		m = (val & GENMASK(1, 0)) + 1;
+		p = 1 << ((val & GENMASK(17, 16)) >> 16);
+
+		/*
+		 * Known mainline U-Boot revisions never uses
+		 * divider p, and it will only use m when k = 3 or 4.
+		 * Specially judge for these cases, to satisfy
+		 * what will most possibly happen.
+		 * For m = 2 and k = 3, fractional change will be
+		 * applied to n, to mostly keep the clock rate.
+		 * For m = 2 and k = 4, just change to m = 1 and k = 2.
+		 * For other cases, just try to divide it from N.
+		 */
+		if (p >= 2) {
+			n /= p;
+			p = 1;
+		}
+
+		if (m == 2) {
+			if (k == 3) {
+				k = 2;
+				n = n * 3 / 4;
+				m = 1;
+			}
+			if (k == 4) {
+				k = 2;
+				m = 1;
+			}
+		}
+
+		if (m >= 2) {
+			n /= m;
+			m = 1;
+		}
+
+		/* The user manual constrains n*k >= 10 */
+		if (n * k < 10) {
+			n = 10;
+			k = 1;
+		}
+
+		/* Switch CPUX clock to osc24M temporarily */
+		val = readl(reg + SUN50I_A64_CPUX_AXI_REG);
+		val &= ~GENMASK(17, 16);
+		val |= (1 << 16);
+		writel(val, reg + SUN50I_A64_CPUX_AXI_REG);
+		udelay(1);
+
+		/* Setup PLL-CPUX with new factors */
+		val = ((n - 1) << 8) | ((k - 1) << 4);
+		writel(val, reg + SUN50I_A64_PLL_CPUX_REG);
+		val |= BIT(31);
+		writel(val, reg + SUN50I_A64_PLL_CPUX_REG);
+		do {
+			/* Wait the PLL to lock */
+			val = readl(reg + SUN50I_A64_PLL_CPUX_REG);
+		} while (!(val & BIT(28)));
+
+		/* Switch CPUX clock back to PLL-CPUX */
+		val = readl(reg + SUN50I_A64_CPUX_AXI_REG);
+		val &= ~GENMASK(17, 16);
+		val |= (2 << 16);
+		writel(val, reg + SUN50I_A64_CPUX_AXI_REG);
+	}
+
 	ret = sunxi_ccu_probe(pdev->dev.of_node, reg, &sun50i_a64_ccu_desc);
 	if (ret)
 		return ret;
-- 
2.28.0