[PATCH v3] soc: mediatek: Introduce mediatek-regulator-coupler driver

[AngeloGioacchino Del Regno]

This driver currently deals with GPU-SRAM regulator coupling, ensuring
that the SRAM voltage is always between a specific range of distance to
the GPU voltage, depending on the SoC, necessary in order to achieve
system stability across the full range of supported GPU frequencies.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxx>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
---

Changes in v3:
 - Added braces to else-if branch

Changes in v2:
 - Added check for n_coupled
 - Added check for vgpu to enforce attaching to vgpu<->sram coupling only

Context:
This driver is one of the pieces of a bigger puzzle, aiming to finally
enable Dynamic Voltage/Frequency Scaling for Mali GPUs found on MediaTek
SoCs on the fully open source graphics stack (Panfrost driver).

No devicetree bindings are provided because this does not require any
driver-specific binding at all.

Last but not least: it was chosen to have this driver enabled for
( ARCH_MEDIATEK && REGULATOR ) without really giving a free configuration
choice because, once the DVFS mechanism will be fully working, using one
of the listed MediaTek SoCs *without* this coupling mechanism *will* lead
to unstabilities and system crashes.
For COMPILE_TEST, choice is given for obvious reasons.

 drivers/soc/mediatek/Kconfig                 |   5 +
 drivers/soc/mediatek/Makefile                |   1 +
 drivers/soc/mediatek/mtk-regulator-coupler.c | 159 +++++++++++++++++++
 3 files changed, 165 insertions(+)
 create mode 100644 drivers/soc/mediatek/mtk-regulator-coupler.c

diff --git a/drivers/soc/mediatek/Kconfig b/drivers/soc/mediatek/Kconfig
index 3c3eedea35f7..cfa24fc6287b 100644
--- a/drivers/soc/mediatek/Kconfig
+++ b/drivers/soc/mediatek/Kconfig
@@ -43,6 +43,11 @@ config MTK_PMIC_WRAP
 	  on different MediaTek SoCs. The PMIC wrapper is a proprietary
 	  hardware to connect the PMIC.
 
+config MTK_REGULATOR_COUPLER
+	bool "MediaTek SoC Regulator Coupler" if COMPILE_TEST
+	default ARCH_MEDIATEK
+	depends on REGULATOR
+
 config MTK_SCPSYS
 	bool "MediaTek SCPSYS Support"
 	default ARCH_MEDIATEK
diff --git a/drivers/soc/mediatek/Makefile b/drivers/soc/mediatek/Makefile
index 0e9e703c931a..8c0ddacbcde8 100644
--- a/drivers/soc/mediatek/Makefile
+++ b/drivers/soc/mediatek/Makefile
@@ -3,6 +3,7 @@ obj-$(CONFIG_MTK_CMDQ) += mtk-cmdq-helper.o
 obj-$(CONFIG_MTK_DEVAPC) += mtk-devapc.o
 obj-$(CONFIG_MTK_INFRACFG) += mtk-infracfg.o
 obj-$(CONFIG_MTK_PMIC_WRAP) += mtk-pmic-wrap.o
+obj-$(CONFIG_MTK_REGULATOR_COUPLER) += mtk-regulator-coupler.o
 obj-$(CONFIG_MTK_SCPSYS) += mtk-scpsys.o
 obj-$(CONFIG_MTK_SCPSYS_PM_DOMAINS) += mtk-pm-domains.o
 obj-$(CONFIG_MTK_MMSYS) += mtk-mmsys.o
diff --git a/drivers/soc/mediatek/mtk-regulator-coupler.c b/drivers/soc/mediatek/mtk-regulator-coupler.c
new file mode 100644
index 000000000000..ad2ed42aa697
--- /dev/null
+++ b/drivers/soc/mediatek/mtk-regulator-coupler.c
@@ -0,0 +1,159 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Voltage regulators coupler for MediaTek SoCs
+ *
+ * Copyright (C) 2022 Collabora, Ltd.
+ * Author: AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxx>
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/of.h>
+#include <linux/regulator/coupler.h>
+#include <linux/regulator/driver.h>
+#include <linux/regulator/machine.h>
+#include <linux/suspend.h>
+
+#define to_mediatek_coupler(x)	container_of(x, struct mediatek_regulator_coupler, coupler)
+
+struct mediatek_regulator_coupler {
+	struct regulator_coupler coupler;
+	struct regulator_dev *vsram_rdev;
+};
+
+/*
+ * We currently support only couples of not more than two vregs and
+ * modify the vsram voltage only when changing voltage of vgpu.
+ *
+ * This function is limited to the GPU<->SRAM voltages relationships.
+ */
+static int mediatek_regulator_balance_voltage(struct regulator_coupler *coupler,
+					      struct regulator_dev *rdev,
+					      suspend_state_t state)
+{
+	struct mediatek_regulator_coupler *mrc = to_mediatek_coupler(coupler);
+	int max_spread = rdev->constraints->max_spread[0];
+	int vsram_min_uV = mrc->vsram_rdev->constraints->min_uV;
+	int vsram_max_uV = mrc->vsram_rdev->constraints->max_uV;
+	int vsram_target_min_uV, vsram_target_max_uV;
+	int min_uV = 0;
+	int max_uV = INT_MAX;
+	int ret;
+
+	/*
+	 * If the target device is on, setting the SRAM voltage directly
+	 * is not supported as it scales through its coupled supply voltage.
+	 *
+	 * An exception is made in case the use_count is zero: this means
+	 * that this is the first time we power up the SRAM regulator, which
+	 * implies that the target device has yet to perform initialization
+	 * and setting a voltage at that time is harmless.
+	 */
+	if (rdev == mrc->vsram_rdev) {
+		if (rdev->use_count == 0)
+			return regulator_do_balance_voltage(rdev, state, true);
+
+		return -EPERM;
+	}
+
+	ret = regulator_check_consumers(rdev, &min_uV, &max_uV, state);
+	if (ret < 0)
+		return ret;
+
+	if (min_uV == 0) {
+		ret = regulator_get_voltage_rdev(rdev);
+		if (ret < 0)
+			return ret;
+		min_uV = ret;
+	}
+
+	ret = regulator_check_voltage(rdev, &min_uV, &max_uV);
+	if (ret < 0)
+		return ret;
+
+	/*
+	 * If we're asked to set a voltage less than VSRAM min_uV, set
+	 * the minimum allowed voltage on VSRAM, as in this case it is
+	 * safe to ignore the max_spread parameter.
+	 */
+	vsram_target_min_uV = max(vsram_min_uV, min_uV + max_spread);
+	vsram_target_max_uV = min(vsram_max_uV, vsram_target_min_uV + max_spread);
+
+	/* Make sure we're not out of range */
+	vsram_target_min_uV = min(vsram_target_min_uV, vsram_max_uV);
+
+	pr_debug("Setting voltage %d-%duV on %s (minuV %d)\n",
+		 vsram_target_min_uV, vsram_target_max_uV,
+		 rdev_get_name(mrc->vsram_rdev), min_uV);
+
+	ret = regulator_set_voltage_rdev(mrc->vsram_rdev, vsram_target_min_uV,
+					 vsram_target_max_uV, state);
+	if (ret)
+		return ret;
+
+	/* The sram voltage is now balanced: update the target vreg voltage */
+	return regulator_do_balance_voltage(rdev, state, true);
+}
+
+static int mediatek_regulator_attach(struct regulator_coupler *coupler,
+				     struct regulator_dev *rdev)
+{
+	struct mediatek_regulator_coupler *mrc = to_mediatek_coupler(coupler);
+	const char *rdev_name = rdev_get_name(rdev);
+
+	/*
+	 * If we're getting a coupling of more than two regulators here and
+	 * this means that this is surely not a GPU<->SRAM couple: in that
+	 * case, we may want to use another coupler implementation, if any,
+	 * or the generic one: the regulator core will keep walking through
+	 * the list of couplers when any .attach_regulator() cb returns 1.
+	 */
+	if (rdev->coupling_desc.n_coupled > 2)
+		return 1;
+
+	if (strstr(rdev_name, "sram")) {
+		if (mrc->vsram_rdev)
+			return -EINVAL;
+		mrc->vsram_rdev = rdev;
+	} else if (!strstr(rdev_name, "vgpu") && !strstr(rdev_name, "Vgpu")) {
+		return 1;
+	}
+
+	return 0;
+}
+
+static int mediatek_regulator_detach(struct regulator_coupler *coupler,
+				     struct regulator_dev *rdev)
+{
+	struct mediatek_regulator_coupler *mrc = to_mediatek_coupler(coupler);
+
+	if (rdev == mrc->vsram_rdev)
+		mrc->vsram_rdev = NULL;
+
+	return 0;
+}
+
+static struct mediatek_regulator_coupler mediatek_coupler = {
+	.coupler = {
+		.attach_regulator = mediatek_regulator_attach,
+		.detach_regulator = mediatek_regulator_detach,
+		.balance_voltage = mediatek_regulator_balance_voltage,
+	},
+};
+
+static int mediatek_regulator_coupler_init(void)
+{
+	if (!of_machine_is_compatible("mediatek,mt8183") &&
+	    !of_machine_is_compatible("mediatek,mt8186") &&
+	    !of_machine_is_compatible("mediatek,mt8192"))
+		return 0;
+
+	return regulator_coupler_register(&mediatek_coupler.coupler);
+}
+arch_initcall(mediatek_regulator_coupler_init);
+
+MODULE_AUTHOR("AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxx>");
+MODULE_DESCRIPTION("MediaTek Regulator Coupler driver");
+MODULE_LICENSE("GPL");
-- 
2.35.1