clk: qcom: Support attaching GDSCs to multiple parents

When a clock-controller lists multiple power-domains we need make each GDSC a
subdomain of each of the clock-controller's listed power-domains.

GDSCs without an explicitly defined parent should be a subdomain of each of
the clock-controller's listed power-domains.

GDSCs with an explicitly defined parent should attach only to the parent
GDSC and not the listed power-domains. Any votes will trickle through the
hierarchy up to the external power-domains.

========================================
:: arch/arm64/boot/dts/example.dtsi ::
========================================

clockcc: clock-controller@0 {
compat ="qcom,example-clockcc";
power-domains = <&pd_a, &pd_b>;
}

========================================
:: drivers/clk/qcom/example-clockcc.c ::
========================================

static struct gdsc parent_gdsc = {
.pd = {
.name = "parent_gdsc",
},
};

static struct gdsc child0_gdsc = {
.pd = {
.name = "child0_gdsc",
},
.parent = &parent_gdsc.pd,
};

static struct gdsc child1_gdsc = {
.pd = {
.name = "child1_gdsc",
},
.parent = &parent_gdsc.pd,
};

========================================
:: power-subdomains ::
========================================

pm-domain::pd_a
└── pm-subdomain::clockcc::parent_gdsc
├── pm-subdomain::clockcc::child0_gdsc
└── pm-subdomain::clockcc::child1_gdsc

pm-domain::pd_b
└── pm-subdomain::clockcc::parent_gdsc
├── pm-subdomain::clockcc::child1_gdsc
└── pm-subdomain::clockcc::child2_gdsc

The performance states will percolate through the pm-domain hierarchy to
the domains that handle the relevant states.

Signed-off-by: Bryan O'Donoghue <[e-mail]>
Link: https://lore.kernel.org/r/20250117-b4-lin[e-mail]
Signed-off-by: Bjorn Andersson <[e-mail]>

drivers/clk/qcom/common.c

···
 		scd->dev = dev;
 		scd->scs = desc->gdscs;
 		scd->num = desc->num_gdscs;
+		scd->pd_list = cc->pd_list;
 		ret = gdsc_register(scd, &reset->rcdev, regmap);
 		if (ret)
 			return ret;

drivers/clk/qcom/gdsc.c

···
 	return ret;
 }
 
+static int gdsc_add_subdomain_list(struct dev_pm_domain_list *pd_list,
+				   struct generic_pm_domain *subdomain)
+{
+	int i, ret;
+
+	for (i = 0; i < pd_list->num_pds; i++) {
+		struct device *dev = pd_list->pd_devs[i];
+		struct generic_pm_domain *genpd = pd_to_genpd(dev->pm_domain);
+
+		ret = pm_genpd_add_subdomain(genpd, subdomain);
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+
+static void gdsc_remove_subdomain_list(struct dev_pm_domain_list *pd_list,
+				       struct generic_pm_domain *subdomain)
+{
+	int i;
+
+	for (i = 0; i < pd_list->num_pds; i++) {
+		struct device *dev = pd_list->pd_devs[i];
+		struct generic_pm_domain *genpd = pd_to_genpd(dev->pm_domain);
+
+		pm_genpd_remove_subdomain(genpd, subdomain);
+	}
+}
+
 static void gdsc_pm_subdomain_remove(struct gdsc_desc *desc, size_t num)
 {
 	struct device *dev = desc->dev;
···
 			pm_genpd_remove_subdomain(scs[i]->parent, &scs[i]->pd);
 		else if (!IS_ERR_OR_NULL(dev->pm_domain))
 			pm_genpd_remove_subdomain(pd_to_genpd(dev->pm_domain), &scs[i]->pd);
+		else if (desc->pd_list)
+			gdsc_remove_subdomain_list(desc->pd_list, &scs[i]->pd);
 	}
 }
 
···
 			ret = pm_genpd_add_subdomain(scs[i]->parent, &scs[i]->pd);
 		else if (!IS_ERR_OR_NULL(dev->pm_domain))
 			ret = pm_genpd_add_subdomain(pd_to_genpd(dev->pm_domain), &scs[i]->pd);
+		else if (desc->pd_list)
+			ret = gdsc_add_subdomain_list(desc->pd_list, &scs[i]->pd);
+
 		if (ret)
 			goto err_pm_subdomain_remove;
 	}

drivers/clk/qcom/gdsc.h

···
 	struct device *dev;
 	struct gdsc **scs;
 	size_t num;
+	struct dev_pm_domain_list *pd_list;
 };
 
 #ifdef CONFIG_QCOM_GDSC