PSCI: Add pwr_domain_pwr_down_wfi() hook in plat_psci_ops

This patch adds a new optional platform hook `pwr_domain_pwr_down_wfi()` in
the plat_psci_ops structure. This hook allows the platform to perform platform
specific actions including the wfi invocation to enter powerdown. This hook
is invoked by both psci_do_cpu_off() and psci_cpu_suspend_start() functions.
The porting-guide.md is also updated for the same.

This patch also modifies the `psci_power_down_wfi()` function to invoke
`plat_panic_handler` incase of panic instead of the busy while loop.

Fixes ARM-Software/tf-issues#375

Change-Id: Iba104469a1445ee8d59fb3a6fdd0a98e7f24dfa3

docs/porting-guide.md

@@ -1715,6 +1715,22 @@ latter case, the power domain is expected to save enough state so that it can
 resume execution by restoring this state when its powered on (see
 `pwr_domain_suspend_finish()`).
 
+#### plat_psci_ops.pwr_domain_pwr_down_wfi()
+
+This is an optional function and, if implemented, is expected to perform
+platform specific actions including the `wfi` invocation which allows the
+CPU to powerdown. Since this function is invoked outside the PSCI locks,
+the actions performed in this hook must be local to the CPU or the platform
+must ensure that races between multiple CPUs cannot occur.
+
+The `target_state` has a similar meaning as described in the `pwr_domain_off()`
+operation and it encodes the platform coordinated target local power states for
+the CPU power domain and its parent power domain levels. This function must
+not return back to the caller.
+
+If this function is not implemented by the platform, PSCI generic
+implementation invokes `psci_power_down_wfi()` for power down.
+
 #### plat_psci_ops.pwr_domain_on_finish()
 
 This function is called by the PSCI implementation after the calling CPU is

include/bl31/services/psci.h

@@ -265,6 +265,8 @@ typedef struct plat_psci_ops {
 	void (*pwr_domain_on_finish)(const psci_power_state_t *target_state);
 	void (*pwr_domain_suspend_finish)(
 				const psci_power_state_t *target_state);
+	void (*pwr_domain_pwr_down_wfi)(
+				const psci_power_state_t *target_state) __dead2;
 	void (*system_off)(void) __dead2;
 	void (*system_reset)(void) __dead2;
 	int (*validate_power_state)(unsigned int power_state,

services/std_svc/psci/psci_entry.S

@@ -106,7 +106,6 @@ endfunc psci_entrypoint
 func psci_power_down_wfi
 	dsb	sy		// ensure write buffer empty
 	wfi
-wfi_spill:
-	b	wfi_spill
+	bl	plat_panic_handler
 endfunc psci_power_down_wfi
 

services/std_svc/psci/psci_off.c

@@ -138,12 +138,17 @@ exit:
 		dsbish();
 		inv_cpu_data(psci_svc_cpu_data.aff_info_state);
 
+		if (psci_plat_pm_ops->pwr_domain_pwr_down_wfi) {
+			/* This function must not return */
+			psci_plat_pm_ops->pwr_domain_pwr_down_wfi(&state_info);
+		} else {
 			/*
-		 * Enter a wfi loop which will allow the power controller to
-		 * physically power down this cpu.
+			 * Enter a wfi loop which will allow the power
+			 * controller to physically power down this cpu.
 			 */
 			psci_power_down_wfi();
 		}
+	}
 
 	return rc;
 }

services/std_svc/psci/psci_suspend.c

@@ -189,8 +189,13 @@ exit:
 	if (skip_wfi)
 		return;
 
-	if (is_power_down_state)
+	if (is_power_down_state) {
+		/* The function calls below must not return */
+		if (psci_plat_pm_ops->pwr_domain_pwr_down_wfi)
+			psci_plat_pm_ops->pwr_domain_pwr_down_wfi(state_info);
+		else
 			psci_power_down_wfi();
+	}
 
 	/*
 	 * We will reach here if only retention/standby states have been