acpi_spmc(4): Gracefully support a standalone Microsoft DSM

In acpi_spmc_get_constraints(), stop assuming that if there is no AMD
DSM, then the Intel one is present.  Although this is likely to be the
overwhelming majority of cases on amd64, there is no technical reason
nor constraint in our code that really needs assuming that.  On (so far
hypothetical) machines with only the Microsoft DSM, this assumption
would cause a cryptic and irrelevant error message (and, prior to the
previous commit, a panic on INVARIANTS).

Warn the user if both the Intel and AMD DSMs are present, and use the
constraints reported by the Intel one (see the comment for why).

Reviewed by:    imp (older version), obiwac
Sponsored by:   The FreeBSD Foundation
Differential Revision:  https://reviews.freebsd.org/D56813

sys/dev/acpica/acpi_spmc.c

@@ -333,7 +333,7 @@ static void	acpi_spmc_probe_dsm(struct acpi_spmc_softc *sc,
 static void	acpi_spmc_dsm_print_functions(
 		    const struct acpi_spmc_softc *const sc,
 		    const struct dsm_desc *const dsm);
-static int	acpi_spmc_get_constraints(device_t dev);
+static int	acpi_spmc_get_constraints(struct acpi_spmc_softc *const sc);
 static void	acpi_spmc_free_constraints(struct acpi_spmc_softc *const sc);
 
 static void	acpi_spmc_suspend(device_t dev, enum power_stype stype);
@@ -399,7 +399,7 @@ acpi_spmc_attach(device_t dev)
 			acpi_spmc_dsm_print_functions(sc, dsms[i]);
 
 	/* Get device constraints. We can only call this once so do this now. */
-	acpi_spmc_get_constraints(dev);
+	acpi_spmc_get_constraints(sc);
 
 	sc->eh_suspend = EVENTHANDLER_REGISTER(acpi_post_dev_suspend,
 	    acpi_spmc_suspend, dev, 0);
@@ -597,24 +597,36 @@ acpi_spmc_parse_constraints_amd(struct acpi_spmc_softc *sc, ACPI_OBJECT *object)
 }
 
 static int
-acpi_spmc_get_constraints(device_t dev)
+acpi_spmc_get_constraints(struct acpi_spmc_softc *const sc)
 {
-	struct acpi_spmc_softc	*sc;
-	struct dsm_desc		*dsm;
-	ACPI_STATUS		status;
-	ACPI_BUFFER		result;
-	ACPI_OBJECT		*object;
-	bool			is_amd;
-	int			rv;
+	struct dsm_desc *dsm;
+	ACPI_STATUS status;
+	ACPI_BUFFER result;
+	ACPI_OBJECT *object;
+	int rv;
 	struct acpi_spmc_constraint *constraint;
 
-	sc = device_get_softc(dev);
 
 	MPASS(!sc->get_constraints_succeeded);
+	/*
+	 * Constraints are not supported by the Microsoft DSM.  Since we do not
+	 * expect both Intel and AMD DSMs to be present at once, we only have
+	 * a single storage for common information ('min_d_state').  In case
+	 * some day both happen to be present, warn the user so that he can
+	 * report that condition to us, and somewhat arbitrarily favor the Intel
+	 * one because it at least has a written specification.
+	 */
+	if (has_dsm(sc, DSM_INTEL)) {
+		dsm = &dsm_intel;
 
-	/* The Microsoft DSM doesn't have this function. */
-	is_amd = has_dsm(sc, DSM_AMD);
-	dsm = is_amd ? &dsm_amd : &dsm_intel;
+		if (has_dsm(sc, DSM_AMD))
+			device_printf(sc->dev, "Constraints: Both Intel and "
+			    "AMD DSMs are present!\n"
+			    "Using constraints from Intel.\nPlease report.\n");
+	} else if (has_dsm(sc, DSM_AMD))
+		dsm = &dsm_amd;
+	else
+		return (0);
 
 	/* It seems like this DSM can fail if called more than once. */
 	status = acpi_EvaluateDSMTyped(sc->handle, (uint8_t *)&dsm->uuid,
@@ -626,10 +638,12 @@ acpi_spmc_get_constraints(device_t dev)
 	}
 
 	object = (ACPI_OBJECT *)result.Pointer;
-	if (is_amd)
-		rv = acpi_spmc_parse_constraints_amd(sc, object);
-	else
+	if (dsm == &dsm_intel)
 		rv = acpi_spmc_parse_constraints_intel(sc, object);
+	else {
+		MPASS(dsm == &dsm_amd);
+		rv = acpi_spmc_parse_constraints_amd(sc, object);
+	}
 	AcpiOsFree(object);
 	if (rv != 0)
 		return (rv);
@@ -641,7 +655,7 @@ acpi_spmc_get_constraints(device_t dev)
 		status = acpi_GetHandleInScope(sc->handle,
 		    __DECONST(char *, constraint->name), &constraint->handle);
 		if (ACPI_FAILURE(status)) {
-			device_printf(dev,
+			device_printf(sc->dev,
 			    "Constraints: Cannot get handle for %s, ignoring\n",
 			    constraint->name);
 			constraint->handle = NULL;