acpi, nfit: fix acpi_check_dsm() vs zero functions implemented

QEMU 2.6 implements nascent support for nvdimm DSMs. Depending on
configuration it may only implement the function0 dsm to indicate that
no other DSMs are available. Commit 31eca76 "nfit, libnvdimm:
limited/whitelisted dimm command marshaling mechanism" breaks QEMU, but
QEMU is spec compliant.  Per the spec the way to indicate that no
functions are supported is:

    If Function Index is zero, the return is a buffer containing one bit
    for each function index, starting with zero. Bit 0 indicates whether
    there is support for any functions other than function 0 for the
    specified UUID and Revision ID. If set to zero, no functions are
    supported (other than function zero) for the specified UUID and
    Revision ID.

Update the nfit driver to determine the family (interface UUID) without
requiring the implementation to define any other functions, i.e.
short-circuit acpi_check_dsm() to succeed per the spec.  The nfit driver
appears to be the only user passing funcs==0 to acpi_check_dsm(), so
this behavior change of the common routine should be limited to the
probing done by the nfit driver.

Cc: Len Brown <lenb@kernel.org>
Cc: Jerry Hoemann <jerry.hoemann@hpe.com>
Acked-by: "Rafael J. Wysocki" <rafael@kernel.org>
Fixes: 31eca76 ("nfit, libnvdimm: limited/whitelisted dimm command marshaling mechanism")
Reported-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Tested-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/acpi/nfit.c

@@ -1131,11 +1131,11 @@ static int acpi_nfit_add_dimm(struct acpi_nfit_desc *acpi_desc,
 
 	/*
 	 * Until standardization materializes we need to consider up to 3
-	 * different command sets.  Note, that checking for function0 (bit0)
-	 * tells us if any commands are reachable through this uuid.
+	 * different command sets.  Note, that checking for zero functions
+	 * tells us if any commands might be reachable through this uuid.
 	 */
 	for (i = NVDIMM_FAMILY_INTEL; i <= NVDIMM_FAMILY_HPE2; i++)
-		if (acpi_check_dsm(adev_dimm->handle, to_nfit_uuid(i), 1, 1))
+		if (acpi_check_dsm(adev_dimm->handle, to_nfit_uuid(i), 1, 0))
 			break;
 
 	/* limit the supported commands to those that are publicly documented */

drivers/acpi/utils.c

@@ -680,9 +680,6 @@ bool acpi_check_dsm(acpi_handle handle, const u8 *uuid, u64 rev, u64 funcs)
 	u64 mask = 0;
 	union acpi_object *obj;
 
-	if (funcs == 0)
-		return false;
-
 	obj = acpi_evaluate_dsm(handle, uuid, rev, 0, NULL);
 	if (!obj)
 		return false;
@@ -695,6 +692,9 @@ bool acpi_check_dsm(acpi_handle handle, const u8 *uuid, u64 rev, u64 funcs)
 			mask |= (((u64)obj->buffer.pointer[i]) << (i * 8));
 	ACPI_FREE(obj);
 
+	if (funcs == 0)
+		return true;
+
 	/*
 	 * Bit 0 indicates whether there's support for any functions other than
 	 * function 0 for the specified UUID and revision.