acpi/nfit: Require opt-in for read-only label configurations

Recent fixes to command handling enabled Linux to read label
configurations that it could not before. Unfortunately that means that
configurations that were operating in label-less mode will be broken as
the kernel ignores the existing namespace configuration and tries to
honor the new found labels.

Fortunately this seems limited to a case where Linux can quirk the
behavior and maintain the existing label-less semantics by default.
When the platform does not emit an _LSW method, disable all label access
methods. Provide a 'force_labels' module parameter to allow read-only
label operation.

Fixes: 11189c10 ("acpi/nfit: Fix command-supported detection")
Reported-by: Dexuan Cui <decui@microsoft.com>
Reviewed-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/acpi/nfit/core.c

@@ -56,6 +56,10 @@ static bool no_init_ars;
 module_param(no_init_ars, bool, 0644);
 MODULE_PARM_DESC(no_init_ars, "Skip ARS run at nfit init time");
 
+static bool force_labels;
+module_param(force_labels, bool, 0444);
+MODULE_PARM_DESC(force_labels, "Opt-in to labels despite missing methods");
+
 LIST_HEAD(acpi_descs);
 DEFINE_MUTEX(acpi_desc_lock);
 
@@ -1916,6 +1920,19 @@ static int acpi_nfit_add_dimm(struct acpi_nfit_desc *acpi_desc,
 			dev_dbg(dev, "%s: has _LSW\n", dev_name(&adev_dimm->dev));
 			set_bit(NFIT_MEM_LSW, &nfit_mem->flags);
 		}
+
+		/*
+		 * Quirk read-only label configurations to preserve
+		 * access to label-less namespaces by default.
+		 */
+		if (!test_bit(NFIT_MEM_LSW, &nfit_mem->flags)
+				&& !force_labels) {
+			dev_dbg(dev, "%s: No _LSW, disable labels\n",
+					dev_name(&adev_dimm->dev));
+			clear_bit(NFIT_MEM_LSR, &nfit_mem->flags);
+		} else
+			dev_dbg(dev, "%s: Force enable labels\n",
+					dev_name(&adev_dimm->dev));
 	}
 
 	populate_shutdown_status(nfit_mem);