media: atomisp_gmin_platform: stop abusing efivar API

As the code comment already suggests, using the efivar API in this way
is not how it is intended, and so let's switch to the right one, which
is simply to call efi.get_variable() directly after checking whether or
not the GetVariable() runtime service is supported.

Link: https://lore.kernel.org/linux-media/20220620100819.1682995-1-ardb@kernel.org
Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/staging/media/atomisp/pci/atomisp_gmin_platform.c

@@ -1284,7 +1284,7 @@ static int gmin_get_config_var(struct device *maindev,
 	const struct dmi_system_id *id;
 	struct device *dev = maindev;
 	char var8[CFG_VAR_NAME_MAX];
-	struct efivar_entry *ev;
+	efi_status_t status;
 	int i, ret;
 
 	/* For sensors, try first to use the _DSM table */
@@ -1326,24 +1326,11 @@ static int gmin_get_config_var(struct device *maindev,
 	for (i = 0; i < sizeof(var8) && var8[i]; i++)
 		var16[i] = var8[i];
 
-	/* Not sure this API usage is kosher; efivar_entry_get()'s
-	 * implementation simply uses VariableName and VendorGuid from
-	 * the struct and ignores the rest, but it seems like there
-	 * ought to be an "official" efivar_entry registered
-	 * somewhere?
-	 */
-	ev = kzalloc(sizeof(*ev), GFP_KERNEL);
-	if (!ev)
-		return -ENOMEM;
-	memcpy(&ev->var.VariableName, var16, sizeof(var16));
-	ev->var.VendorGuid = GMIN_CFG_VAR_EFI_GUID;
-	ev->var.DataSize = *out_len;
-
-	ret = efivar_entry_get(ev, &ev->var.Attributes,
-			       &ev->var.DataSize, ev->var.Data);
-	if (ret == 0) {
-		memcpy(out, ev->var.Data, ev->var.DataSize);
-		*out_len = ev->var.DataSize;
+	status = EFI_UNSUPPORTED;
+	if (efi_rt_services_supported(EFI_RT_SUPPORTED_GET_VARIABLE))
+		status = efi.get_variable(var16, &GMIN_CFG_VAR_EFI_GUID, NULL,
+					  (unsigned long *)out_len, out);
+	if (status == EFI_SUCCESS) {
 		dev_info(maindev, "found EFI entry for '%s'\n", var8);
 	} else if (is_gmin) {
 		dev_info(maindev, "Failed to find EFI gmin variable %s\n", var8);
@@ -1351,8 +1338,6 @@ static int gmin_get_config_var(struct device *maindev,
 		dev_info(maindev, "Failed to find EFI variable %s\n", var8);
 	}
 
-	kfree(ev);
-
 	return ret;
 }