firmware: arm_ffa: Handle compatibility with different firmware versions

The driver currently just support v1.0 of Arm FFA specification. It also
expects the firmware implementation to match the same and bail out if it
doesn't match. This is causing issue when running with higher version of
firmware implementation(e.g. v1.1 which will released soon).

In order to support compatibility with different firmware versions, let
us add additional checks and find the compatible version the driver can
work with.

Link: https://lore.kernel.org/r/20211013091127.990992-1-sudeep.holla@arm.com
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_ffa/driver.c

@@ -167,6 +167,28 @@ struct ffa_drv_info {
 
 static struct ffa_drv_info *drv_info;
 
+/*
+ * The driver must be able to support all the versions from the earliest
+ * supported FFA_MIN_VERSION to the latest supported FFA_DRIVER_VERSION.
+ * The specification states that if firmware supports a FFA implementation
+ * that is incompatible with and at a greater version number than specified
+ * by the caller(FFA_DRIVER_VERSION passed as parameter to FFA_VERSION),
+ * it must return the NOT_SUPPORTED error code.
+ */
+static u32 ffa_compatible_version_find(u32 version)
+{
+	u32 compat_version;
+	u16 major = MAJOR_VERSION(version), minor = MINOR_VERSION(version);
+	u16 drv_major = MAJOR_VERSION(FFA_DRIVER_VERSION);
+	u16 drv_minor = MINOR_VERSION(FFA_DRIVER_VERSION);
+
+	if ((major < drv_major) || (major == drv_major && minor <= drv_minor))
+		return version;
+
+	pr_info("Firmware version higher than driver version, downgrading\n");
+	return FFA_DRIVER_VERSION;
+}
+
 static int ffa_version_check(u32 *version)
 {
 	ffa_value_t ver;
@@ -180,15 +202,20 @@ static int ffa_version_check(u32 *version)
 		return -EOPNOTSUPP;
 	}
 
-	if (ver.a0 < FFA_MIN_VERSION || ver.a0 > FFA_DRIVER_VERSION) {
-		pr_err("Incompatible version %d.%d found\n",
-		       MAJOR_VERSION(ver.a0), MINOR_VERSION(ver.a0));
+	if (ver.a0 < FFA_MIN_VERSION) {
+		pr_err("Incompatible v%d.%d! Earliest supported v%d.%d\n",
+		       MAJOR_VERSION(ver.a0), MINOR_VERSION(ver.a0),
+		       MAJOR_VERSION(FFA_MIN_VERSION),
+		       MINOR_VERSION(FFA_MIN_VERSION));
 		return -EINVAL;
 	}
 
-	*version = ver.a0;
-	pr_info("Version %d.%d found\n", MAJOR_VERSION(ver.a0),
+	pr_info("Driver version %d.%d\n", MAJOR_VERSION(FFA_DRIVER_VERSION),
+		MINOR_VERSION(FFA_DRIVER_VERSION));
+	pr_info("Firmware version %d.%d found\n", MAJOR_VERSION(ver.a0),
 		MINOR_VERSION(ver.a0));
+	*version = ffa_compatible_version_find(ver.a0);
+
 	return 0;
 }