firmware: arm_ffa: Explicitly cast return value from FFA_VERSION before comparison

The return value ver.a0 is unsigned long type and FFA_RET_NOT_SUPPORTED
is a negative value.

Since the return value from the firmware can be just 32-bit even on
64-bit systems as FFA specification mentions it as int32 error code in
w0 register, explicitly casting to s32 ensures correct sign interpretation
when comparing against a signed error code FFA_RET_NOT_SUPPORTED.

Without casting, comparison between unsigned long and a negative
constant could lead to unintended results due to type promotions.

Fixes: 3bbfe98 ("firmware: arm_ffa: Add initial Arm FFA driver support")
Reported-by: Andrei Homescu <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Sudeep Holla <[email protected]>

Author: sudeep-holla

drivers/firmware/arm_ffa/driver.c

@@ -144,7 +144,7 @@ static int ffa_version_check(u32 *version)
 		      .a0 = FFA_VERSION, .a1 = FFA_DRIVER_VERSION,
 		      }, &ver);
 
-	if (ver.a0 == FFA_RET_NOT_SUPPORTED) {
+	if ((s32)ver.a0 == FFA_RET_NOT_SUPPORTED) {
 		pr_info("FFA_VERSION returned not supported\n");
 		return -EOPNOTSUPP;
 	}