kselftest/arm64: Expand the set of ZA writes fp-ptrace does

Currently our test for implementable ZA writes is written in a bit of a
convoluted fashion which excludes all changes where we clear SVCR.SM even
though we can actually support that since changing the vector length resets
SVCR. Make the logic more direct, enabling us to actually run these cases.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241112-arm64-fp-ptrace-fpmr-v2-2-250b57c61254@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: broonie

tools/testing/selftests/arm64/fp/fp-ptrace.c

@@ -1078,21 +1078,19 @@ static void sve_write(pid_t child, struct test_config *config)
 
 static bool za_write_supported(struct test_config *config)
 {
-	if (config->svcr_expected & SVCR_SM) {
-		if (!(config->svcr_in & SVCR_SM))
+	if (config->sme_vl_in != config->sme_vl_expected) {
+		/* Changing the SME VL exits streaming mode. */
+		if (config->svcr_expected & SVCR_SM) {
 			return false;
-
-		/* Changing the SME VL exits streaming mode */
-		if (config->sme_vl_in != config->sme_vl_expected) {
+		}
+	} else {
+		/* Otherwise we can't change streaming mode */
+		if ((config->svcr_in & SVCR_SM) !=
+		    (config->svcr_expected & SVCR_SM)) {
 			return false;
 		}
 	}
 
-	/* Can't disable SM outside a VL change */
-	if ((config->svcr_in & SVCR_SM) &&
-	    !(config->svcr_expected & SVCR_SM))
-		return false;
-
 	return true;
 }