[PATCH v3] firmware: qcom_scm: use 64-bit calling convention only when client is 64-bit

[Kathiravan Thirumoorthy]

Per the "SMC calling convention specification", the 64-bit calling
convention can only be used when the client is 64-bit. Whereas the
32-bit calling convention can be used by either a 32-bit or a 64-bit
client.

Currently during SCM probe, irrespective of the client, 64-bit calling
convention is made, which is incorrect and may lead to the undefined
behaviour when the client is 32-bit. Let's fix it.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 9a434cee773a ("firmware: qcom_scm: Dynamically support SMCCC and legacy conventions")
Reviewed-By: Elliot Berman <quic_eberman@xxxxxxxxxxx>
Signed-off-by: Kathiravan Thirumoorthy <quic_kathirav@xxxxxxxxxxx>
---
Changes in V3:
	- reworded the commit title and msg
	- pick up the R-b tag

Changes in V2:
	- Added the Fixes tag and cc'd stable mailing list
---
 drivers/firmware/qcom_scm.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/firmware/qcom_scm.c b/drivers/firmware/qcom_scm.c
index c2c7fafef34b..520de9b5633a 100644
--- a/drivers/firmware/qcom_scm.c
+++ b/drivers/firmware/qcom_scm.c
@@ -215,6 +215,12 @@ static enum qcom_scm_convention __get_convention(void)
 	if (likely(qcom_scm_convention != SMC_CONVENTION_UNKNOWN))
 		return qcom_scm_convention;
 
+	/*
+	 * Per the "SMC calling convention specification", the 64-bit calling
+	 * convention can only be used when the client is 64-bit, otherwise
+	 * system will encounter the undefined behaviour.
+	 */
+#if IS_ENABLED(CONFIG_ARM64)
 	/*
 	 * Device isn't required as there is only one argument - no device
 	 * needed to dma_map_single to secure world
@@ -235,6 +241,7 @@ static enum qcom_scm_convention __get_convention(void)
 		forced = true;
 		goto found;
 	}
+#endif
 
 	probed_convention = SMC_CONVENTION_ARM_32;
 	ret = __scm_smc_call(NULL, &desc, probed_convention, &res, true);

---
base-commit: 8fff9184d1b5810dca5dd1a02726d4f844af88fc
change-id: 20230925-scm-d62c6cd1947b

Best regards,
-- 
Kathiravan Thirumoorthy <quic_kathirav@xxxxxxxxxxx>