MdePkg/BaseLib: RISC-V: Add function to update stimecmp register

stimecmp is a CSR supported only when Sstc extension is supported by the
platform. This register can be used to set the timer interrupt directly in
S-mode instead of going via SBI call. Add a function to update this
register.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Cc: Andrei Warkentin <andrei.warkentin@intel.com>
Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Andrei Warkentin <andrei.warkentin@intel.com>

MdePkg/Include/Library/BaseLib.h

@@ -191,6 +191,11 @@ RiscVReadTimer (
   VOID
   );
 
+VOID
+RiscVSetSupervisorTimeCompareRegister (
+  IN UINT64
+  );
+
 VOID
 RiscVEnableTimerInterrupt (
   VOID

MdePkg/Include/Register/RiscV64/RiscVEncoding.h

@@ -96,6 +96,9 @@
 /* Supervisor Protection and Translation */
 #define CSR_SATP  0x180
 
+/* Sstc extension */
+#define CSR_STIMECMP  0x14D
+
 /* Trap/Exception Causes */
 #define CAUSE_MISALIGNED_FETCH          0x0
 #define CAUSE_FETCH_ACCESS              0x1

MdePkg/Library/BaseLib/RiscV64/ReadTimer.S

@@ -21,3 +21,10 @@
 ASM_FUNC (RiscVReadTimer)
     csrr a0, CSR_TIME
     ret
+
+//
+// Set Supervisor Time Compare Register
+//
+ASM_FUNC (RiscVSetSupervisorTimeCompareRegister)
+    csrw  CSR_STIMECMP, a0
+    ret