riscv: Add support for timer interrupts

RISCV requires that timer interrupts be handled in machine
mode and delegated as necessary. Also you can only reset the
timer interrupt by writing to mtimecmp. Further, you must
write a number > mtime, not just != mtime. This rather clumsy
situation requires that we write some value into the future
into mtimecmp lest we never be able to leave machine mode as
the interrupt either is not cleared or instantly reoccurs.

This current code is tested and works for harvey (Plan 9)
timer interrupts.

Change-Id: I8538d5fd8d80d9347773c638f5cbf0da18dc1cae
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: https://review.coreboot.org/17807
Tested-by: build bot (Jenkins)
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>

src/arch/riscv/include/arch/encoding.h

@@ -48,6 +48,11 @@
 #define MSTATUS32_SD        0x80000000
 #define MSTATUS64_SD        0x8000000000000000
 
+#define MIE_UTIE        0x00000010
+#define MIE_STIE        0x00000020
+#define MIE_HTIE        0x00000040
+#define MIE_MTIE        0x00000080
+
 #define SSTATUS_UIE         0x00000001
 #define SSTATUS_SIE         0x00000002
 #define SSTATUS_UPIE        0x00000010
@@ -126,6 +131,7 @@
 
 #define SIP_SSIP MIP_SSIP
 #define SIP_STIP MIP_STIP
+#define SIE_STIE MIE_STIE
 
 #define PRV_U 0
 #define PRV_S 1

src/arch/riscv/trap_handler.c

@@ -20,6 +20,10 @@
 #include <mcall.h>
 #include <string.h>
 #include <vm.h>
+#include <commonlib/configstring.h>
+
+static uint64_t *time;
+static uint64_t *timecmp;
 
 void handle_supervisor_call(trapframe *tf) {
 	uintptr_t call = tf->gpr[17]; /* a7 */
@@ -70,7 +74,7 @@ void handle_supervisor_call(trapframe *tf) {
 			returnValue = mcall_query_memory(arg0, (memory_block_info*) arg1);
 			break;
 		default:
-			printk(BIOS_DEBUG, "ERROR! Unrecognized system call\n");
+			printk(BIOS_DEBUG, "ERROR! Unrecognized SBI call\n");
 			returnValue = 0;
 			break; // note: system call we do not know how to handle
 	}
@@ -130,8 +134,76 @@ static void print_trap_information(const trapframe *tf)
 	printk(BIOS_DEBUG, "Stored sp:          %p\n", (void*) tf->gpr[2]);
 }
 
-void trap_handler(trapframe *tf) {
+static void gettimer(void)
+{
+	query_result res;
+	const char *config;
+
+	config = configstring();
+	query_rtc(config, (uintptr_t *)&time);
+	if (!time)
+		die("Got timer interrupt but found no timer.");
+	res = query_config_string(config, "core{0{0{timecmp");
+	timecmp = (void *)get_uint(res);
+	if (!timecmp)
+		die("Got a timer interrupt but found no timecmp.");
+}
+
+static void interrupt_handler(trapframe *tf)
+{
+	uint64_t cause = tf->cause & ~0x8000000000000000ULL;
+	uint32_t ssip, ssie;
+
+	switch (cause) {
+	case IRQ_M_TIMER:
+		// The only way to reset the timer interrupt is to
+		// write mtimecmp. But we also have to ensure the
+		// comparison fails, for a long time, to let
+		// supervisor interrupt handler compute a new value
+		// and set it. Finally, it fires if mtimecmp is <=
+		// mtime, not =, so setting mtimecmp to 0 won't work
+		// to clear the interrupt and disable a new one. We
+		// have to set the mtimecmp far into the future.
+		// Akward!
+		//
+		// Further, maybe the platform doesn't have the
+		// hardware or the payload never uses it. We hold off
+		// querying some things until we are sure we need
+		// them. What to do if we can not find them? There are
+		// no good options.
+
+		// This hart may have disabled timer interrupts.  If
+		// so, just return. Kernels should only enable timer
+		// interrupts on one hart, and that should be hart 0
+		// at present, as we only search for
+		// "core{0{0{timecmp" above.
+		ssie = read_csr(sie);
+		if (!(ssie & SIE_STIE))
+			break;
+
+		if (!timecmp)
+			gettimer();
+		*timecmp = (uint64_t) -1;
+		ssip = read_csr(sip);
+		ssip |= SIP_STIP;
+		write_csr(sip, ssip);
+		break;
+	default:
+		printk(BIOS_EMERG, "======================================\n");
+		printk(BIOS_EMERG, "Coreboot: Unknown machine interrupt: 0x%llx\n",
+		       cause);
+		printk(BIOS_EMERG, "======================================\n");
+		print_trap_information(tf);
+		break;
+	}
+}
+void trap_handler(trapframe *tf)
+{
 	write_csr(mscratch, tf);
+	if (tf->cause & 0x8000000000000000ULL) {
+		interrupt_handler(tf);
+		return;
+	}
 
 	switch(tf->cause) {
 		case CAUSE_MISALIGNED_FETCH:
@@ -159,6 +231,9 @@ void trap_handler(trapframe *tf) {
 			handle_supervisor_call(tf);
 			break;
 		default:
+			printk(BIOS_EMERG, "================================\n");
+			printk(BIOS_EMERG, "Coreboot: can not handle a trap:\n");
+			printk(BIOS_EMERG, "================================\n");
 			print_trap_information(tf);
 			break;
 	}

src/arch/riscv/virtual_memory.c

@@ -292,12 +292,21 @@ void initVirtualMemory(void) {
 void mstatus_init(void)
 {
 	uintptr_t ms = 0;
+
 	ms = INSERT_FIELD(ms, MSTATUS_FS, 3);
 	ms = INSERT_FIELD(ms, MSTATUS_XS, 3);
 	write_csr(mstatus, ms);
 
-	clear_csr(mip, MIP_MSIP);
+	// clear any pending timer interrupts.
-	set_csr(mie, MIP_MSIP);
+	clear_csr(mip,  MIP_STIP | MIP_SSIP);
+
+	// enable machine and supervisor timer and
+	// all other supervisor interrupts.
+	set_csr(mie, MIP_MTIP | MIP_STIP | MIP_SSIP);
+
+	// Delegate supervisor timer and other interrupts
+	// to supervisor mode.
+	set_csr(mideleg,  MIP_STIP | MIP_SSIP);
 
 	set_csr(medeleg, delegate);