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>

1 files changed, 77 insertions, 2 deletions

diff --git a/src/arch/riscv/trap_handler.c b/src/arch/riscv/trap_handler.c
index c7a11c6b9c..56812090c5 100644
--- a/src/arch/riscv/trap_handler.c
+++ b/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;
 	}