path: root/drivers
diff options
authorPalmer Dabbelt <palmer@rivosinc.com>2022-08-11 14:41:52 -0700
committerPalmer Dabbelt <palmer@rivosinc.com>2022-08-11 14:41:52 -0700
commit7ab52f75a9cf7fed7ba85802b986825dd72df641 (patch)
tree7925addedc2d883212d80f000d16a683c0984681 /drivers
parent3dbe5829408bc1586f75b4667ef60e5aab0209c7 (diff)
parent9f7a8ff6391fd5363363b8e5c8b1462a07922368 (diff)
RISC-V: Add Sstc extension support
This series implements Sstc extension support which was ratified recently. Before the Sstc extension, an SBI call is necessary to generate timer interrupts as only M-mode have access to the timecompare registers. Thus, there is significant latency to generate timer interrupts at kernel. For virtualized enviornments, its even worse as the KVM handles the SBI call and uses a software timer to emulate the timecomapre register. Sstc extension solves both these problems by defining a stimecmp/vstimecmp at supervisor (host/guest) level. It allows kernel to program a timer and recieve interrupt without supervisor execution enviornment (M-mode/HS mode) intervention. * palmer/riscv-sstc: RISC-V: Prefer sstc extension if available RISC-V: Enable sstc extension parsing from DT RISC-V: Add SSTC extension CSR details
Diffstat (limited to 'drivers')
1 files changed, 24 insertions, 1 deletions
diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
index e460df7ed799..969a552da8d2 100644
--- a/drivers/clocksource/timer-riscv.c
+++ b/drivers/clocksource/timer-riscv.c
@@ -7,6 +7,9 @@
* either be read from the "time" and "timeh" CSRs, and can use the SBI to
* setup events, or directly accessed using MMIO registers.
+#define pr_fmt(fmt) "riscv-timer: " fmt
#include <linux/clocksource.h>
#include <linux/clockchips.h>
#include <linux/cpu.h>
@@ -20,14 +23,28 @@
#include <linux/of_irq.h>
#include <clocksource/timer-riscv.h>
#include <asm/smp.h>
+#include <asm/hwcap.h>
#include <asm/sbi.h>
#include <asm/timex.h>
+static DEFINE_STATIC_KEY_FALSE(riscv_sstc_available);
static int riscv_clock_next_event(unsigned long delta,
struct clock_event_device *ce)
+ u64 next_tval = get_cycles64() + delta;
csr_set(CSR_IE, IE_TIE);
- sbi_set_timer(get_cycles64() + delta);
+ if (static_branch_likely(&riscv_sstc_available)) {
+#if defined(CONFIG_32BIT)
+ csr_write(CSR_STIMECMP, next_tval & 0xFFFFFFFF);
+ csr_write(CSR_STIMECMPH, next_tval >> 32);
+ csr_write(CSR_STIMECMP, next_tval);
+ } else
+ sbi_set_timer(next_tval);
return 0;
@@ -166,6 +183,12 @@ static int __init riscv_timer_init_dt(struct device_node *n)
if (error)
pr_err("cpu hp setup state failed for RISCV timer [%d]\n",
+ if (riscv_isa_extension_available(NULL, SSTC)) {
+ pr_info("Timer interrupt in S-mode is available via sstc extension\n");
+ static_branch_enable(&riscv_sstc_available);
+ }
return error;

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