kernel/linux.git/drivers/clocksource/timer-riscv.c, branch v6.1.168

riscv: clocksource: Fix stimecmp update hazard on RV32

2026-02-06T15:44:08+00:00

[ Upstream commit eaa9bb1d39d59e7c17b06cec12622b7c586ab629 ] On RV32, updating the 64-bit stimecmp (or vstimecmp) CSR requires two separate 32-bit writes. A race condition exists if the timer triggers during these two writes. The RISC-V Privileged Specification (e.g., Section 3.2.1 for mtimecmp) recommends a specific 3-step sequence to avoid spurious interrupts when updating 64-bit comparison registers on 32-bit systems: 1. Set the low-order bits (stimecmp) to all ones (ULONG_MAX). 2. Set the high-order bits (stimecmph) to the desired value. 3. Set the low-order bits (stimecmp) to the desired value. Current implementation writes the LSB first without ensuring a future value, which may lead to a transient state where the 64-bit comparison is incorrectly evaluated as "expired" by the hardware. This results in spurious timer interrupts. This patch adopts the spec-recommended 3-step sequence to ensure the intermediate 64-bit state is never smaller than the current time. Fixes: 9f7a8ff6391f ("RISC-V: Prefer sstc extension if available") Signed-off-by: Naohiko Shimizu Reviewed-by: Anup Patel Link: https://patch.msgid.link/20260104135938.524-2-naohiko.shimizu@gmail.com Signed-off-by: Paul Walmsley Signed-off-by: Sasha Levin

clocksource/drivers/riscv: Patch riscv_clock_next_event() jump before first use

2023-03-10T08:33:03+00:00

[ Upstream commit 225b9596cb0227c1c1b1e4a836dad43595c3e61a ] A static key is used to select between SBI and Sstc timer usage in riscv_clock_next_event(), but currently the direction is resolved after cpuhp_setup_state() is called (which sets the next event). The first event will therefore fall through the sbi_set_timer() path; this breaks Sstc-only systems. So, apply the jump patching before first use. Fixes: 9f7a8ff6391f ("RISC-V: Prefer sstc extension if available") Signed-off-by: Matt Evans Reviewed-by: Palmer Dabbelt Acked-by: Palmer Dabbelt Reviewed-by: Anup Patel Link: https://lore.kernel.org/r/CDDAB2D0-264E-42F3-8E31-BA210BEB8EC1@rivosinc.com Signed-off-by: Daniel Lezcano Signed-off-by: Sasha Levin

Revert "clocksource/drivers/riscv: Events are stopped during CPU suspend"

2022-12-01T11:05:29+00:00

This reverts commit 232ccac1bd9b5bfe73895f527c08623e7fa0752d. On the subject of suspend, the RISC-V SBI spec states: This does not cover whether any given events actually reach the hart or not, just what the hart will do if it receives an event. On PolarFire SoC, and potentially other SiFive based implementations, events from the RISC-V timer do reach a hart during suspend. This is not the case for the implementation on the Allwinner D1 - there timer events are not received during suspend. To fix this, the CLOCK_EVT_FEAT_C3STOP (mis)feature was enabled for the timer driver - but this has broken both RCU stall detection and timers generally on PolarFire SoC and potentially other SiFive based implementations. If an AXI read to the PCIe controller on PolarFire SoC times out, the system will stall, however, with CLOCK_EVT_FEAT_C3STOP active, the system just locks up without RCU stalling: io scheduler mq-deadline registered io scheduler kyber registered microchip-pcie 2000000000.pcie: host bridge /soc/pcie@2000000000 ranges: microchip-pcie 2000000000.pcie: MEM 0x2008000000..0x2087ffffff -> 0x0008000000 microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer microchip-pcie 2000000000.pcie: axi read request error microchip-pcie 2000000000.pcie: axi read timeout microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer Freeing initrd memory: 7332K Similarly issues were reported with clock_nanosleep() - with a test app that sleeps each cpu for 6, 5, 4, 3 ms respectively, HZ=250 & the blamed commit in place, the sleep times are rounded up to the next jiffy: == CPU: 1 == == CPU: 2 == == CPU: 3 == == CPU: 4 == Mean: 7.974992 Mean: 7.976534 Mean: 7.962591 Mean: 3.952179 Std Dev: 0.154374 Std Dev: 0.156082 Std Dev: 0.171018 Std Dev: 0.076193 Hi: 9.472000 Hi: 10.495000 Hi: 8.864000 Hi: 4.736000 Lo: 6.087000 Lo: 6.380000 Lo: 4.872000 Lo: 3.403000 Samples: 521 Samples: 521 Samples: 521 Samples: 521 Fortunately, the D1 has a second timer, which is "currently used in preference to the RISC-V/SBI timer driver" so a revert here does not hurt operation of D1 in its current form. Ultimately, a DeviceTree property (or node) will be added to encode the behaviour of the timers, but until then revert the addition of CLOCK_EVT_FEAT_C3STOP. Fixes: 232ccac1bd9b ("clocksource/drivers/riscv: Events are stopped during CPU suspend") Signed-off-by: Conor Dooley Signed-off-by: Thomas Gleixner Reviewed-by: Palmer Dabbelt Acked-by: Palmer Dabbelt Acked-by: Samuel Holland Link: https://lore.kernel.org/linux-riscv/YzYTNQRxLr7Q9JR0@spud/ Link: https://github.com/riscv-non-isa/riscv-sbi-doc/issues/98/ Link: https://lore.kernel.org/linux-riscv/bf6d3b1f-f703-4a25-833e-972a44a04114@sholland.org/ Link: https://lore.kernel.org/r/20221122121620.3522431-1-conor.dooley@microchip.com

RISC-V: Add Sstc extension support

2022-08-11T21:41:52+00:00

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

RISC-V: Prefer sstc extension if available

2022-08-11T21:36:55+00:00

RISC-V ISA has sstc extension which allows updating the next clock event via a CSR (stimecmp) instead of an SBI call. This should happen dynamically if sstc extension is available. Otherwise, it will fallback to SBI call to maintain backward compatibility. Reviewed-by: Anup Patel Signed-off-by: Atish Patra Reviewed-by: Guo Ren Link: https://lore.kernel.org/r/20220722165047.519994-4-atishp@rivosinc.com Signed-off-by: Palmer Dabbelt

riscv: cpu: Add 64bit hartid support on RV64

2022-07-19T23:39:14+00:00

The hartid can be a 64bit value on RV64 platforms. Add support for 64bit hartid in riscv_of_processor_hartid() and update its callers. Signed-off-by: Sunil V L Reviewed-by: Atish Patra Link: https://lore.kernel.org/r/20220527051743.2829940-5-sunilvl@ventanamicro.com Signed-off-by: Palmer Dabbelt

clocksource/drivers/riscv: Events are stopped during CPU suspend

2022-05-18T09:08:52+00:00

Some implementations of the SBI time extension depend on hart-local state (for example, CSRs) that are lost or hardware that is powered down when a CPU is suspended. To be safe, the clockevents driver cannot assume that timer IRQs will be received during CPU suspend. Fixes: 62b019436814 ("clocksource: new RISC-V SBI timer driver") Signed-off-by: Samuel Holland Reviewed-by: Anup Patel Link: https://lore.kernel.org/r/20220509012121.40031-1-samuel@sholland.org Signed-off-by: Daniel Lezcano

RISC-V: KVM: Add timer functionality

2021-10-04T10:37:16+00:00

The RISC-V hypervisor specification doesn't have any virtual timer feature. Due to this, the guest VCPU timer will be programmed via SBI calls. The host will use a separate hrtimer event for each guest VCPU to provide timer functionality. We inject a virtual timer interrupt to the guest VCPU whenever the guest VCPU hrtimer event expires. This patch adds guest VCPU timer implementation along with ONE_REG interface to access VCPU timer state from user space. Signed-off-by: Atish Patra Signed-off-by: Anup Patel Acked-by: Paolo Bonzini Reviewed-by: Paolo Bonzini Acked-by: Daniel Lezcano Acked-by: Palmer Dabbelt

RISC-V: Remove CLINT related code from timer and arch

2020-08-20T17:58:13+00:00

Right now the RISC-V timer driver is convoluted to support: 1. Linux RISC-V S-mode (with MMU) where it will use TIME CSR for clocksource and SBI timer calls for clockevent device. 2. Linux RISC-V M-mode (without MMU) where it will use CLINT MMIO counter register for clocksource and CLINT MMIO compare register for clockevent device. We now have a separate CLINT timer driver which also provide CLINT based IPI operations so let's remove CLINT MMIO related code from arch/riscv directory and RISC-V timer driver. Signed-off-by: Anup Patel Tested-by: Emil Renner Berhing Acked-by: Daniel Lezcano Reviewed-by: Atish Patra Reviewed-by: Palmer Dabbelt Signed-off-by: Palmer Dabbelt

clocksource/drivers/timer-riscv: Use per-CPU timer interrupt

2020-06-10T02:11:22+00:00

Instead of directly calling RISC-V timer interrupt handler from RISC-V local interrupt conntroller driver, this patch implements RISC-V timer interrupt as a per-CPU interrupt using per-CPU APIs of Linux IRQ subsystem. Signed-off-by: Anup Patel Reviewed-by: Atish Patra Reviewed-by: Marc Zyngier Signed-off-by: Palmer Dabbelt