kernel/linux.git/drivers/clocksource/timer-riscv.c, branch v6.6.132

riscv: clocksource: Fix stimecmp update hazard on RV32

2026-01-30T09:27:36+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/timer-riscv: Add ACPI support

2023-06-01T15:45:12+00:00

Initialize the timer driver based on RHCT table on ACPI based platforms. Currently, ACPI doesn't support a flag to indicate that the timer interrupt can wake up the cpu irrespective of its power state. It will be added in future update. Signed-off-by: Sunil V L Acked-by: Rafael J. Wysocki Reviewed-by: Andrew Jones Reviewed-by: Conor Dooley Link: https://lore.kernel.org/r/20230515054928.2079268-19-sunilvl@ventanamicro.com Signed-off-by: Palmer Dabbelt

clocksource/timer-riscv: Refactor riscv_timer_init_dt()

2023-06-01T15:45:11+00:00

Refactor the timer init function such that few things can be shared by both DT and ACPI based platforms. Co-developed-by: Anup Patel Signed-off-by: Anup Patel Signed-off-by: Sunil V L Acked-by: Rafael J. Wysocki Reviewed-by: Andrew Jones Reviewed-by: Conor Dooley Link: https://lore.kernel.org/r/20230515054928.2079268-18-sunilvl@ventanamicro.com Signed-off-by: Palmer Dabbelt

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

2023-02-13T12:10:17+00:00

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

clocksource/drivers/riscv: Get rid of clocksource_arch_init() callback

2023-02-13T12:10:17+00:00

Having a clocksource_arch_init() callback always sets vdso_clock_mode to VDSO_CLOCKMODE_ARCHTIMER if GENERIC_GETTIMEOFDAY is enabled, this is required for the riscv-timer. This works for platforms where just riscv-timer clocksource is present. On platforms where other clock sources are available we want them to register with vdso_clock_mode set to VDSO_CLOCKMODE_NONE. On the Renesas RZ/Five SoC OSTM block can be used as clocksource [0], to avoid multiple clock sources being registered as VDSO_CLOCKMODE_ARCHTIMER move setting of vdso_clock_mode in the riscv-timer driver instead of doing this in clocksource_arch_init() callback as done similarly for ARM/64 architecture. [0] drivers/clocksource/renesas-ostm.c Signed-off-by: Lad Prabhakar Tested-by: Samuel Holland Reviewed-by: Conor Dooley Reviewed-by: Samuel Holland Link: https://lore.kernel.org/r/20221229224601.103851-1-prabhakar.mahadev-lad.rj@bp.renesas.com Signed-off-by: Daniel Lezcano

clocksource/drivers/riscv: Increase the clock source rating

2023-02-13T12:10:16+00:00

RISC-V provides an architectural clock source via the time CSR. This clock source exposes a 64-bit counter synchronized across all CPUs. Because it is accessed using a CSR, it is much more efficient to read than MMIO clock sources. For example, on the Allwinner D1, reading the sun4i timer in a loop takes 131 cycles/iteration, while reading the RISC-V time CSR takes only 5 cycles/iteration. Adjust the RISC-V clock source rating so it is preferred over the various platform-specific MMIO clock sources. Signed-off-by: Samuel Holland Acked-by: Palmer Dabbelt Reviewed-by: Palmer Dabbelt Reviewed-by: Anup Patel Reviewed-by: Lad Prabhakar Link: https://lore.kernel.org/r/20221228004444.61568-1-samuel@sholland.org Signed-off-by: Daniel Lezcano

clocksource/drivers/timer-riscv: Set CLOCK_EVT_FEAT_C3STOP based on DT

2023-02-13T12:10:16+00:00

We should set CLOCK_EVT_FEAT_C3STOP for a clock_event_device only when riscv,timer-cannot-wake-cpu DT property is present in the RISC-V timer DT node. This way CLOCK_EVT_FEAT_C3STOP feature is set for clock_event_device based on RISC-V platform capabilities rather than having it set for all RISC-V platforms. Signed-off-by: Anup Patel Reviewed-by: Conor Dooley Acked-by: Palmer Dabbelt Link: https://lore.kernel.org/r/20230103141102.772228-4-apatel@ventanamicro.com Signed-off-by: Daniel Lezcano

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