Re: [PATCH 2/2] ACPI: processor: Add support for ACPI C4 state

Rafael J. Wysocki
2025-09-16T11:24:14Z

On Mon, Sep 15, 2025 at 10:43 PM Mario Limonciello (AMD)
<superm1@kernel.org> wrote:
>
> The _CST object described in the ACPI specification [1] allows processors
> to support any arbitrary number of power states that are described by
> a package describing the register used, the type of C-state, latency
> and the power consumption.
>
> Currently the Linux kernel supports up to ACPI C3, and if a system
> supports any further states they are ignored. This causes problems on
> some AMD hardware which can support up to ACPI C4.
>
> AMD systems that support up to C3 will enter CPUOFF and VDDOFF
> when entering C3. Systems that support up to C4 will enter CPUOFF
> when going to C3 and will enter VDDOFF when entering into C4.
>
> The existing semantics for bus mastering around C3 are also valid for C4,
> so instead of hardcoding to C3, map then >= C3. In the case of s2idle
> detect the deepest C-state supported and enter the deepest.
>
> Link: https://uefi.org/htmlspecs/ACPI_Spec_6_4_html/08_Processor_Configuration_and_Control/declaring-processors.html?#cst-c-states [1]
> Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>
> ---
> drivers/acpi/processor_idle.c | 10 +++++++---
> include/acpi/actypes.h | 5 +++--
> 2 files changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> index 5dacf41d7cc0a..537b0119535ea 100644
> --- a/drivers/acpi/processor_idle.c
> +++ b/drivers/acpi/processor_idle.c
> @@ -444,6 +444,10 @@ static int acpi_processor_power_verify(struct acpi_processor *pr)
> case ACPI_STATE_C3:
> acpi_processor_power_verify_c3(pr, cx);
> break;
> + case ACPI_STATE_C4:
> + if (!cx->latency || !cx->address)
> + break;
> + cx->valid = 1;
> }
> if (!cx->valid)
> continue;
> @@ -685,7 +689,7 @@ static int __cpuidle acpi_idle_enter(struct cpuidle_device *dev,
> return -EINVAL;
>
> if (cx->type != ACPI_STATE_C1) {
> - if (cx->type == ACPI_STATE_C3 && pr->flags.bm_check)
> + if (cx->type >= ACPI_STATE_C3 && pr->flags.bm_check)
> return acpi_idle_enter_bm(drv, pr, cx, index);
>
> /* C2 to C1 demotion. */
> @@ -708,7 +712,7 @@ static int __cpuidle acpi_idle_enter_s2idle(struct cpuidle_device *dev,
> {
> struct acpi_processor_cx *cx = per_cpu(acpi_cstate[index], dev->cpu);
>
> - if (cx->type == ACPI_STATE_C3) {
> + if (cx->index == drv->state_count - 1) {
> struct acpi_processor *pr = __this_cpu_read(processors);
>
> if (unlikely(!pr))
> @@ -754,7 +758,7 @@ static int acpi_processor_setup_cpuidle_cx(struct acpi_processor *pr,
> if (lapic_timer_needs_broadcast(pr, cx))
> state->flags |= CPUIDLE_FLAG_TIMER_STOP;
>
> - if (cx->type == ACPI_STATE_C3) {
> + if (cx->type >= ACPI_STATE_C3) {
> state->flags |= CPUIDLE_FLAG_TLB_FLUSHED;
> if (pr->flags.bm_check)
> state->flags |= CPUIDLE_FLAG_RCU_IDLE;
> diff --git a/include/acpi/actypes.h b/include/acpi/actypes.h
> index 8fe893d776dde..6c9f472af482c 100644
> --- a/include/acpi/actypes.h
> +++ b/include/acpi/actypes.h
> @@ -600,8 +600,9 @@ typedef u64 acpi_integer;
> #define ACPI_STATE_C1 (u8) 1
> #define ACPI_STATE_C2 (u8) 2
> #define ACPI_STATE_C3 (u8) 3
> -#define ACPI_C_STATES_MAX ACPI_STATE_C3
> -#define ACPI_C_STATE_COUNT 4
> +#define ACPI_STATE_C4 (u8) 4
> +#define ACPI_C_STATES_MAX ACPI_STATE_C4
> +#define ACPI_C_STATE_COUNT 5

Don't you need to update ACPICA accordingly?

>
> /*
> * Sleep type invalid value
> --

The current point in the cycle is kind of unsuitable for changes like
this. I'd prefer to revisit it after 6.18-rc1.