arch/x86/lib/delay.c

Source file repositories/reference/linux-study-clean/arch/x86/lib/delay.c

File Facts

System
Linux kernel
Corpus path
arch/x86/lib/delay.c
Extension
.c
Size
5168 bytes
Lines
232
Domain
Architecture Layer
Bucket
arch/x86
Inferred role
Architecture Layer: exported/initcall integration point
Status
integration implementation candidate

Why This File Exists

CPU and platform-specific kernel glue: boot entry, traps, syscall entry, interrupts, page tables, context switch, and low-level barriers.

Dependency Surface

Detected Declarations

Annotated Snippet

if (unlikely(cpu != smp_processor_id())) {
			cycles -= (now - bclock);
			cpu = smp_processor_id();
			bclock = rdtsc_ordered();
		}
	}
	preempt_enable();
}

/*
 * On Intel the TPAUSE instruction waits until any of:
 * 1) the TSC counter exceeds the value provided in EDX:EAX
 * 2) global timeout in IA32_UMWAIT_CONTROL is exceeded
 * 3) an external interrupt occurs
 */
static void delay_halt_tpause(u64 start, u64 cycles)
{
	u64 until = start + cycles;
	u32 eax, edx;

	eax = lower_32_bits(until);
	edx = upper_32_bits(until);

	/*
	 * Hard code the deeper (C0.2) sleep state because exit latency is
	 * small compared to the "microseconds" that usleep() will delay.
	 */
	__tpause(TPAUSE_C02_STATE, edx, eax);
}

/*
 * On some AMD platforms, MWAITX has a configurable 32-bit timer, that
 * counts with TSC frequency. The input value is the number of TSC cycles
 * to wait. MWAITX will also exit when the timer expires.
 */
static void delay_halt_mwaitx(u64 unused, u64 cycles)
{
	u64 delay;

	delay = min_t(u64, MWAITX_MAX_WAIT_CYCLES, cycles);
	/*
	 * Use cpu_tss_rw as a cacheline-aligned, seldom accessed per-cpu
	 * variable as the monitor target.
	 */
	__monitorx(raw_cpu_ptr(&cpu_tss_rw), 0, 0);

	/*
	 * AMD, like Intel, supports the EAX hint and EAX=0xf means, do not
	 * enter any deep C-state and we use it here in delay() to minimize
	 * wakeup latency.
	 */
	__mwaitx(MWAITX_DISABLE_CSTATES, delay, MWAITX_ECX_TIMER_ENABLE);
}

/*
 * Call a vendor specific function to delay for a given amount of time. Because
 * these functions may return earlier than requested, check for actual elapsed
 * time and call again until done.
 */
static void delay_halt(u64 __cycles)
{
	u64 start, end, cycles = __cycles;

	/*
	 * Timer value of 0 causes MWAITX to wait indefinitely, unless there
	 * is a store on the memory monitored by MONITORX.
	 */
	if (!cycles)
		return;

	start = rdtsc_ordered();

	for (;;) {
		delay_halt_fn(start, cycles);
		end = rdtsc_ordered();

		if (cycles <= end - start)
			break;

		cycles -= end - start;
		start = end;
	}
}

void __init use_tsc_delay(void)
{
	if (delay_fn == delay_loop)
		delay_fn = delay_tsc;
}

Annotation

Implementation Notes