linux-kernel - [PATCH v3 1/3] x86/delay: Preparatory code cleanup

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Date:   Fri, 10 Apr 2020 16:29:53 -0700
From:   Kyung Min Park <kyung.min.park@...el.com>
To:     x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        gregkh@...uxfoundation.org, ak@...ux.intel.com,
        tony.luck@...el.com, ashok.raj@...el.com, ravi.v.shankar@...el.com,
        fenghua.yu@...el.com, kyung.min.park@...el.com
Subject: [PATCH v3 1/3] x86/delay: Preparatory code cleanup

From: Thomas Gleixner <tglx@...utronix.de>

From: Thomas Gleixner <tglx@...utronix.de>

The naming conventions in the delay code are confusing at best.

All delay variants use a loops argument and or variable which originates
from the original delay_loop() implementation. But all variants except
delay_loop() are based on TSC cycles.

Rename the argument to cycles and make it type u64 to avoid these weird
expansions to u64 in the functions.

Rename MWAITX_MAX_LOOPS to MWAITX_MAX_WAIT_CYCLES for the same reason
and fixup the comment of delay_mwaitx() as well.

Mark the delay_fn function pointer __ro_after_init and fixup the comment
for it.

No functional change and preparation for the upcoming TPAUSE based delay
variant.

[Kyung Min Park: Added __init to use_tsc_delay()]

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Kyung Min Park <kyung.min.park@...el.com>
---
 arch/x86/include/asm/delay.h |  2 +-
 arch/x86/include/asm/mwait.h |  2 +-
 arch/x86/lib/delay.c         | 45 +++++++++++++++++++++++---------------------
 3 files changed, 26 insertions(+), 23 deletions(-)

diff --git a/arch/x86/include/asm/delay.h b/arch/x86/include/asm/delay.h
index de9e784..bb91d7c 100644
--- a/arch/x86/include/asm/delay.h
+++ b/arch/x86/include/asm/delay.h
@@ -4,7 +4,7 @@
 
 #include <asm-generic/delay.h>
 
-void use_tsc_delay(void);
+void __init use_tsc_delay(void);
 void use_mwaitx_delay(void);
 
 #endif /* _ASM_X86_DELAY_H */
diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
index b809f11..a43b35b 100644
--- a/arch/x86/include/asm/mwait.h
+++ b/arch/x86/include/asm/mwait.h
@@ -20,7 +20,7 @@
 
 #define MWAIT_ECX_INTERRUPT_BREAK	0x1
 #define MWAITX_ECX_TIMER_ENABLE		BIT(1)
-#define MWAITX_MAX_LOOPS		((u32)-1)
+#define MWAITX_MAX_WAIT_CYCLES		UINT_MAX
 #define MWAITX_DISABLE_CSTATES		0xf0
 
 u32 get_umwait_control_msr(void);
diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index c126571..887d52d 100644
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -27,9 +27,19 @@
 # include <asm/smp.h>
 #endif
 
+static void delay_loop(u64 __loops);
+
+/*
+ * Calibration and selection of the delay mechanism happens only once
+ * during boot.
+ */
+static void (*delay_fn)(u64) __ro_after_init = delay_loop;
+
 /* simple loop based delay: */
-static void delay_loop(unsigned long loops)
+static void delay_loop(u64 __loops)
 {
+	unsigned long loops = (unsigned long)__loops;
+
 	asm volatile(
 		"	test %0,%0	\n"
 		"	jz 3f		\n"
@@ -49,9 +59,9 @@ static void delay_loop(unsigned long loops)
 }
 
 /* TSC based delay: */
-static void delay_tsc(unsigned long __loops)
+static void delay_tsc(u64 cycles)
 {
-	u64 bclock, now, loops = __loops;
+	u64 bclock, now;
 	int cpu;
 
 	preempt_disable();
@@ -59,7 +69,7 @@ static void delay_tsc(unsigned long __loops)
 	bclock = rdtsc_ordered();
 	for (;;) {
 		now = rdtsc_ordered();
-		if ((now - bclock) >= loops)
+		if ((now - bclock) >= cycles)
 			break;
 
 		/* Allow RT tasks to run */
@@ -77,7 +87,7 @@ static void delay_tsc(unsigned long __loops)
 		 * counter for this CPU.
 		 */
 		if (unlikely(cpu != smp_processor_id())) {
-			loops -= (now - bclock);
+			cycles -= (now - bclock);
 			cpu = smp_processor_id();
 			bclock = rdtsc_ordered();
 		}
@@ -87,24 +97,24 @@ static void delay_tsc(unsigned long __loops)
 
 /*
  * On some AMD platforms, MWAITX has a configurable 32-bit timer, that
- * counts with TSC frequency. The input value is the loop of the
- * counter, it will exit when the timer expires.
+ * 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_mwaitx(unsigned long __loops)
+static void delay_mwaitx(u64 cycles)
 {
-	u64 start, end, delay, loops = __loops;
+	u64 start, end, delay;
 
 	/*
 	 * Timer value of 0 causes MWAITX to wait indefinitely, unless there
 	 * is a store on the memory monitored by MONITORX.
 	 */
-	if (loops == 0)
+	if (!cycles)
 		return;
 
 	start = rdtsc_ordered();
 
 	for (;;) {
-		delay = min_t(u64, MWAITX_MAX_LOOPS, loops);
+		delay = min_t(u64, MWAITX_MAX_WAIT_CYCLES, cycles);
 
 		/*
 		 * Use cpu_tss_rw as a cacheline-aligned, seldomly
@@ -121,22 +131,15 @@ static void delay_mwaitx(unsigned long __loops)
 
 		end = rdtsc_ordered();
 
-		if (loops <= end - start)
+		if (cycles <= end - start)
 			break;
 
-		loops -= end - start;
-
+		cycles -= end - start;
 		start = end;
 	}
 }
 
-/*
- * Since we calibrate only once at boot, this
- * function should be set once at boot and not changed
- */
-static void (*delay_fn)(unsigned long) = delay_loop;
-
-void use_tsc_delay(void)
+void __init use_tsc_delay(void)
 {
 	if (delay_fn == delay_loop)
 		delay_fn = delay_tsc;
-- 
2.7.4