[v4,11/13] mips: Add udelay lpj numbers adjustment

Commit Message

Serge Semin May 21, 2020, 2:07 p.m. UTC

Loops-per-jiffies is a special number which represents a number of
noop-loop cycles per CPU-scheduler quantum - jiffies. As you
understand aside from CPU-specific implementation it depends on
the CPU frequency. So when a platform has the CPU frequency fixed,
we have no problem and the current udelay interface will work
just fine. But as soon as CPU-freq driver is enabled and the cores
frequency changes, we'll end up with distorted udelay's. In order
to fix this we have to accordinly adjust the per-CPU udelay_val
(the same as the global loops_per_jiffy) number. This can be done
in the CPU-freq transition event handler. We subscribe to that event
in the MIPS arch time-inititalization method.

Co-developed-by: Alexey Malahov <[e-mail]>
Signed-off-by: Alexey Malahov <[e-mail]>
Signed-off-by: Serge Semin <[e-mail]>
Reviewed-by: Jiaxun Yang <[e-mail]>
Cc: Thomas Bogendoerfer <[e-mail]>
Cc: Paul Burton <[e-mail]>
Cc: Ralf Baechle <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Rob Herring <[e-mail]>
Cc: [e-mail]
---
 arch/mips/kernel/time.c | 70 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)

Comments

Thomas Bogendoerfer May 22, 2020, 7:29 a.m. UTC | #1

On Thu, May 21, 2020 at 05:07:22PM +0300, Serge Semin wrote:
> Loops-per-jiffies is a special number which represents a number of
> noop-loop cycles per CPU-scheduler quantum - jiffies. As you
> understand aside from CPU-specific implementation it depends on
> the CPU frequency. So when a platform has the CPU frequency fixed,
> we have no problem and the current udelay interface will work
> just fine. But as soon as CPU-freq driver is enabled and the cores
> frequency changes, we'll end up with distorted udelay's. In order
> to fix this we have to accordinly adjust the per-CPU udelay_val
> (the same as the global loops_per_jiffy) number. This can be done
> in the CPU-freq transition event handler. We subscribe to that event
> in the MIPS arch time-inititalization method.
> 
> Co-developed-by: Alexey Malahov <[e-mail]>
> Signed-off-by: Alexey Malahov <[e-mail]>
> Signed-off-by: Serge Semin <[e-mail]>
> Reviewed-by: Jiaxun Yang <[e-mail]>
> Cc: Thomas Bogendoerfer <[e-mail]>
> Cc: Paul Burton <[e-mail]>
> Cc: Ralf Baechle <[e-mail]>
> Cc: Arnd Bergmann <[e-mail]>
> Cc: Rob Herring <[e-mail]>
> Cc: [e-mail]
> ---
>  arch/mips/kernel/time.c | 70 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 70 insertions(+)

applied to mips-next.

Thomas.

Patch

diff --git a/arch/mips/kernel/time.c b/arch/mips/kernel/time.c
index 37e9413a393d..caa01457dce6 100644
--- a/arch/mips/kernel/time.c
+++ b/arch/mips/kernel/time.c
@@ -18,12 +18,82 @@ 
 #include <linux/smp.h>
 #include <linux/spinlock.h>
 #include <linux/export.h>
+#include <linux/cpufreq.h>
+#include <linux/delay.h>
 
 #include <asm/cpu-features.h>
 #include <asm/cpu-type.h>
 #include <asm/div64.h>
 #include <asm/time.h>
 
+#ifdef CONFIG_CPU_FREQ
+
+static DEFINE_PER_CPU(unsigned long, pcp_lpj_ref);
+static DEFINE_PER_CPU(unsigned long, pcp_lpj_ref_freq);
+static unsigned long glb_lpj_ref;
+static unsigned long glb_lpj_ref_freq;
+
+static int cpufreq_callback(struct notifier_block *nb,
+			    unsigned long val, void *data)
+{
+	struct cpufreq_freqs *freq = data;
+	struct cpumask *cpus = freq->policy->cpus;
+	unsigned long lpj;
+	int cpu;
+
+	/*
+	 * Skip lpj numbers adjustment if the CPU-freq transition is safe for
+	 * the loops delay. (Is this possible?)
+	 */
+	if (freq->flags & CPUFREQ_CONST_LOOPS)
+		return NOTIFY_OK;
+
+	/* Save the initial values of the lpjes for future scaling. */
+	if (!glb_lpj_ref) {
+		glb_lpj_ref = boot_cpu_data.udelay_val;
+		glb_lpj_ref_freq = freq->old;
+
+		for_each_online_cpu(cpu) {
+			per_cpu(pcp_lpj_ref, cpu) =
+				cpu_data[cpu].udelay_val;
+			per_cpu(pcp_lpj_ref_freq, cpu) = freq->old;
+		}
+	}
+
+	/*
+	 * Adjust global lpj variable and per-CPU udelay_val number in
+	 * accordance with the new CPU frequency.
+	 */
+	if ((val == CPUFREQ_PRECHANGE  && freq->old < freq->new) ||
+	    (val == CPUFREQ_POSTCHANGE && freq->old > freq->new)) {
+		loops_per_jiffy = cpufreq_scale(glb_lpj_ref,
+						glb_lpj_ref_freq,
+						freq->new);
+
+		for_each_cpu(cpu, cpus) {
+			lpj = cpufreq_scale(per_cpu(pcp_lpj_ref, cpu),
+					    per_cpu(pcp_lpj_ref_freq, cpu),
+					    freq->new);
+			cpu_data[cpu].udelay_val = (unsigned int)lpj;
+		}
+	}
+
+	return NOTIFY_OK;
+}
+
+static struct notifier_block cpufreq_notifier = {
+	.notifier_call  = cpufreq_callback,
+};
+
+static int __init register_cpufreq_notifier(void)
+{
+	return cpufreq_register_notifier(&cpufreq_notifier,
+					 CPUFREQ_TRANSITION_NOTIFIER);
+}
+core_initcall(register_cpufreq_notifier);
+
+#endif /* CONFIG_CPU_FREQ */
+
 /*
  * forward reference
  */