MIPS: Rework smt cmdline parameters

Provide a generic smt parameters interface aligned with s390
to allow users to limit smt usage and threads per core.

It replaced previous undocumented "nothreads" parameter for
smp-cps which is ambiguous and does not cover smp-mt.

Signed-off-by: Jiaxun Yang <[email protected]>
Signed-off-by: Thomas Bogendoerfer <[email protected]>

Author: FlyGoat

Documentation/admin-guide/kernel-parameters.txt

@@ -3838,7 +3838,7 @@
 	nosmp		[SMP] Tells an SMP kernel to act as a UP kernel,
 			and disable the IO APIC.  legacy for "maxcpus=0".
 
-	nosmt		[KNL,S390] Disable symmetric multithreading (SMT).
+	nosmt		[KNL,MIPS,S390] Disable symmetric multithreading (SMT).
 			Equivalent to smt=1.
 
 			[KNL,X86] Disable symmetric multithreading (SMT).
@@ -5735,7 +5735,7 @@
 				1: Fast pin select (default)
 				2: ATC IRMode
 
-	smt=		[KNL,S390] Set the maximum number of threads (logical
+	smt=		[KNL,MIPS,S390] Set the maximum number of threads (logical
 			CPUs) to use per physical CPU on systems capable of
 			symmetric multithreading (SMT). Will be capped to the
 			actual hardware limit.

arch/mips/include/asm/smp.h

@@ -57,6 +57,8 @@ extern int __cpu_logical_map[NR_CPUS];
 /* Mask of CPUs which are currently definitely operating coherently */
 extern cpumask_t cpu_coherent_mask;
 
+extern unsigned int smp_max_threads __initdata;
+
 extern asmlinkage void smp_bootstrap(void);
 
 extern void calculate_cpu_foreign_map(void);

arch/mips/kernel/smp-cps.c

@@ -25,24 +25,13 @@
 #include <asm/time.h>
 #include <asm/uasm.h>
 
-static bool threads_disabled;
 static DECLARE_BITMAP(core_power, NR_CPUS);
 
 struct core_boot_config *mips_cps_core_bootcfg;
 
-static int __init setup_nothreads(char *s)
-{
-	threads_disabled = true;
-	return 0;
-}
-early_param("nothreads", setup_nothreads);
-
 static unsigned core_vpe_count(unsigned int cluster, unsigned core)
 {
-	if (threads_disabled)
-		return 1;
-
-	return mips_cps_numvps(cluster, core);
+	return min(smp_max_threads, mips_cps_numvps(cluster, core));
 }
 
 static void __init cps_smp_setup(void)

arch/mips/kernel/smp-mt.c

@@ -46,7 +46,8 @@ static void __init smvp_copy_vpe_config(void)
 static unsigned int __init smvp_vpe_init(unsigned int tc, unsigned int mvpconf0,
 	unsigned int ncpu)
 {
-	if (tc > ((mvpconf0 & MVPCONF0_PVPE) >> MVPCONF0_PVPE_SHIFT))
+	if (tc >= smp_max_threads ||
+		(tc > ((mvpconf0 & MVPCONF0_PVPE) >> MVPCONF0_PVPE_SHIFT)))
 		return ncpu;
 
 	/* Deactivate all but VPE 0 */

arch/mips/kernel/smp.c

@@ -73,6 +73,24 @@ static cpumask_t cpu_core_setup_map;
 
 cpumask_t cpu_coherent_mask;
 
+unsigned int smp_max_threads __initdata = UINT_MAX;
+
+static int __init early_nosmt(char *s)
+{
+	smp_max_threads = 1;
+	return 0;
+}
+early_param("nosmt", early_nosmt);
+
+static int __init early_smt(char *s)
+{
+	get_option(&s, &smp_max_threads);
+	/* Ensure at least one thread is available */
+	smp_max_threads = clamp_val(smp_max_threads, 1U, UINT_MAX);
+	return 0;
+}
+early_param("smt", early_smt);
+
 #ifdef CONFIG_GENERIC_IRQ_IPI
 static struct irq_desc *call_desc;
 static struct irq_desc *sched_desc;