powerpc/tau: Convert from timer to workqueue

Since commit 19dbdcb8 ("smp: Warn on function calls from softirq
context") the Thermal Assist Unit driver causes a warning like the
following when CONFIG_SMP is enabled.

  ------------[ cut here ]------------
  WARNING: CPU: 0 PID: 0 at kernel/smp.c:428 smp_call_function_many_cond+0xf4/0x38c
  Modules linked in:
  CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.7.0-pmac #3
  NIP:  c00b37a8 LR: c00b3abc CTR: c001218c
  REGS: c0799c60 TRAP: 0700   Not tainted  (5.7.0-pmac)
  MSR:  00029032 <EE,ME,IR,DR,RI>  CR: 42000224  XER: 00000000
  GPR00: c00b3abc c0799d18 c076e300 c079ef5c c0011fec 00000000 00000000 00000000
  GPR08: 00000100 00000100 00008000 ffffffff 42000224 00000000 c079d040 c079d044
  GPR16: 00000001 00000000 00000004 c0799da0 c079f054 c07a0000 c07a0000 00000000
  GPR24: c0011fec 00000000 c079ef5c c079ef5c 00000000 00000000 00000000 00000000
  NIP [c00b37a8] smp_call_function_many_cond+0xf4/0x38c
  LR [c00b3abc] on_each_cpu+0x38/0x68
  Call Trace:
  [c0799d18] [ffffffff] 0xffffffff (unreliable)
  [c0799d68] [c00b3abc] on_each_cpu+0x38/0x68
  [c0799d88] [c0096704] call_timer_fn.isra.26+0x20/0x7c
  [c0799d98] [c0096b40] run_timer_softirq+0x1d4/0x3fc
  [c0799df8] [c05b4368] __do_softirq+0x118/0x240
  [c0799e58] [c0039c44] irq_exit+0xc4/0xcc
  [c0799e68] [c000ade8] timer_interrupt+0x1b0/0x230
  [c0799ea8] [c0013520] ret_from_except+0x0/0x14
  --- interrupt: 901 at arch_cpu_idle+0x24/0x6c
      LR = arch_cpu_idle+0x24/0x6c
  [c0799f70] [00000001] 0x1 (unreliable)
  [c0799f80] [c0060990] do_idle+0xd8/0x17c
  [c0799fa0] [c0060ba8] cpu_startup_entry+0x24/0x28
  [c0799fb0] [c072d220] start_kernel+0x434/0x44c
  [c0799ff0] [00003860] 0x3860
  Instruction dump:
  8129f204 2f890000 40beff98 3d20c07a 8929eec4 2f890000 40beff88 0fe00000
  81220000 552805de 550802ef 4182ff84 <0fe00000> 3860ffff 7f65db78 7f44d378
  ---[ end trace 34a886e47819c2eb ]---

Don't call on_each_cpu() from a timer callback, call it from a worker
thread instead.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/bb61650bea4f4c91fb8e24b9a6f130a1438651a7.1599260540.git.fthain@telegraphics.com.au

arch/powerpc/kernel/tau_6xx.c

@@ -13,13 +13,14 @@
  */
 
 #include <linux/errno.h>
-#include <linux/jiffies.h>
 #include <linux/kernel.h>
 #include <linux/param.h>
 #include <linux/string.h>
 #include <linux/mm.h>
 #include <linux/interrupt.h>
 #include <linux/init.h>
+#include <linux/delay.h>
+#include <linux/workqueue.h>
 
 #include <asm/io.h>
 #include <asm/reg.h>
@@ -39,8 +40,6 @@ static struct tau_temp
 	unsigned char grew;
 } tau[NR_CPUS];
 
-struct timer_list tau_timer;
-
 #undef DEBUG
 
 /* TODO: put these in a /proc interface, with some sanity checks, and maybe
@@ -50,7 +49,7 @@ struct timer_list tau_timer;
 #define step_size		2	/* step size when temp goes out of range */
 #define window_expand		1	/* expand the window by this much */
 /* configurable values for shrinking the window */
-#define shrink_timer	2*HZ	/* period between shrinking the window */
+#define shrink_timer	2000	/* period between shrinking the window */
 #define min_window	2	/* minimum window size, degrees C */
 
 static void set_thresholds(unsigned long cpu)
@@ -187,14 +186,18 @@ static void tau_timeout(void * info)
 	local_irq_restore(flags);
 }
 
-static void tau_timeout_smp(struct timer_list *unused)
-{
+static struct workqueue_struct *tau_workq;
 
-	/* schedule ourselves to be run again */
-	mod_timer(&tau_timer, jiffies + shrink_timer) ;
+static void tau_work_func(struct work_struct *work)
+{
+	msleep(shrink_timer);
 	on_each_cpu(tau_timeout, NULL, 0);
+	/* schedule ourselves to be run again */
+	queue_work(tau_workq, work);
 }
 
+DECLARE_WORK(tau_work, tau_work_func);
+
 /*
  * setup the TAU
  *
@@ -227,21 +230,16 @@ static int __init TAU_init(void)
 		return 1;
 	}
 
-
-	/* first, set up the window shrinking timer */
-	timer_setup(&tau_timer, tau_timeout_smp, 0);
-	tau_timer.expires = jiffies + shrink_timer;
-	add_timer(&tau_timer);
+	tau_workq = alloc_workqueue("tau", WQ_UNBOUND, 1, 0);
+	if (!tau_workq)
+		return -ENOMEM;
 
 	on_each_cpu(TAU_init_smp, NULL, 0);
 
-	printk("Thermal assist unit ");
-#ifdef CONFIG_TAU_INT
-	printk("using interrupts, ");
-#else
-	printk("using timers, ");
-#endif
-	printk("shrink_timer: %d jiffies\n", shrink_timer);
+	queue_work(tau_workq, &tau_work);
+
+	pr_info("Thermal assist unit using %s, shrink_timer: %d ms\n",
+		IS_ENABLED(CONFIG_TAU_INT) ? "interrupts" : "workqueue", shrink_timer);
 	tau_initialized = 1;
 
 	return 0;