powerpc/tau: Disable TAU between measurements

Enabling CONFIG_TAU_INT causes random crashes:

Unrecoverable exception 1700 at c0009414 (msr=1000)
Oops: Unrecoverable exception, sig: 6 [#1]
BE PAGE_SIZE=4K MMU=Hash SMP NR_CPUS=2 PowerMac
Modules linked in:
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.7.0-pmac-00043-gd5f545e1a8593 #5
NIP:  c0009414 LR: c0009414 CTR: c00116fc
REGS: c0799eb8 TRAP: 1700   Not tainted  (5.7.0-pmac-00043-gd5f545e1a8593)
MSR:  00001000 <ME>  CR: 22000228  XER: 00000100

GPR00: 00000000 c0799f70 c076e300 00800000 0291c0ac 00e00000 c076e300 00049032
GPR08: 00000001 c00116fc 00000000 dfbd3200 ffffffff 007f80a8 00000000 00000000
GPR16: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 c075ce04
GPR24: c075ce04 dfff8880 c07b0000 c075ce04 00080000 00000001 c079ef98 c079ef5c
NIP [c0009414] arch_cpu_idle+0x24/0x6c
LR [c0009414] arch_cpu_idle+0x24/0x6c
Call Trace:
[c0799f70] [00000001] 0x1 (unreliable)
[c0799f80] [c0060990] do_idle+0xd8/0x17c
[c0799fa0] [c0060ba4] cpu_startup_entry+0x20/0x28
[c0799fb0] [c072d220] start_kernel+0x434/0x44c
[c0799ff0] [00003860] 0x3860
Instruction dump:
XXXXXXXX XXXXXXXX XXXXXXXX 3d20c07b XXXXXXXX XXXXXXXX XXXXXXXX 7c0802a6
XXXXXXXX XXXXXXXX XXXXXXXX 4e800421 XXXXXXXX XXXXXXXX XXXXXXXX 7d2000a6
---[ end trace 3a0c9b5cb216db6b ]---

Resolve this problem by disabling each THRMn comparator when handling
the associated THRMn interrupt and by disabling the TAU entirely when
updating THRMn thresholds.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/5a0ba3dc5612c7aac596727331284a3676c08472.1599260540.git.fthain@telegraphics.com.au

arch/powerpc/kernel/tau_6xx.c

@@ -42,8 +42,6 @@ static struct tau_temp
 
 static bool tau_int_enable;
 
-#undef DEBUG
-
 /* TODO: put these in a /proc interface, with some sanity checks, and maybe
  * dynamic adjustment to minimize # of interrupts */
 /* configurable values for step size and how much to expand the window when
@@ -67,42 +65,33 @@ static void set_thresholds(unsigned long cpu)
 
 static void TAUupdate(int cpu)
 {
-	unsigned thrm;
-
-#ifdef DEBUG
-	printk("TAUupdate ");
-#endif
+	u32 thrm;
+	u32 bits = THRM1_TIV | THRM1_TIN | THRM1_V;
 
 	/* if both thresholds are crossed, the step_sizes cancel out
 	 * and the window winds up getting expanded twice. */
-	if((thrm = mfspr(SPRN_THRM1)) & THRM1_TIV){ /* is valid? */
-		if(thrm & THRM1_TIN){ /* crossed low threshold */
-			if (tau[cpu].low >= step_size){
-				tau[cpu].low -= step_size;
-				tau[cpu].high -= (step_size - window_expand);
-			}
-			tau[cpu].grew = 1;
-#ifdef DEBUG
-			printk("low threshold crossed ");
-#endif
+	thrm = mfspr(SPRN_THRM1);
+	if ((thrm & bits) == bits) {
+		mtspr(SPRN_THRM1, 0);
+
+		if (tau[cpu].low >= step_size) {
+			tau[cpu].low -= step_size;
+			tau[cpu].high -= (step_size - window_expand);
 		}
+		tau[cpu].grew = 1;
+		pr_debug("%s: low threshold crossed\n", __func__);
 	}
-	if((thrm = mfspr(SPRN_THRM2)) & THRM1_TIV){ /* is valid? */
-		if(thrm & THRM1_TIN){ /* crossed high threshold */
-			if (tau[cpu].high <= 127-step_size){
-				tau[cpu].low += (step_size - window_expand);
-				tau[cpu].high += step_size;
-			}
-			tau[cpu].grew = 1;
-#ifdef DEBUG
-			printk("high threshold crossed ");
-#endif
+	thrm = mfspr(SPRN_THRM2);
+	if ((thrm & bits) == bits) {
+		mtspr(SPRN_THRM2, 0);
+
+		if (tau[cpu].high <= 127 - step_size) {
+			tau[cpu].low += (step_size - window_expand);
+			tau[cpu].high += step_size;
 		}
+		tau[cpu].grew = 1;
+		pr_debug("%s: high threshold crossed\n", __func__);
 	}
-
-#ifdef DEBUG
-	printk("grew = %d\n", tau[cpu].grew);
-#endif
 }
 
 #ifdef CONFIG_TAU_INT
@@ -127,17 +116,17 @@ void TAUException(struct pt_regs * regs)
 static void tau_timeout(void * info)
 {
 	int cpu;
-	unsigned long flags;
 	int size;
 	int shrink;
 
-	/* disabling interrupts *should* be okay */
-	local_irq_save(flags);
 	cpu = smp_processor_id();
 
 	if (!tau_int_enable)
 		TAUupdate(cpu);
 
+	/* Stop thermal sensor comparisons and interrupts */
+	mtspr(SPRN_THRM3, 0);
+
 	size = tau[cpu].high - tau[cpu].low;
 	if (size > min_window && ! tau[cpu].grew) {
 		/* do an exponential shrink of half the amount currently over size */
@@ -159,18 +148,12 @@ static void tau_timeout(void * info)
 
 	set_thresholds(cpu);
 
-	/*
-	 * Do the enable every time, since otherwise a bunch of (relatively)
-	 * complex sleep code needs to be added. One mtspr every time
-	 * tau_timeout is called is probably not a big deal.
-	 *
+	/* Restart thermal sensor comparisons and interrupts.
 	 * The "PowerPC 740 and PowerPC 750 Microprocessor Datasheet"
 	 * recommends that "the maximum value be set in THRM3 under all
 	 * conditions."
 	 */
 	mtspr(SPRN_THRM3, THRM3_SITV(0x1fff) | THRM3_E);
-
-	local_irq_restore(flags);
 }
 
 static struct workqueue_struct *tau_workq;

arch/powerpc/platforms/Kconfig

@@ -227,7 +227,7 @@ config TAU
 	  don't assume the cpu temp is actually what /proc/cpuinfo says it is.
 
 config TAU_INT
-	bool "Interrupt driven TAU driver (DANGEROUS)"
+	bool "Interrupt driven TAU driver (EXPERIMENTAL)"
 	depends on TAU
 	help
 	  The TAU supports an interrupt driven mode which causes an interrupt
@@ -235,12 +235,7 @@ config TAU_INT
 	  to get notified the temp has exceeded a range. With this option off,
 	  a timer is used to re-check the temperature periodically.
 
-	  However, on some cpus it appears that the TAU interrupt hardware
-	  is buggy and can cause a situation which would lead unexplained hard
-	  lockups.
-
-	  Unless you are extending the TAU driver, or enjoy kernel/hardware
-	  debugging, leave this option off.
+	  If in doubt, say N here.
 
 config TAU_AVERAGE
 	bool "Average high and low temp"