powerpc/tau: Check processor type before enabling TAU interrupt

According to Freescale's documentation, MPC74XX processors have an
erratum that prevents the TAU interrupt from working, so don't try to
use it when running on those processors.

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/c281611544768e758bd58fe812cf702a5bd2d042.1599260540.git.fthain@telegraphics.com.au

arch/powerpc/kernel/tau_6xx.c

@@ -40,6 +40,8 @@ static struct tau_temp
 	unsigned char grew;
 } tau[NR_CPUS];
 
+static bool tau_int_enable;
+
 #undef DEBUG
 
 /* TODO: put these in a /proc interface, with some sanity checks, and maybe
@@ -54,22 +56,13 @@ static struct tau_temp
 
 static void set_thresholds(unsigned long cpu)
 {
-#ifdef CONFIG_TAU_INT
-	/*
-	 * setup THRM1,
-	 * threshold, valid bit, enable interrupts, interrupt when below threshold
-	 */
-	mtspr(SPRN_THRM1, THRM1_THRES(tau[cpu].low) | THRM1_V | THRM1_TIE | THRM1_TID);
+	u32 maybe_tie = tau_int_enable ? THRM1_TIE : 0;
 
-	/* setup THRM2,
-	 * threshold, valid bit, enable interrupts, interrupt when above threshold
-	 */
-	mtspr (SPRN_THRM2, THRM1_THRES(tau[cpu].high) | THRM1_V | THRM1_TIE);
-#else
-	/* same thing but don't enable interrupts */
-	mtspr(SPRN_THRM1, THRM1_THRES(tau[cpu].low) | THRM1_V | THRM1_TID);
-	mtspr(SPRN_THRM2, THRM1_THRES(tau[cpu].high) | THRM1_V);
-#endif
+	/* setup THRM1, threshold, valid bit, interrupt when below threshold */
+	mtspr(SPRN_THRM1, THRM1_THRES(tau[cpu].low) | THRM1_V | maybe_tie | THRM1_TID);
+
+	/* setup THRM2, threshold, valid bit, interrupt when above threshold */
+	mtspr(SPRN_THRM2, THRM1_THRES(tau[cpu].high) | THRM1_V | maybe_tie);
 }
 
 static void TAUupdate(int cpu)
@@ -142,9 +135,8 @@ static void tau_timeout(void * info)
 	local_irq_save(flags);
 	cpu = smp_processor_id();
 
-#ifndef CONFIG_TAU_INT
+	if (!tau_int_enable)
 		TAUupdate(cpu);
-#endif
 
 	size = tau[cpu].high - tau[cpu].low;
 	if (size > min_window && ! tau[cpu].grew) {
@@ -225,6 +217,9 @@ static int __init TAU_init(void)
 		return 1;
 	}
 
+	tau_int_enable = IS_ENABLED(CONFIG_TAU_INT) &&
+			 !strcmp(cur_cpu_spec->platform, "ppc750");
+
 	tau_workq = alloc_workqueue("tau", WQ_UNBOUND, 1, 0);
 	if (!tau_workq)
 		return -ENOMEM;
@@ -234,7 +229,7 @@ static int __init TAU_init(void)
 	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_int_enable ? "interrupts" : "workqueue", shrink_timer);
 	tau_initialized = 1;
 
 	return 0;

arch/powerpc/platforms/Kconfig

@@ -223,9 +223,8 @@ config TAU
 	  temperature within 2-4 degrees Celsius. This option shows the current
 	  on-die temperature in /proc/cpuinfo if the cpu supports it.
 
-	  Unfortunately, on some chip revisions, this sensor is very inaccurate
-	  and in many cases, does not work at all, so don't assume the cpu
-	  temp is actually what /proc/cpuinfo says it is.
+	  Unfortunately, this sensor is very inaccurate when uncalibrated, so
+	  don't assume the cpu temp is actually what /proc/cpuinfo says it is.
 
 config TAU_INT
 	bool "Interrupt driven TAU driver (DANGEROUS)"