powerpc/[booke|4xx]: Don't clobber TCR[WP] when setting TCR[DIE]

Prevent a kernel panic caused by unintentionally clearing TCR watchdog
bits. At this point in the kernel boot, the watchdog may have already
been enabled by u-boot. The original code's attempt to write to the TCR
register results in an inadvertent clearing of the watchdog
configuration bits, causing the 476 to reset.

Signed-off-by: Ivan Mikhaylov <ivan@de.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 2b33cfa..60714b8 100644 (file)
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -739,12 +739,20 @@ static int __init get_freq(char *name, int cells, unsigned long *val)
 static void start_cpu_decrementer(void)
 {
 #if defined(CONFIG_BOOKE) || defined(CONFIG_40x)
+       unsigned int tcr;
+
        /* Clear any pending timer interrupts */
        mtspr(SPRN_TSR, TSR_ENW | TSR_WIS | TSR_DIS | TSR_FIS);
 
-       /* Enable decrementer interrupt */
-       mtspr(SPRN_TCR, TCR_DIE);
-#endif /* defined(CONFIG_BOOKE) || defined(CONFIG_40x) */
+       tcr = mfspr(SPRN_TCR);
+       /*
+        * The watchdog may have already been enabled by u-boot. So leave
+        * TRC[WP] (Watchdog Period) alone.
+        */
+       tcr &= TCR_WP_MASK;     /* Clear all bits except for TCR[WP] */
+       tcr |= TCR_DIE;         /* Enable decrementer */
+       mtspr(SPRN_TCR, tcr);
+#endif
 }
 
 void __init generic_calibrate_decr(void)