[PATCH 4.9 153/310] powerpc/[booke|4xx]: Dont clobber TCR[WP] when setting TCR[DIE]

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ivan Mikhaylov <ivan@xxxxxxxxxx>


[ Upstream commit 6e2f03e292ef46eed2b31b0a344a91d514f9cd81 ]

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@xxxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/powerpc/kernel/time.c |   14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -719,12 +719,20 @@ static int __init get_freq(char *name, i
 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)