[PATCH AUTOSEL 4.9 18/24] openrisc: start CPU timer early in boot

[Sasha Levin <sashal@kernel.org>]

From: "Jason A. Donenfeld" <Jason@zx2c4.com>

[ Upstream commit 516dd4aacd67a0f27da94f3fe63fe0f4dbab6e2b ]

In order to measure the boot process, the timer should be switched on as
early in boot as possible. As well, the commit defines the get_cycles
macro, like the previous patches in this series, so that generic code is
aware that it's implemented by the platform, as is done on other archs.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Acked-by: Stafford Horne <shorne@gmail.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/openrisc/include/asm/timex.h | 1 +
 arch/openrisc/kernel/head.S       | 9 +++++++++
 2 files changed, 10 insertions(+)

diff --git a/arch/openrisc/include/asm/timex.h b/arch/openrisc/include/asm/timex.h
index 9935cad1b9b9..34d015bf0462 100644
--- a/arch/openrisc/include/asm/timex.h
+++ b/arch/openrisc/include/asm/timex.h
@@ -27,6 +27,7 @@ static inline cycles_t get_cycles(void)
 {
 	return mfspr(SPR_TTCR);
 }
+#define get_cycles get_cycles
 
 /* This isn't really used any more */
 #define CLOCK_TICK_RATE 1000
diff --git a/arch/openrisc/kernel/head.S b/arch/openrisc/kernel/head.S
index 98dd6860bc0b..0b6be5b3522b 100644
--- a/arch/openrisc/kernel/head.S
+++ b/arch/openrisc/kernel/head.S
@@ -452,6 +452,15 @@ _start:
 	l.ori	r3,r0,0x1
 	l.mtspr	r0,r3,SPR_SR
 
+	/*
+	 * Start the TTCR as early as possible, so that the RNG can make use of
+	 * measurements of boot time from the earliest opportunity. Especially
+	 * important is that the TTCR does not return zero by the time we reach
+	 * rand_initialize().
+	 */
+	l.movhi r3,hi(SPR_TTMR_CR)
+	l.mtspr r0,r3,SPR_TTMR
+
 	CLEAR_GPR(r1)
 	CLEAR_GPR(r2)
 	CLEAR_GPR(r3)
-- 
2.35.1