RISC-V: Check clint_time_val before use

The NoMMU kernel is broken for QEMU virt machine from Linux-5.9-rc6
because clint_time_val is used even before CLINT driver is probed
at following places:
1. rand_initialize() calls get_cycles() which in-turn uses
   clint_time_val
2. boot_init_stack_canary() calls get_cycles() which in-turn
   uses clint_time_val

The issue#1 (above) is fixed by providing custom random_get_entropy()
for RISC-V NoMMU kernel. For issue#2 (above), we remove dependency of
boot_init_stack_canary() on get_cycles() and this is aligned with the
boot_init_stack_canary() implementations of ARM, ARM64 and MIPS kernel.

Fixes: d5be89a ("RISC-V: Resurrect the MMIO timer implementation for M-mode systems")
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Tested-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/stackprotector.h

@@ -5,7 +5,6 @@
 
 #include <linux/random.h>
 #include <linux/version.h>
-#include <asm/timex.h>
 
 extern unsigned long __stack_chk_guard;
 
@@ -18,12 +17,9 @@ extern unsigned long __stack_chk_guard;
 static __always_inline void boot_init_stack_canary(void)
 {
 	unsigned long canary;
-	unsigned long tsc;
 
 	/* Try to get a semi random initial value. */
 	get_random_bytes(&canary, sizeof(canary));
-	tsc = get_cycles();
-	canary += tsc + (tsc << BITS_PER_LONG/2);
 	canary ^= LINUX_VERSION_CODE;
 	canary &= CANARY_MASK;
 

arch/riscv/include/asm/timex.h

@@ -33,6 +33,19 @@ static inline u32 get_cycles_hi(void)
 #define get_cycles_hi get_cycles_hi
 #endif /* CONFIG_64BIT */
 
+/*
+ * Much like MIPS, we may not have a viable counter to use at an early point
+ * in the boot process. Unfortunately we don't have a fallback, so instead
+ * we just return 0.
+ */
+static inline unsigned long random_get_entropy(void)
+{
+	if (unlikely(clint_time_val == NULL))
+		return 0;
+	return get_cycles();
+}
+#define random_get_entropy()	random_get_entropy()
+
 #else /* CONFIG_RISCV_M_MODE */
 
 static inline cycles_t get_cycles(void)