powerpc/time: Expose boot_tb via accessor

- Define accessor function get_boot_tb() to safely return boot_tb value,
this is only needed when running in SPLPAR environments, so the
accessor is built conditionally under CONFIG_PPC_SPLPAR.

- Tag boot_tb as __ro_after_init since it is written once at initialized
and never updated afterwards.

Signed-off-by: Aboorva Devarajan <[e-mail]>
Tested-by: Tejas Manhas <[e-mail]>
Tested-by: Venkat Rao Bagalkote <[e-mail]>
Signed-off-by: Madhavan Srinivasan <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

arch/powerpc/include/asm/time.h

···
 
 extern void generic_calibrate_decr(void);
 
+#ifdef CONFIG_PPC_SPLPAR
+extern u64 get_boot_tb(void);
+#endif
+
 /* Some sane defaults: 125 MHz timebase, 1GHz processor */
 extern unsigned long ppc_proc_freq;
 #define DEFAULT_PROC_FREQ	(DEFAULT_TB_FREQ * 8)

arch/powerpc/kernel/time.c

···
 
 static u64 tb_to_ns_scale __read_mostly;
 static unsigned tb_to_ns_shift __read_mostly;
-static u64 boot_tb __read_mostly;
+static u64 boot_tb __ro_after_init;
 
 extern struct timezone sys_tz;
 static long timezone_offset;
···
 	return mulhdu(get_tb() - boot_tb, tb_to_ns_scale) << tb_to_ns_shift;
 }
 
+#ifdef CONFIG_PPC_SPLPAR
+u64 get_boot_tb(void)
+{
+	return boot_tb;
+}
+#endif
 
 #ifdef CONFIG_PPC_PSERIES