[RFC 2/2] x86, tsc: Enable clock for ealry printk timestamp

[Feng Tang]

To show time info in kernel log earlier and help optimizing kernel
boot time, printk adds a debug hook "boot_printk_clock_fn()"  for
capable platform which has accurate clock in early boot phase.

This patch will add early param setup option, so that user can
chose to provide a tsc based early printk clock simply by adding
in command line: "boot_tsc=xxxxM" (xxxxM is the stable TSC freq).

Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
---
 arch/x86/kernel/tsc.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 74392d9..d99eb70 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -281,6 +281,57 @@ static int __init tsc_setup(char *str)
 
 __setup("tsc=", tsc_setup);
 
+
+/*
+ * This is to provide a not-so-accurate clock for printk timestamp in
+ * early boot phase (before sched_clock is setup).It could be used for
+ * analyzing/optimising kernel boot time and tools like bootchart.
+ *
+ * User can use it by simply append something like
+ *	"boot_tsc=1881M"
+ * to cmdline for a platform with a stable 1881MHz TSC.
+ */
+static u64 boot_tsc_mhz;
+static u64 boot_tsc_offset;
+
+static u64 boot_tsc_clock(void)
+{
+	u64 cur_tsc, cur_ns;
+
+	cur_tsc	= rdtsc();
+	cur_tsc -= boot_tsc_offset;
+
+	/* return value in ns */
+	cur_ns = div64_u64(cur_tsc * 1000, boot_tsc_mhz);
+	return cur_ns;
+}
+
+static int __init boot_tsc_setup(char *p)
+{
+	u64 tsc_hz;
+
+	if (!p)
+		return -EINVAL;
+
+	boot_tsc_offset = rdtsc();
+
+	tsc_hz = memparse(p, &p);
+	boot_tsc_mhz = div64_u64(tsc_hz, 1024 * 1024);
+	if (boot_tsc_mhz == 0)
+		return -EINVAL;
+
+	pr_info("TSC has run for %lld us\n",
+		div64_u64(boot_tsc_offset, boot_tsc_mhz));
+
+	/* Setup the early printk clock */
+	boot_printk_clock_fn = boot_tsc_clock;
+	pr_info("TSC: Setup early printk timestamp with %lldM TSC.",
+		boot_tsc_mhz);
+
+	return 0;
+}
+early_param("boot_tsc", boot_tsc_setup);
+
 #define MAX_RETRIES     5
 #define SMI_TRESHOLD    50000
 
-- 
2.7.4