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Subject[RFC 2/2] x86, tsc: Enable clock for ealry printk timestamp
Date
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@intel.com>
---
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
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