[syslinux:master] core: add a ms-denominated timer

[syslinux-bot for H. Peter Anvin]

Commit-ID:  4a770eb97fc40ec8d9f394337614ac3c2074ee01
Gitweb:     http://syslinux.zytor.com/commit/4a770eb97fc40ec8d9f394337614ac3c2074ee01
Author:     H. Peter Anvin <hpa at zytor.com>
AuthorDate: Mon, 5 Jul 2010 15:41:36 -0700
Committer:  H. Peter Anvin <hpa at zytor.com>
CommitDate: Mon, 5 Jul 2010 15:41:36 -0700

core: add a ms-denominated timer

Add a timer denominated in milliseconds.  This is still driven by the
18.2 Hz timer interrupt, but counts "real" milliseconds, including
handing the adjustment factor (which in reality means it advances by
55 for most timer ticks and 54 for some.)

Signed-off-by: H. Peter Anvin <hpa at zytor.com>


---
 com32/include/syslinux/pmapi.h |    4 ++++
 core/include/core.h            |    6 +++++-
 core/pmapi.c                   |    3 +++
 core/timer.inc                 |   15 +++++++++++++--
 4 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/com32/include/syslinux/pmapi.h b/com32/include/syslinux/pmapi.h
index c325b62..fa39018 100644
--- a/com32/include/syslinux/pmapi.h
+++ b/com32/include/syslinux/pmapi.h
@@ -70,6 +70,10 @@ struct com32_pmapi {
 
     int (*chdir)(const char *);
     char *(*getcwd)(char *, size_t);
+
+    /* Should be "const volatile", but gcc miscompiles that sometimes */
+    volatile uint32_t *jiffies;
+    volatile uint32_t *ms_timer;
 };
 
 #endif /* _SYSLINUX_PMAPI_H */
diff --git a/core/include/core.h b/core/include/core.h
index eb7bfcd..7db5daf 100644
--- a/core/include/core.h
+++ b/core/include/core.h
@@ -65,10 +65,14 @@ __noreturn _kaboom(void);
 /*
  * Basic timer function...
  */
-extern volatile uint32_t __jiffies;
+extern volatile uint32_t __jiffies, __ms_timer;
 static inline uint32_t jiffies(void)
 {
     return __jiffies;
 }
+static inline uint32_t ms_timer(void)
+{
+    return __ms_timer;
+}
 
 #endif /* CORE_H */
diff --git a/core/pmapi.c b/core/pmapi.c
index ff65533..4b1ccbb 100644
--- a/core/pmapi.c
+++ b/core/pmapi.c
@@ -37,4 +37,7 @@ const struct com32_pmapi pm_api_vector =
 
     .chdir	= chdir,
     .getcwd	= getcwd,
+
+    .jiffies	= &__jiffies,
+    .ms_timer	= &__ms_timer,
 };
diff --git a/core/timer.inc b/core/timer.inc
index 728812b..b01ff91 100644
--- a/core/timer.inc
+++ b/core/timer.inc
@@ -19,6 +19,9 @@
 ;; about the BIOS_timer variable wrapping around at "midnight" and other
 ;; weird things.
 ;;
+;; This also maintains a timer variable calibrated in milliseconds
+;; (wraparound time = 49.7 days!)
+;;
 
 		section .text16
 
@@ -35,12 +38,20 @@ timer_cleanup:
 		mov [BIOS_timer_hook],eax
 		ret
 
+;
+; The specified frequency is 14.31818 MHz/12/65536; this turns out
+; to be a period of 54.92542 ms, or 0x36.ece8(187c) hexadecimal.
+;
 timer_irq:
 		inc dword [cs:__jiffies]
+		add word  [cs:__ms_timer_adj],0xece8
+		adc dword [cs:__ms_timer],0x36
 		jmp 0:0
 BIOS_timer_next	equ $-4
 
 		section .data16
 		alignz 4
-		global __jiffies
-__jiffies	dd 0			; The actual timer variable
+		global __jiffies, __ms_timer
+__jiffies	dd 0			; Clock tick timer
+__ms_timer	dd 0			; Millisecond timer
+__ms_timer_adj	dw 0			; Millisecond timer correction factor