x86/time: CMOS RTC may run in binary mode

Indicating it would always use BCD mode is just wrong (and then the
comment there said the opposite). All halfway recent (and really all 64-
bit capable) systems having a CMOS RTC should properly indicate the mode
in control register B.

Make use of the flag, but provide a fallback mechanism in case people run
into systems not matching the above assumption. Additionally, when binary
mode is indicated and when "cmos-rtc-probe" is in use (but "cmos-rtc-bcd"
isn't), probe whether the clock really runs in binary mode. (This probing,
sadly, can take up to 10 seconds.)

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger@xenproject.org>

docs/misc/xen-command-line.pandoc

@@ -339,6 +339,14 @@ parameter to "stable:socket".
 Specify the event count threshold for raising Corrected Machine Check
 Interrupts.  Specifying zero disables CMCI handling.
 
+### cmos-rtc-bcd (x86)
+> `= <boolean>`
+
+> Default: `false`
+
+Flag to indicate the CMOS Real Time Clock uses BCD mode irrespective of
+control register B indicating binary mode.
+
 ### cmos-rtc-probe (x86)
 > `= <boolean>`
 

xen/arch/x86/include/asm/mc146818rtc.h

@@ -96,7 +96,6 @@ bool is_cmos_port(unsigned int port, unsigned int bytes,
 
 #ifndef RTC_PORT
 #define RTC_PORT(x)	(0x70 + (x))
-#define RTC_ALWAYS_BCD	1	/* RTC operates in binary mode */
 #endif
 
 /*

xen/arch/x86/time.c

@@ -1250,6 +1250,9 @@ mktime (unsigned int year, unsigned int mon,
         )*60 + sec; /* finally seconds */
 }
 
+static bool __ro_after_init opt_cmos_rtc_bcd;
+boolean_param("cmos-rtc-bcd", opt_cmos_rtc_bcd);
+
 struct rtc_time {
     unsigned int year, mon, day, hour, min, sec;
 };
@@ -1285,7 +1288,7 @@ static bool __get_cmos_time(struct rtc_time *rtc)
     if ( acpi_gbl_FADT.century && acpi_gbl_FADT.century < 0x80 )
         century = CMOS_READ(acpi_gbl_FADT.century);
 
-    bcd = RTC_ALWAYS_BCD || !(CMOS_READ(RTC_CONTROL) & RTC_DM_BINARY);
+    bcd = opt_cmos_rtc_bcd || !(CMOS_READ(RTC_CONTROL) & RTC_DM_BINARY);
 
     spin_unlock_irqrestore(&rtc_lock, flags);
 
@@ -1353,6 +1356,48 @@ static bool __init cmos_rtc_probe(void)
     return false;
 }
 
+static inline bool __init attr_const is_bcd(unsigned int x)
+{
+    return (x & 0xf) < 10 && (x >> 4) < 10;
+}
+
+static void __init cmos_rtc_probe_bcd(void)
+{
+    bool bcd;
+    unsigned long flags;
+
+    if ( opt_cmos_rtc_bcd )
+        return;
+
+    spin_lock_irqsave(&rtc_lock, flags);
+    bcd = !(CMOS_READ(RTC_CONTROL) & RTC_DM_BINARY);
+    spin_unlock_irqrestore(&rtc_lock, flags);
+
+    if ( bcd )
+        return;
+
+    for ( unsigned int seclo = 0; ; )
+    {
+        struct rtc_time rtc;
+
+        if ( !__get_cmos_time(&rtc) ||
+             !is_bcd(rtc.sec) ||
+             !is_bcd(rtc.min) ||
+             !is_bcd(rtc.hour) ||
+             !is_bcd(rtc.day) ||
+             !is_bcd(rtc.mon) )
+            return;
+
+        if ( seclo > (rtc.sec & 0xf) )
+            break;
+
+        seclo = rtc.sec & 0xf;
+    }
+
+    printk(XENLOG_WARNING "CMOS RTC indicates binary mode but uses BCD\n");
+
+    opt_cmos_rtc_bcd = true;
+}
 
 static unsigned long cmos_rtc_read(void)
 {
@@ -1614,6 +1659,10 @@ static void __init probe_wallclock(void)
     if ( cmos_rtc_probe() )
     {
         wallclock_source = WALLCLOCK_CMOS;
+
+        if ( opt_cmos_rtc_probe )
+            cmos_rtc_probe_bcd();
+
         return;
     }
     if ( efi_enabled(EFI_RS) && efi_get_time() )