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Thu, 02 Jul 2026 02:29:11 -0700 (PDT) Message-ID: <5945d8f4-aece-4572-8e89-60408dd7ac32@suse.com> Date: Thu, 2 Jul 2026 11:29:11 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: [PATCH v2 1/4] x86/time: CMOS RTC may run in binary mode From: Jan Beulich To: "xen-devel@lists.xenproject.org" Cc: Andrew Cooper , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Teddy Astie References: <79d50725-3892-4643-b854-bfec9c0c0d79@suse.com> Content-Language: en-US Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: <79d50725-3892-4643-b854-bfec9c0c0d79@suse.com> Content-Transfer-Encoding: quoted-printable X-purgate-ID: tlsNG-16d1c6/1782984552-C453168D-9984B018/0/0 X-purgate-type: clean X-purgate-size: 3621 X-ZohoMail-DKIM: pass (identity @suse.com) X-ZM-MESSAGEID: 1782984571439158500 Content-Type: text/plain; charset="utf-8" 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 --- v2: New. --- a/docs/misc/xen-command-line.pandoc +++ b/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. =20 +### cmos-rtc-bcd (x86) +> `=3D ` + +> 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) > `=3D ` =20 --- a/xen/arch/x86/include/asm/mc146818rtc.h +++ b/xen/arch/x86/include/asm/mc146818rtc.h @@ -96,7 +96,6 @@ bool is_cmos_port(unsigned int port, uns =20 #ifndef RTC_PORT #define RTC_PORT(x) (0x70 + (x)) -#define RTC_ALWAYS_BCD 1 /* RTC operates in binary mode */ #endif =20 /* --- a/xen/arch/x86/time.c +++ b/xen/arch/x86/time.c @@ -1250,6 +1250,9 @@ mktime (unsigned int year, unsigned int )*60 + sec; /* finally seconds */ } =20 +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_t if ( acpi_gbl_FADT.century && acpi_gbl_FADT.century < 0x80 ) century =3D CMOS_READ(acpi_gbl_FADT.century); =20 - bcd =3D RTC_ALWAYS_BCD || !(CMOS_READ(RTC_CONTROL) & RTC_DM_BINARY); + bcd =3D opt_cmos_rtc_bcd || !(CMOS_READ(RTC_CONTROL) & RTC_DM_BINARY); =20 spin_unlock_irqrestore(&rtc_lock, flags); =20 @@ -1353,6 +1356,48 @@ static bool __init cmos_rtc_probe(void) return false; } =20 +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 =3D !(CMOS_READ(RTC_CONTROL) & RTC_DM_BINARY); + spin_unlock_irqrestore(&rtc_lock, flags); + + if ( bcd ) + return; + + for ( unsigned int seclo =3D 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 =3D rtc.sec & 0xf; + } + + printk(XENLOG_WARNING "CMOS RTC indicates binary mode but uses BCD\n"); + + opt_cmos_rtc_bcd =3D true; +} =20 static unsigned long cmos_rtc_read(void) { @@ -1614,6 +1659,10 @@ static void __init probe_wallclock(void) if ( cmos_rtc_probe() ) { wallclock_source =3D WALLCLOCK_CMOS; + + if ( opt_cmos_rtc_probe ) + cmos_rtc_probe_bcd(); + return; } if ( efi_enabled(EFI_RS) && efi_get_time() )