• Mateusz Jończyk's avatar
    rtc: mc146818-lib: fix RTC presence check · ea6fa496
    Mateusz Jończyk authored
    To prevent an infinite loop in mc146818_get_time(),
    commit 211e5db1 ("rtc: mc146818: Detect and handle broken RTCs")
    added a check for RTC availability. Together with a later fix, it
    checked if bit 6 in register 0x0d is cleared.
    
    This, however, caused a false negative on a motherboard with an AMD
    SB710 southbridge; according to the specification [1], bit 6 of register
    0x0d of this chipset is a scratchbit. This caused a regression in Linux
    5.11 - the RTC was determined broken by the kernel and not used by
    rtc-cmos.c [3]. This problem was also reported in Fedora [4].
    
    As a better alternative, check whether the UIP ("Update-in-progress")
    bit is set for longer then 10ms. If that is the case, then apparently
    the RTC is either absent (and all register reads return 0xff) or broken.
    Also limit the number of loop iterations in mc146818_get_time() to 10 to
    prevent an infinite loop there.
    
    The functions mc146818_get_time() and mc146818_does_rtc_work() will be
    refactored later in this patch series, in order to fix a separate
    problem with reading / setting the RTC alarm time. This is done so to
    avoid a confusion about what is being fixed when.
    
    In a previous approach to this problem, I implemented a check whether
    the RTC_HOURS register contains a value <= 24. This, however, sometimes
    did not work correctly on my Intel Kaby Lake laptop. According to
    Intel's documentation [2], "the time and date RAM locations (0-9) are
    disconnected from the external bus" during the update cycle so reading
    this register without checking the UIP bit is incorrect.
    
    [1] AMD SB700/710/750 Register Reference Guide, page 308,
    https://developer.amd.com/wordpress/media/2012/10/43009_sb7xx_rrg_pub_1.00.pdf
    
    [2] 7th Generation Intel ® Processor Family I/O for U/Y Platforms [...] Datasheet
    Volume 1 of 2, page 209
    Intel's Document Number: 334658-006,
    https://www.intel.com/content/dam/www/public/us/en/documents/datasheets/7th-and-8th-gen-core-family-mobile-u-y-processor-lines-i-o-datasheet-vol-1.pdf
    
    [3] Functions in arch/x86/kernel/rtc.c apparently were using it.
    
    [4] https://bugzilla.redhat.com/show_bug.cgi?id=1936688
    
    Fixes: 211e5db1 ("rtc: mc146818: Detect and handle broken RTCs")
    Fixes: ebb22a05 ("rtc: mc146818: Dont test for bit 0-5 in Register D")
    Signed-off-by: default avatarMateusz Jończyk <mat.jonczyk@o2.pl>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Alessandro Zummo <a.zummo@towertech.it>
    Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
    Signed-off-by: default avatarAlexandre Belloni <alexandre.belloni@bootlin.com>
    Link: https://lore.kernel.org/r/20211210200131.153887-5-mat.jonczyk@o2.pl
    ea6fa496
rtc-cmos.c 36.6 KB