• Geert Uytterhoeven's avatar
    serial: sh-sci: Remove cpufreq notifier to fix crash/deadlock · 13ead617
    Geert Uytterhoeven authored
    commit ff1cab37 upstream.
    
    The BSP team noticed that there is spin/mutex lock issue on sh-sci when
    CPUFREQ is used.  The issue is that the notifier function may call
    mutex_lock() while the spinlock is held, which can lead to a BUG().
    This may happen if CPUFREQ is changed while another CPU calls
    clk_get_rate().
    
    Taking the spinlock was added to the notifier function in commit
    e552de24 ("sh-sci: add platform device private data"), to
    protect the list of serial ports against modification during traversal.
    At that time the Common Clock Framework didn't exist yet, and
    clk_get_rate() just returned clk->rate without taking a mutex.
    Note that since commit d535a230 ("serial: sh-sci: Require a
    device per port mapping."), there's no longer a list of serial ports to
    traverse, and taking the spinlock became superfluous.
    
    To fix the issue, just remove the cpufreq notifier:
      1. The notifier doesn't work correctly: all it does is update the
         stored clock rate; it does not update the divider in the hardware.
         The divider will only be updated when calling sci_set_termios().
         I believe this was broken back in 2004, when the old
         drivers/char/sh-sci.c driver (where the notifier did update the
         divider) was replaced by drivers/serial/sh-sci.c (where the
         notifier just updated port->uartclk).
         Cfr. full-history-linux commits 6f8deaef2e9675d9 ("[PATCH] sh: port
         sh-sci driver to the new API") and 3f73fe878dc9210a ("[PATCH]
         Remove old sh-sci driver").
      2. On modern SoCs, the sh-sci parent clock rate is no longer related
         to the CPU clock rate anyway, so using a cpufreq notifier is
         futile.
    Signed-off-by: default avatarGeert Uytterhoeven <geert+renesas@glider.be>
    Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
    13ead617
sh-sci.c 60.5 KB