[PATCH 0/2] serial: amba-pl011: fix console clock lifetime

[Karl Mehltretter]

Patch 1 fixes an independent bug: pl011_console_setup() prepares the UART
clock but nothing releases it when the console is unregistered, so the
clock's prepare count leaks one reference per registration cycle (via
the sysfs "console" attribute or a driver unbind). It adds the missing
console .exit() and stands on its own.

Patch 2 fixes a PREEMPT_RT failure: pl011_console_write_atomic() runs in
nbcon atomic context but calls clk_enable(), which under RT can acquire a
sleeping lock, so an atomic-context printk on a clk-backed pl011 can hit
"sleeping function called from invalid context". The same lock
acquisition would also be unsafe if write_atomic() is invoked from NMI
context. It keeps the clock enabled while the console is registered and
releases it in .exit(); it depends on patch 1.

Tested on QEMU: the prepare-count leak was reproduced and fixed on
bcm2835 (a gateable CPRMAN UART clock), and the PREEMPT_RT splat was
reproduced and fixed on versatilepb. With the series applied, recycling
the console via the sysfs attribute keeps both the prepare and enable
counts balanced, and the console keeps working after re-registration.

Karl Mehltretter (2):
  serial: amba-pl011: unprepare console clock on unregister
  serial: amba-pl011: keep console clock enabled for atomic writes

 drivers/tty/serial/amba-pl011.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

-- 
2.53.0