Commits on Sep 21, 2023

1. serial: Reduce spinlocked portion of uart_rs485_config() …

   Commit 44b27ae ("serial: core, 8250: set RS485 termination GPIO in
   serial core") enabled support for RS485 termination GPIOs behind i2c
   expanders by setting the GPIO outside of the critical section protected
   by the port spinlock.  Access to the i2c expander may sleep, which
   caused a splat with the port spinlock held.
   
   Commit 7c7f9bc ("serial: Deassert Transmit Enable on probe in
   driver-specific way") erroneously regressed that by spinlocking the
   GPIO manipulation again.
   
   Fix by moving uart_rs485_config() (the function manipulating the GPIO)
   outside of the spinlocked section and acquiring the spinlock inside of
   uart_rs485_config() for the invocation of ->rs485_config() only.
   
   This gets us one step closer to pushing the spinlock down into the
   ->rs485_config() callbacks which actually need it.  (Some callbacks
   do not want to be spinlocked because they perform sleepable register
   accesses, see e.g. sc16is7xx_config_rs485().)
   
   Stack trace for posterity:
   
    Voluntary context switch within RCU read-side critical section!
    WARNING: CPU: 0 PID: 56 at kernel/rcu/tree_plugin.h:318 rcu_note_context_switch
    Call trace:
    rcu_note_context_switch
    __schedule
    schedule
    schedule_timeout
    wait_for_completion_timeout
    bcm2835_i2c_xfer
    __i2c_transfer
    i2c_transfer
    i2c_transfer_buffer_flags
    regmap_i2c_write
    _regmap_raw_write_impl
    _regmap_bus_raw_write
    _regmap_write
    _regmap_update_bits
    regmap_update_bits_base
    pca953x_gpio_set_value
    gpiod_set_raw_value_commit
    gpiod_set_value_nocheck
    gpiod_set_value_cansleep
    uart_rs485_config
    uart_add_one_port
    pl011_register_port
    pl011_probe
   
   Fixes: 7c7f9bc ("serial: Deassert Transmit Enable on probe in driver-specific way")
   Suggested-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
   Signed-off-by: Lukas Wunner <lukas@wunner.de>
   Cc: stable@vger.kernel.org # v6.1+
   Signed-off-by: Lino Sanfilippo <l.sanfilippo@kunbus.com>

   

   l1k authored and linosanfilippo-kunbus committed Sep 21, 2023
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