Re: [PATCH v2 1/4] serial: 8250_dw: Keep the BUSY-safe divisor hook when DLF is present

[sashiko-bot@kernel.org]

> dw8250_probe() installs dw8250_set_divisor(), which brackets the
> DLAB/DLL/DLH sequence with dw8250_idle_enter() and dw8250_idle_exit()
> so a BUSY UART cannot drop the divisor LCR write.
> 
> dw8250_setup_port() runs afterwards and replaces that hook with the
> dwlib divisor hook whenever it detects DLF. The dwlib hook programs the
> divisor through serial8250_do_set_divisor() without entering idle,
> leaving the LCR|DLAB write exposed to BUSY.
> 
> When termios changes while the UART is transmitting, that LCR|DLAB
> write can be ignored and dw8250_check_lcr() retries it between
> dw8250_idle_enter() and dw8250_idle_exit(). dw8250_idle_exit() then
> restores IER while DLAB is still set, so the value lands in DLH. The
> real IER stays at the zero written by dw8250_idle_enter(), and the port
> gets no further TX or RX interrupts while up->ier says they're enabled.
> 
> Do not replace a divisor hook the driver has already installed. Program
> DLF from the BUSY-safe hook, inside the idle section, through a dwlib
> helper that checks dlf_size.
> 
> Observed on an Allwinner A733 (Radxa Cubie A7Z), where
> dw8250_setup_port() mistakes the RS485 control register for DLF (see the
> next patch): the first interactive tcsetattr() during console output
> left hardware IER at 0 and up->ier at 0x07. The shell never printed its
> prompt or read input. With this change alone the same boot reached the
> prompt and echoed input, and hardware IER matched up->ier afterwards.
> 
> Fixes: a7b9ce39fbe4 ("serial: 8250_dw: Ensure BUSY is deasserted")
> Cc: stable@vger.kernel.org
> Signed-off-by: Vinicius Pedrosa <vinicius.eduardo.pedrosa@gmail.com>

Sashiko has reviewed this patch and found no issues.

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