Re: [PATCH] serial: 8250: restore FIFO in serial8250_console_restore()

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Nov 02, 2026 at 05:36:09PM +0800, fuzheng.meng wrote:
> When console_suspend is disabled, the serial console stays enabled
> over a system suspend, and serial8250_suspend_port() writes a canary
> value to the SCR register (if the port has one) to detect whether the
> UART lost power during the sleep cycle.
> 
> If the hardware did lose power, all registers are reset, and the
> first console write after resume detects the canary mismatch and
> calls serial8250_console_restore(), which reprograms divisor, LCR and
> MCR, but not the FCR.
> 
> serial8250_console_write() decides whether to take the FIFO write
> path from the cached up->fcr, which still carries
> UART_FCR_ENABLE_FIFO. With the hardware FIFO left disabled by the
> reset, the transmit holding register holds only one byte, while
> serial8250_console_fifo_write() pushes up to tx_loadsz bytes into the
> port after waiting for THRE only once. The burst overwrites itself
> and the console output right after resume is garbled.
> 
> Factor the FIFO write path condition out into a new
> serial8250_console_use_fifo() helper and use it from both
> serial8250_console_write() and serial8250_console_restore(). Restore
> the FIFO in serial8250_console_restore() when the FIFO write path is
> going to be used: first write UART_FCR_ENABLE_FIFO, then the cached
> fcr, as dw8250_idle_exit() does after its registers are reset.
> 
> Tested on a DesignWare 8250 based platform with console_suspend=0
> over a suspend/resume cycle.
> 
> Fixes: 8f3631f0f6eb ("serial/8250: Use fifo in 8250 console driver")
> Cc: stable@vger.kernel.org
> Signed-off-by: fuzheng.meng <fuzheng.meng@semidrive.com>

Real name please, not an email alias.

thanks,

greg k-h