linux-kernel - Re: [PATCH] Revert "serial: fsl_lpuart: Reset prior to registration"

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Date:   Mon, 10 Oct 2022 11:09:30 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Sherry Sun <sherry.sun@....com>
Cc:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-serial <linux-serial@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>
Subject: Re: [PATCH] Revert "serial: fsl_lpuart: Reset prior to registration"

On Sun, Oct 09, 2022 at 10:23:13AM +0000, Sherry Sun wrote:
> I am not familiar with 8250 serial, but at least for imx uart driver
> and lpuart driver, the following behavior is same.
> For the "real" consoles (everything which is not a bootconsole),
> the (port)->cons and (port)->cons->index are initialized through
> uart_add_one_port()->uart_configure_port()->register_console()->
> try_enable_new_console(), here the console index is assigned by
> the console cmdline parameters.

Hm, uart_add_one_port() does the following *before* calling
uart_configure_port():

	/*
	 * If this port is in use as a console then the spinlock is already
	 * initialised.
	 */
	if (!uart_console_enabled(uport))
		uart_port_spin_lock_init(uport);

It sounds like in the case of fsl_lpuart.c, the spin lock is *always*
initialized, even though a concurrent lpuart_console_write() may be
holding it.  That's not solved by moving lpuart_global_reset() around.

The problem with performing lpuart_global_reset() after UART registration
is that as soon as uart_add_one_port() returns, the port is available
for user space to use.  So resetting it is a no-go.

Thanks,

Lukas