Re: Regression: serial: imx: overrun errors on debug UART

From: Sergey Organov <sorganov@gmail.com>
To: Stefan Wahren <stefan.wahren@i2se.com>
Cc: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	"Fabio Estevam" <festevam@gmail.com>,
	"Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>,
	"Stefan Wahren" <stefan.wahren@chargebyte.com>,
	linux-serial@vger.kernel.org,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Sascha Hauer" <s.hauer@pengutronix.de>,
	"NXP Linux Team" <linux-imx@nxp.com>,
	"Pengutronix Kernel Team" <kernel@pengutronix.de>,
	"Shawn Guo" <shawnguo@kernel.org>,
	"Jiri Slaby" <jirislaby@kernel.org>,
	"Tomasz Moń" <tomasz.mon@camlingroup.com>,
	"Linux ARM" <linux-arm-kernel@lists.infradead.org>
Subject: Re: Regression: serial: imx: overrun errors on debug UART
Date: Mon, 27 Mar 2023 18:11:12 +0300	[thread overview]
Message-ID: <87mt3ynsa7.fsf@osv.gnss.ru> (raw)
In-Reply-To: <cb16ddb7-f22f-d637-8670-bccc77add0af@i2se.com> (Stefan Wahren's message of "Mon, 27 Mar 2023 16:42:23 +0200")

Stefan Wahren <stefan.wahren@i2se.com> writes:

> Hi,
>
> Am 25.03.23 um 16:11 schrieb Uwe Kleine-König:

[...]

> today i had time to do some testing. At first i tested with different RXTL_DEFAULT values.
>
> 1 No overrun
> 2 No overrun
> 4 No overrun
> 8 Overruns
>
> After that i look at the # echo 0 > /proc/sys/kernel/printk approach,
> but this didn't change anything. The kernel is usually silent about
> log message after boot and the console works still with echo.
> Enforcing some driver to call printk periodically would make the
> console unusuable.

As you figured that printk() is not the cause, it must be something else
that causes overruns, so there is no need to check printk case further.

>
> Finally i tried to disabled the spin_lock in imx_uart_console_write:
>
> diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
> index f07c4f9ff13c..c342559ff1a2 100644
> --- a/drivers/tty/serial/imx.c
> +++ b/drivers/tty/serial/imx.c
> @@ -2007,14 +2007,12 @@ imx_uart_console_write(struct console *co, const char *s, unsigned int count)
>  	struct imx_port_ucrs old_ucr;
>  	unsigned long flags;
>  	unsigned int ucr1;
> -	int locked = 1;
> +	int locked = 0;
>
>  	if (sport->port.sysrq)
>  		locked = 0;
>  	else if (oops_in_progress)
>  		locked = spin_trylock_irqsave(&sport->port.lock, flags);
> -	else
> -		spin_lock_irqsave(&sport->port.lock, flags);
>
>  	/*
>  	 *	First, save UCR1/2/3 and then disable interrupts
>
> But the overruns still occured. Is this because the serial core
> already helds a lock?

This probably isn't even called when there is no printk() output, as
user-space writes to /dev/console are rather performed through regular
generic code, AFAIK.

Best regards,
-- Sergey Organov

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