linux-kernel - Re: Linux 3.15: serial console regression

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Message-ID: <20140613201831.GA2360@drone.musicnaut.iki.fi>
Date:	Fri, 13 Jun 2014 23:18:31 +0300
From:	Aaro Koskinen <aaro.koskinen@....fi>
To:	Peter Hurley <peter@...leysoftware.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	sparclinux@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Seth Bollinger <sethb@...i.com>
Subject: Re: Linux 3.15: serial console regression

Hi,

On Mon, Jun 09, 2014 at 07:48:17PM -0400, Peter Hurley wrote:
> On 06/09/2014 04:59 PM, Aaro Koskinen wrote:> Hi,
> > While testing the final 3.15, I noticed the serial console (sunsab)
> > does not work anymore on Sun Ultra.
> > 
> > It will tx fine the console output during the boot. But then I try
> > to type something e.g. at the login: prompt, it behaves oddly:
> > it re-transmits some old output, and then stalls for a few secs, usually
> > for each character. Typed characters are however received correctly.
> > But the console is impossible to use...
> > 
> > I bisected this to:
> > 
> > 717f3bbab3c7628736ef738fdbf3d9a28578c26c is the first bad commit
> > commit 717f3bbab3c7628736ef738fdbf3d9a28578c26c
> > Author: Seth Bollinger <sethb@...i.com>
> > Date:   Tue Mar 25 12:55:37 2014 -0500
> > 
> >      serial_core: Fix conditional start_tx on ring buffer not empty
> 
> [Let's try that again. This time without the mess]
> 
> Thanks for the report Aaro.
> 
> Looks like Seth's fix exposes some broken assumptions in the Sun
> serial drivers.

The same problem is present also on PowerPC G5 Xserve (pmac_zilog driver),
and the below type of fix works there too.

A.

> --- %< ---
> From: Peter Hurley <peter@...leysoftware.com>
> Date: Mon, 9 Jun 2014 19:21:43 -0400
> Subject: [PATCH] serial: sunsab: Test for no tx data on tx restart
> 
> Commit 717f3bbab3c7628736ef738fdbf3d9a28578c26c,
> 'serial_core: Fix conditional start_tx on ring buffer not empty'
> exposes an incorrect assumption in sunsab's start_tx method; the
> tx ring buffer can, in fact, be empty when restarting tx when
> performing flow control.
> 
> Test for empty tx ring buffer when in sunsab's start_tx method.
> 
> Signed-off-by: Peter Hurley <peter@...leysoftware.com>
> ---
>  drivers/tty/serial/sunsab.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/tty/serial/sunsab.c b/drivers/tty/serial/sunsab.c
> index 5faa8e9..b99a4ea 100644
> --- a/drivers/tty/serial/sunsab.c
> +++ b/drivers/tty/serial/sunsab.c
> @@ -427,6 +427,9 @@ static void sunsab_start_tx(struct uart_port *port)
>  	struct circ_buf *xmit = &up->port.state->xmit;
>  	int i;
>  
> +	if (uart_circ_empty(xmit))
> +		return;
> +
>  	up->interrupt_mask1 &= ~(SAB82532_IMR1_ALLS|SAB82532_IMR1_XPR);
>  	writeb(up->interrupt_mask1, &up->regs->w.imr1);
>  	
> -- 
> 2.0.0
> 
> --
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