linux-kernel - Re: Serial related oops

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Message-ID: <20070219134539.GA27370@flint.arm.linux.org.uk>
Date:	Mon, 19 Feb 2007 13:45:39 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Frederik Deweerdt <deweerdt@...e.fr>
Cc:	jose.goncalves@...v.pt, tie-fei.zang@...escale.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: Serial related oops

On Tue, Feb 20, 2007 at 01:29:09PM +0000, Frederik Deweerdt wrote:
> (Sorry for the resend, I forgot to cc the list)
> Hi Russell,
> 
> It seems that the following change in drivers/serial/8250.c
> 
> +
> +	/*
> +	 * Do a quick test to see if we receive an
> +	 * interrupt when we enable the TX irq.
> +	 */
> +	serial_outp(up, UART_IER, UART_IER_THRI);
> +	lsr = serial_in(up, UART_LSR);
> +	iir = serial_in(up, UART_IIR);
> +	serial_outp(up, UART_IER, 0);
> +
> +	if (lsr & UART_LSR_TEMT && iir & UART_IIR_NO_INT) {
> +		if (!(up->capabilities & UART_BUG_TXEN)) {
> +			up->capabilities |= UART_BUG_TXEN;
> +			pr_debug("ttyS%d - enabling bad tx status workarounds\n",
> +				 port->line);
> +		}
> +	} else {
> +		up->capabilities &= ~UART_BUG_TXEN;
> +	}
> +
> 
> that was introduced in 2.6.12[1], is causing oopses on some hardware. In
> particular Jose Goncalves reported[2] an oops in 2.6.16.38 reproducible

I don't see that.  The oops your referring to is a NULL pointer
dereference.  The only dereferences the above code does is via
'up' and 'port' both of which are provably always non-null here.

> I was wondering:
> - what is the goal of the test?

To detect UARTs which do not assert an interrupt when the transmit
interrupt is enabled, which then causes no data to ever be transmitted
without this work-around.

> - could this be CONFIGed ?

No, it's only runtime because you can't tell which ports might be
affected, and you might have a mixture of ports which are affected
and those which aren't.

> PS: CCing Andrew and Zang Roy-r61911 as they seemed to discuss this in
> http://lkml.org/lkml/2006/6/13/21

I don't see any reference to this problem there.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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