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Re: [PATCH] serial: fix au1xxx UART0 irq setup

To: Jan Nikitenko <jan.nikitenko@gmail.com>
Subject: Re: [PATCH] serial: fix au1xxx UART0 irq setup
From: Ralf Baechle <ralf@linux-mips.org>
Date: Thu, 25 Oct 2007 15:09:40 +0100
Cc: linux-mips@linux-mips.org, linux-serial@vger.kernel.org
In-reply-to: <4720A11E.5060101@gmail.com>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <4720A11E.5060101@gmail.com>
Sender: linux-mips-bounce@linux-mips.org
User-agent: Mutt/1.5.14 (2007-02-12)
On Thu, Oct 25, 2007 at 03:58:54PM +0200, Jan Nikitenko wrote:

> UART0 on Alchemy mips platforms (au1xxx) does not use real uart's hw
> irq, causing 'ttyS0: 1 input overrun(s)' kernel message with data loss,
> when more characters than uart's fifo size were to be received by the uart.
> 
> This problem can be experienced for example when uart0 is used as a
> serial console on au1550 and more than 16 characters are pasted from
> clipboard to the console.
> 
> The is_real_interrupt(irq) macro is defined in drivers/serial/8250.c as
> a check, if the irq number is other than zero.
> Because UART0 on au1xxx platforms uses irq number 0, the
> is_real_interrupt() check fails and serial8250_backup_timeout() is used
> instead of uart's hw irq.
> 
> The patch redefines the is_real_interrupt(irq) macro, as suggested in
> the comment above the macro definition in 8250.c, in the
> asm-mips/serial.h to be always true for CONFIG_SERIAL_8250_AU1X00.
> This allows the irq number 0 to be used as hw irq for the alchemy uart0
> and fixes the overrun problem.
> 
> Signed-off-by: Jan Nikitenko <jan.nikitenko@gmail.com>

Fairly unelegent imho but anyway, for 2.6.24 I've added support for
tickless to MIPS which in turn required a bit of a cleanup on the Alchemy
code so I renumbered the Alchemy interrupt numbers, so what used to be
IRQ 0 is now IRQ 8 which means your patch is no longer needed for master.

That said, irq 0 is imho totally valid (take the good old PIT timer
interrupt of the PC as the classic example) and treating it as an invalid 
interrupt number is broken.

It's however equally pretty crude hack to undefine a symbol in a file other
than the one defining it ...

  Ralf

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