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

To: Jan Nikitenko <>
Subject: Re: [PATCH] serial: fix au1xxx UART0 irq setup
From: Sergei Shtylyov <>
Date: Thu, 25 Oct 2007 18:09:47 +0400
Cc: Ralf Baechle <>,,
In-reply-to: <>
Organization: MontaVista Software Inc.
Original-recipient: rfc822;
References: <>
User-agent: Mozilla/5.0 (X11; U; Linux i686; rv:1.7.2) Gecko/20040803
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 <>


 include/asm-mips/serial.h |    5 +++++
 1 file changed, 5 insertions(+)


diff --git a/include/asm-mips/serial.h b/include/asm-mips/serial.h
index c07ebd8..526bd2e 100644
--- a/include/asm-mips/serial.h
+++ b/include/asm-mips/serial.h
@@ -19,4 +19,9 @@
 #define BASE_BAUD (1843200 / 16)
+#ifdef CONFIG_SERIAL_8250_AU1X00
+#undef is_real_interrupt
+#define is_real_interrupt(irq)  (1)
 #endif /* _ASM_SERIAL_H */

I wonder why such patch hasn't still been checked, despite being suggested several times...

WBR, Sergei

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