linux-kernel - Re: [PATCH] serial: 8250 check iir rdi in interrupt

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Message-ID: <20121026151902.0a382d16@pyramind.ukuu.org.uk>
Date:	Fri, 26 Oct 2012 15:19:02 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Min Zhang <mzhang@...sta.com>
Cc:	gregkh@...uxfoundation.org, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] serial: 8250 check iir rdi in interrupt

> It is racing. For "too much work for irq", here is sequence events
> analyzed by a Motorola engineer:

Thanks - this is enormously helpful in understanding the report.

>         5) The LSR indicates that the transmitter needs data,
> but also indicates the presence of data in the FIFO (0x61 in the LSR)
>         6) The processing function receives the characters, and
> outputs data to the FIFO
>         7) At the exact time (very very small window) that the
> character is read from the FIFO, the FIFO timeout occurs locking in an
> interrupt cause
>         8) The next loop through the interrupt code begins
>         9) The IIR now indicates the data timeout interrupt
> (0xCC in the IIR)
>         10) The processing function is called and it reads the
> LSR
>         11) The LSR is 0 indicating nothing to do

>         12) The interrupt loop continues (the IIR won't clear
> until a character is pulled) until it reaches its max count and
> displays the error.

So we only need to check this in serial8250_handle_irq when IIR indicates
a data timeout interrupt ? 

Can we do

	if ((iir & 0x0F) == 0x0C) {
		/* Expensive RDI check */
	}



Alan
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