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Message-ID: <92fc8b8105080407297d8ced80@mail.gmail.com>
Date: Thu, 4 Aug 2005 07:29:06 -0700
From: Chris Budd <budorola@gmail.com>
Reply-To: Chris Budd <budorola@gmail.com>
To: Chris Budd <budorola@gmail.com>, linux-kernel@vger.kernel.org,
       linux-serial@vger.kernel.org
Subject: Re: 2.4.21: Sharing interrupts with serial console
In-Reply-To: <20050804085440.A22910@flint.arm.linux.org.uk>
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	 <20050804085440.A22910@flint.arm.linux.org.uk>
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On 8/4/05, Russell King <rmk+lkml@arm.linux.org.uk> wrote:
> On Wed, Aug 03, 2005 at 06:36:51PM -0700, Chris Budd wrote:
> > 1.  The rs_init function in ./linux-2.4.21/drivers/char/serial.c
> > explicitly states "The interrupt of the serial console port can't be
> > shared."  Does this include *ALL* interrupts?  The code checks for
> > sharing only with other serial devices, not *ALL* types of devices
> > like I2C, RTC, etc.
<snip>
> > 2.  While the presence of the comment about not sharing was nice, it
> > does not explain "why?"
> 
> Connecting a level-active interrupt output to an edge-triggered
> interrupt controller input is Bad News(tm) for missing interrupts.
> 

Of course.  I thought it was something more serious in the bowels of
the kernel.  All the comment needed was just that one adjective "The
*edge-triggered* interrupt of the serial console port can't be
shared."  I know many programmers do not like to write comments, but
good comments make the code more robust and stable:  the code clearly
shows *what* you did, but comments are necessary to indicate *why*.

<snip>
> If your Intel hardware doesn't have level triggered input capabilities,
> please apply customer pressure to Intel to ensure that they consider it
> for their future ARM-based designs.

You will be happy to know that the Intel IOP80321 has level-sensitive
interrupts.

Thank you for the detailed explanation.
Chris.