linux-kernel - Re: [PATCH 1/2]: serial8250: Use native_io

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Message-ID: <20100317183608.600bcdf0@lxorguk.ukuu.org.uk>
Date:	Wed, 17 Mar 2010 18:36:08 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Simon Kagstrom <simon.kagstrom@...insight.net>, x86@...nel.org,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	mingo@...hat.com, tglx@...utronix.de, alan@...ux.intel.com,
	akpm@...ux-foundation.org
Subject: Re: [PATCH 1/2]: serial8250: Use native_io_delay on the x86

On Wed, 17 Mar 2010 11:25:46 -0700
"H. Peter Anvin" <hpa@...or.com> wrote:

> On 03/17/2010 05:30 AM, Simon Kagstrom wrote:
> >   #ifdef __i386__
> > -		outb(0xff, 0x080);
> > +		native_io_delay();
> >   #endif
> 
> There is something a lot more weird about this.  First of all, it's 
> #ifdef'd out on all but __i386__ including x86-64; second, it looks like 
> this is specific to synchronizing to the IER.
> 
> I'm wondering if the right thing isn't to add a dummy write to the SCR 
> register (or similar.)

You just need a write to something on the ISA bus which is 'safe' so that
you don't end up reading back what you wrote to an non-existant port as
some old chipsets will return the last ISA result when you do this rather
than 0xff.
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