linux-kernel - Re: [PATCH v1 4/6] x86/boot: Assume MMIO if serial base address supplied via earlyprintk

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Date:   Tue, 16 Jan 2018 12:55:04 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Ingo Molnar <mingo@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 4/6] x86/boot: Assume MMIO if serial base address
 supplied via earlyprintk

On Tue, 2018-01-16 at 04:13 +0100, Ingo Molnar wrote:
> * Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:
> 
> > If user supplied serial base address via kernel command line and
> > value
> > is higher than IO space limit (64k boundary), assume for now that
> > MMIO
> > byte access is required.
> > 
> > Later we might expand or modify this if needed.
> 
> Is this a standard pattern for serial code configuration values?

I didn't get what you meant under "standard" here.

IO space limit comes from generic io.h header and AFAIU is a hardware
limitation (outN (%dx), ...; inX (%dx); dx is 16 bit register).

Using mmio8 out of the IO space is dictated by the (modern) x86
platforms with non-standard (okay, high speed) UART location in address
space.

-- 
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy