linux-kernel - Re: [PATCH v1 12/14] x86/boot: tweak a20.c for better code generation

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Message-ID: <alpine.DEB.2.21.2601240244460.6421@angie.orcam.me.uk>
Date: Sat, 24 Jan 2026 03:00:27 +0000 (GMT)
From: "Maciej W. Rozycki" <macro@...am.me.uk>
To: David Laight <david.laight.linux@...il.com>
cc: "H. Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...nel.org>, 
    Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
    Dave Hansen <dave.hansen@...ux.intel.com>, Uros Bizjak <ubizjak@...il.com>, 
    Petr Mladek <pmladek@...e.com>, Andrew Morton <akpm@...ux-foundation.org>, 
    Kees Cook <kees@...nel.org>, 
    "Peter Zijlstra (Intel)" <peterz@...radead.org>, 
    Nathan Chancellor <nathan@...nel.org>, Kiryl Shutsemau <kas@...nel.org>, 
    Rick Edgecombe <rick.p.edgecombe@...el.com>, linux-kernel@...r.kernel.org, 
    linux-coco@...ts.linux.dev, x86@...nel.org
Subject: Re: [PATCH v1 12/14] x86/boot: tweak a20.c for better code
 generation

On Wed, 21 Jan 2026, David Laight wrote:

> No loops needed.

 A loop is needed because there can be a considerable delay from issuing 
the I/O request to flip the A20 gate till the circuitry responding.  This 
is particularly true with the command issued to the 8042 device, which is 
a microcontroller running its own firmware that needs it time to process 
an incoming request to drive one of the microcontroller's GPIOs.  There 
was a reason for port 0x92 circuitry later added to the PC architecture 
with the IBM PS/2 being called the "fast A20 gate".

  Maciej