linux-kernel - Re: [PATCH v2 0/5] minitty: a minimal TTY layer alternative for embedded systems

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Date:   Mon, 3 Apr 2017 13:57:53 -0500
From:   Rob Herring <robh@...nel.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Stuart Longland <stuartl@...glandclan.id.au>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andi Kleen <andi@...stfloor.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        Jiri Slaby <jslaby@...e.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 0/5] minitty: a minimal TTY layer alternative for
 embedded systems

On Mon, Apr 3, 2017 at 1:14 PM, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> On Mon, Apr 3, 2017 at 12:44 AM, Stuart Longland
> <stuartl@...glandclan.id.au> wrote:
>> On 03/04/17 07:41, Nicolas Pitre wrote:
>>>> No PTYs seems like a big limitation. This means no sshd?
>>> Again, my ultimate system target is in the sub-megabyte of RAM.  I
>>> really doubt you'll be able to fit an SSH server in there even if PTYs
>>> were supported, unless sshd (or dropbear) can be made really tiny.
>>> Otherwise you most probably have sufficient resources to run the regular
>>> TTY code.
>>
>> Are we talking small microcontrollers here?  The smallest machine in
>> terms of RAM I ever recall running Linux on was a 386SX/25 MHz with 4MB
>> RAM, and that had a MMU.
>
> Let's halve that. I once tried and ran Linux in 2 MiB, incl. X, twm, and xterm.
> Of course with swap enabled.  And swapping like hell.

These are different target uses. We're talking about fixed function,
statically linked user space at the minimum (some may want no
userspace even). Applications that could use an RTOS instead but
benefit from the Linux hardware support, features and ecosystem. It's
not a whole new code base or environment to learn. Maybe Zephyr will
have traction and improve things, but projects I've been involved with
using RTOSs generally have discussions around needing to re-write the
crappy RTOS.

The absolute amount of RAM target is not so important. What's
important is getting to a size feasible for onchip RAM. That's always
moving (up), but has generally been out of reach for Linux.

Rob