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

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Date:   Sun, 2 Apr 2017 21:01:48 -0400 (EDT)
From:   Nicolas Pitre <nicolas.pitre@...aro.org>
To:     Stuart Longland <stuartl@...glandclan.id.au>
cc:     Andi Kleen <andi@...stfloor.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 0/5] minitty: a minimal TTY layer alternative for
 embedded systems

On Mon, 3 Apr 2017, Stuart Longland 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.

Not to repeat what I've said already, I invite you to have a look at 
https://lkml.org/lkml/2017/3/24/634

> I recall Slackware requiring that you booted with a mounted floppy (no
> ramdisk) and possibly even required that you had a second floppy drive
> formatted as swap so you'd be able to get through the install without
> oomkiller knocking on your door.

Did the oom killer even exist in those days? I don't remember.
All I remember is the stack of 73 flopies or so to install Slackware... 
and of course floppy #68 would have developed a bad sector preventing 
you from completing the installation.

> Sub-megabyte system support is a noble goal, but I'm wondering how
> practical such systems would be, and whether an embedded real-time
> kernel might be a better choice than Linux on such systems.

Obviously, you need to leave the idea of a _distribution_ behind. If you 
think of a single user app, and a kernel that only provides those 
syscalls used by that app, and the minimal subset of kernel services 
that such an app require, then nothing prevents such and app/kernel from 
using the actual Linux API. And that's where you get a big advantage 
over other RTOSes. See the link above for the full rationale.


Nicolas