Embedded Linux?

Hi list,

I'm a newbie, true to its every sense, and hence this question.

I have come across the term "Embedded Linux" a lot of times. Is
embedded linux a different branch of kernel all together? Or is it
that the vanila stock kernel (downloaded from kernel.org) becomes
"Embedded Linux" when compiled for an embedded processor like ARM /
PPC etc?


Thanks,

Dan
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Re: Embedded Linux?

Daniel Rodrick wrote:
> Hi list,
>
> I'm a newbie, true to its every sense, and hence this question.
>
> I have come across the term "Embedded Linux" a lot of times. Is
> embedded linux a different branch of kernel all together? Or is it
> that the vanila stock kernel (downloaded from kernel.org) becomes
> "Embedded Linux" when compiled for an embedded processor like ARM /
> PPC etc?
>
>
> Thanks,

Not quite either of these things. Embedded Linux is an imprecise term,
but it generally refers to specialized distributions that are adapted to
embedded processors and to other limitations of embedded systems (such
as use of flash memory instead of a hard disk).

And it covers more than the kernel. A typical Embedded Linux system
includes the kernel, a suitable cross-compiler (a version of gcc that
runs on i86 hardware, say, but produces output for an ARM5), a few
associated things (an assembler, glibc) that are collectively called the
"toolchain", and some stripped-down apps (like busybox). The kernel may
have non-standard patches needed to get it to work with the target
hardware ... but there is a strong push to get those patches integrated
into the main kernel, and it usually works pretty well (at least for
ARM, the one embedded processor I spend some time working with).

Commercial Embedded Linux distributions (from Lynuxworks, Monte Vista,
and maybe others ... I'm way out of date here) typically include non-OS
tools that the vendors claim facilitate cross development.

All this is really an oversimplification, though ... but probably enough
to get you started.

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Re: Embedded Linux?

> Not quite either of these things. Embedded Linux is an imprecise term,
> but it generally refers to specialized distributions that are adapted to
> embedded processors and to other limitations of embedded systems (such
> as use of flash memory instead of a hard disk).
>
> And it covers more than the kernel. A typical Embedded Linux system
> includes the kernel, a suitable cross-compiler (a version of gcc that
> runs on i86 hardware, say, but produces output for an ARM5), a few
> associated things (an assembler, glibc) that are collectively called the
> "toolchain", and some stripped-down apps (like busybox). The kernel may
> have non-standard patches needed to get it to work with the target
> hardware ... but there is a strong push to get those patches integrated
> into the main kernel, and it usually works pretty well (at least for
> ARM, the one embedded processor I spend some time working with).
>

Thanks. So what I get is that there is no term such as "Embedded Linux
Kernel". There are embedded distributions however, that contain kernel
(compiled for an embedded architecture) with specific patches (by the
ditribution vendor) for the specific embedded platform (ARM / PPC
etc).

But CAN I run standard stock kernel compilerd for an embedded chip
(say ARM) on an ARM processor? Or do I NECESSARILY need the patches by
the distribution?

Thanks,

Dan
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Re: Embedded Linux?

Daniel Rodrick wrote:
>> Not quite either of these things. Embedded Linux is an imprecise term,
>> but it generally refers to specialized distributions that are adapted to
>> embedded processors and to other limitations of embedded systems (such
>> as use of flash memory instead of a hard disk).
>>
>> And it covers more than the kernel. A typical Embedded Linux system
>> includes the kernel, a suitable cross-compiler (a version of gcc that
>> runs on i86 hardware, say, but produces output for an ARM5), a few
>> associated things (an assembler, glibc) that are collectively called the
>> "toolchain", and some stripped-down apps (like busybox). The kernel may
>> have non-standard patches needed to get it to work with the target
>> hardware ... but there is a strong push to get those patches integrated
>> into the main kernel, and it usually works pretty well (at least for
>> ARM, the one embedded processor I spend some time working with).
>>
>
> Thanks. So what I get is that there is no term such as "Embedded Linux
> Kernel". There are embedded distributions however, that contain kernel
> (compiled for an embedded architecture) with specific patches (by the
> ditribution vendor) for the specific embedded platform (ARM / PPC
> etc).
>
> But CAN I run standard stock kernel compilerd for an embedded chip
> (say ARM) on an ARM processor? Or do I NECESSARILY need the patches by
> the distribution?

Id depends. To state the obvious, one doesn't run Linux on a CPU; one
runs it on a computer, which has more in it than a CPU. Some kernel
patches involve, for example, the video framebuffer, or support for
flash memory and associated filesystems (e.g., jffs2), or ... you get
the idea. (It's been a couple of years since I did any ARM work, and my
memory of the details is getting a bit rusty.)

Often, embedded CPUs are part of SoCs (Systems on Chip) that integrate
some of the support stuff into a single chip with the CPU, and that can
impose additional requirements on the kernel or core apps.

In practice, when I worked with ARM systems, we always expected the
board vendor to provide the core ... a working kernel, at least a
command-line video interface, the basic app set needed for the system to
work (mostly busybox, plus a few other things), support for jffs2, and a
native- or cross-compile toolchain. It is possible to do this yourself
for a new board (obviously, since the vendors do it), but when we looked
at that option, it looked like a lot of work.

For ARM, at least, all this stuff was well developed. There was a
standard set of kernel patches that provided most, maybe all (I forget),
of the ARM-specific functionality.

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