Device Tree

[Valdis.Kletnieks at vt.edu]

On Tue, 25 Oct 2016 11:51:38 +0530, Madhu K said:

> If in case Device tree is not there, where and how to pass the hardware
> information to the linux kernel.

For many types of hardware, the bus protocol provides a standard way to
find everything on the bus.  And that sort of bus scanning is how non-embedded
systems find all their devices.  If it's a laptop or a desktop or server,
there's always the possibility that the user has plugged in a new graphics
card or a different network card - so being able to scan and detect is
important there.

However, in an embedded system, the configuration is fixed - all the parts are
soldered in place, and there's no place to add new devices.  So the hardware
designers save the US$0.08 per chip by picking stripped down chips that don't
have full function (for instance, leave off the PCI config circuitry and just
provide a hardwired "this device will live at this address permanently")

And yes, if you're building microwave ovens, and selling 10 million of them,
suddenly saving 8 cents US per chip adds up to some major profits...

> Does all embedded systems contains Device tree?

No, only embedded systems that have I/O devices that cannot be
enumerated by the hardware.  So for instance, usually a PCI device
or a USB device can be detected by a bus scan.  So if that's all you
have, you don't need device tree - the system can find all the hardware
resources.

But if you have GPIO pins, or an audio or ethernet card that's hard-wired in
some non-scannable way, or weird clock chips, or anything like that, you'll
need device tree.

And it's certainly possible to do *both* - let the kernel scan for PCI and
USB devices, *and* provide a device tree for non-scannable resources.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 484 bytes
Desc: not available
Url : http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20161025/3ca3e1e6/attachment.bin