linux-kernel - Re: What does ISA/PCI really mean to ARM architecture?

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Message-Id: <201212271607.24201.arnd@arndb.de>
Date:	Thu, 27 Dec 2012 16:07:23 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	anish kumar <anish198519851985@...il.com>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Woody Wu <narkewoody@...il.com>,
	kernelnewbies@...nelnewbies.org,
	"linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: What does ISA/PCI really mean to ARM architecture?

On Thursday 27 December 2012, anish kumar wrote:
> On Thu, 2012-12-27 at 10:51 +0100, Geert Uytterhoeven wrote:
> > On Thu, Dec 27, 2012 at 9:27 AM, Woody Wu <narkewoody@...il.com> wrote:
> > > Can a peripheral chip that claims to be ISA or PCI device be used in a
> > > ARM based embedded system?  For these kind of chips, I only concern
> > > about the planar kind of devices, means they are not on a dedicated
> > > expansion card.
> > >
> > > From hardware point of view, to attach a ISA or PCI planar chip, is
> > > there any requirement need to fulfill on a ARM bard?
> arm AFAIK is only used in embedded system but ISA/PCI buses are
> generally part of 'big systems' and most of the times it refers to x86
> PC.

In the old days, there were a few ARM based systems that were closer
to what we think of as a PC, and that had actual ISA slots, or
(more commonly) PCMCIA, which is related to ISA.

Aside from that, there are and have been for a long time some ARM
systems that have PCI or PCIe slots.

Note that the distinction between ISA-style platform devices and
ISA add-on cards is a little fuzzy. The Kconfig option CONFIG_ISA
refers to the latter, but there are also ARM systems that only
have the former, e.g. for floppy controller.

> > > From Linux driver point of view, what are needed to support an ISA or
> > > PCI driver in ARM architecture?  More important, is ISA or PCI device a
> > > platform device?  If not, how to add these kind of devices in my board
> > > definition?
>
> AFAIK, Platform device is just a way to add a particular driver whose
> probe can't be called at runtime. Mostly platform device is part of
> system on chip.

Right. Note that some ISA devices (especially those that are typically
on-board) are platform devices as well, but PCI devices never are.

> > An ISA device is typically a platform device. For ARM, which uses device trees,
> Don't know much about ISA device to comment on this but people familiar
> with this can enlighten us as to the reason why it is platform device in
> detail.

Classic ISA has no way to probe the presence of a device, so platform
devices were introduced to handle those. There is also ISAPNP, which adds
PCI-like probing on top of ISA, but most ISA cards precede that standard.

	Arnd
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