Re: [GIT PATCH] char/misc patches for 3.10-rc1

[Arnd Bergmann <arnd@arndb.de>]

On Monday 29 April 2013, Linus Torvalds wrote:
> On Mon, Apr 29, 2013 at 1:50 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> >
> > Fair enough. Of course the distinction here is not based on what it
> > does, but how it gets used.
> 
> Even technically, a "bus" generally has a topology. It has addresses,
> and it has a protocol.
> 
> i2c is a bus. PCI is a bus. And something like SSB is a bus. There is
> a protocol, there's device with identity on the bus, there's stuff
> going on.

Right. I was looking at it from the linux driver model perspective,
where we already call a lot of things a bus that are not at all one in
the engineering sense.

> The SBBI driver has neither addresses nor a protocol. It's literally
> just an embedded on-chip serial device as far as I can tell. There's
> nothing "bus" about it. It's just a hose.
>
> Yeah, yeah, at some point you can call "anything" a bus. I could call
> my little two-seater car a "school bus", because it has wheels, it's
> even yellow exactly like the school buses around here. And I can put a
> child in it. So my little yellow two-seater must be a bus too. It's
> all just how you define your words.
> 
> But it's a damn big reach. I didn't use to call the serial line
> connecting my computer to the modem a "bus". Even if it connected two
> devices.

It seems I looked too briefly. As you point out and David already
confirmed, there is only one device on the other side, which is indeed
a major difference to e.g. SPI, which seems rather similar otherwise
but can use chip-select pins to multiplex between different endpoints.
Certainly this hardware could do the same, but you are right that it's
not relevant because it doesn't do that in practice.

	Arnd