Re: Being more anal about iospace accesses..

[Geert Uytterhoeven <geert@linux-m68k.org>]

On Wed, 15 Sep 2004, Deepak Saxena wrote:
> On Sep 15 2004, at 15:46, Linus Torvalds was caught saying:
> > Quite frankly, of your two suggested interfaces, I would select neither.
> > I'd just say that if your bus is special enough, just write your own
> > drivers, and use
> >
> > 	cookie = ixp4xx_iomap(dev, xx);
> > 	...
> > 	ixp4xx_iowrite(val, cookie + offset);
> >
> > which is perfectly valid. You don't have to make these devices even _look_
> > like a PCI device. Why should you?
>
> Problem is that some of those devices are not that special. For example,
> the on-board 16550 is accessed using readb/writeb in the 8250.c driver.

Just what I was thinking...

> I don't think we want to add that level of low-level detail to that
> driver and instead should just hide it in the platform code. I look
> at it from the point of view that the driver should not care about how
> the access actually occurs on the bus. It just says, write data foo at
> location bar regardless of whether bar is ISA, PCI, on-chip, RapidIO,
> etc and that writing of the data is hidden in the implementation of
> the accessor API.

While 16550 serial is a bad example since it does byte accesses only (and thus
doesn't suffer from endianness), I have no problem to imagine there exist
platforms where you have multiple instances of a `standard' (cfr. 16550 serial)
device block, while each of them must be accessed differently:
  - one of them is in PCI I/O space (little endian)
  - one of them is in PCI MMIO space (little endian)
  - one of them is on-chip MMIO (big endian)
  - one of them is somewhere else, but sparsely addressed (some bytes of
    padding between each register)
and we can for sure come up with a few more examples of weird addressing.

How to solve this, without cluttering each ioread*() with many if clauses?

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds