Undestanding ioread32() / readl() and friends

Undestanding ioread32() / readl() and friends

[Florian Boelstler]

Hi,

I just came across include/asm-ppc/io.h, which revealed that ioread32() 
maps to readl(), which in turn maps to in_le32() (when CONFIG_APUS and 
CONFIG_8260_PCI9 are both not defined).
in_le32() uses a asm instruction lwbrx to do byte swapping.

I don't really get why ioread32() effectively returns a little-endian 
value on a big-endian PPCs.

Is this a sort of general rule that ioread32() always returns 
little-endian values on all architectures?

BTW, "Linux Device Drivers, 3rd edition" by Corbet et al states that 
ioread32() does not perform any endian conversion.
May be they are based on x86...

Thanks,

   Florian

Re: Undestanding ioread32() / readl() and friends

[Arnd Bergmann]

On Wednesday 21 February 2007 10:50, Florian Boelstler wrote:
> I just came across include/asm-ppc/io.h, which revealed that ioread32() 
> maps to readl(), which in turn maps to in_le32() (when CONFIG_APUS and 
> CONFIG_8260_PCI9 are both not defined).
> in_le32() uses a asm instruction lwbrx to do byte swapping.
> 
> I don't really get why ioread32() effectively returns a little-endian 
> value on a big-endian PPCs.
> 
> Is this a sort of general rule that ioread32() always returns 
> little-endian values on all architectures?

the ioread family of functions is used for PCI devices, which
are little-endian by definition.

If you want to access on-chip devices that are not behind PCI,
you should use the in_be family of functions.

	Arnd <><

Re: Undestanding ioread32() / readl() and friends

[Florian Boelstler]

Hi,

Arnd Bergmann schrieb:
>> Is this a sort of general rule that ioread32() always returns 
>> little-endian values on all architectures?
> 
> the ioread family of functions is used for PCI devices, which
> are little-endian by definition.

OK, my knowledge about ioread32() was based on some explanations found 
in "Linux Device Drivers". IMHO they showed the ioread family of 
functions as a sort of general purpose functions for __iomem-alike 
resources.

Thanks, anyway.

   Florian