Re: Accessing MMIO PCI space - crossplatform

Jakub Jelinek (jj@sunsite.ms.mff.cuni.cz)
Thu, 12 Nov 1998 23:50:38 +0100 (MET)

> c) virt_to_phys and virt_to_bus should walk through pagetables instead
> of anding/oring with something with something
> (or explicitly say that they cannot be used for shared (adaptor)
> memory... after reading up and down through IO-mapping it looks
> clear to me, but...)
> (after this change, you can do DMA I/O directly to/from userspace
> much easier)

I think it is not wise to slow everything down for no real reason.

> P.P.S.: For Dave and/or Jakub (and others): On __sparc__, __sparc64__
> (one PCI sparc is probably coming here next year) and other non-listed
> ports (__m68k__), what is better, safe and working -
> (*(volatile u_int32_t*)(x)) or readl(x) ?

Both are good, safe and working on SPARC/PCI (ie. JavaStation) and
SPARC64/PCI. The only difference is that the former does no endian
conversion, while the latter swaps the bytes (on Ultra it has no performance
degradation as it is done during the load from memory (but volatile asm
impact)). So you have to know, if you can work with big endian apperture or
how are those mappings called, or if you work with little endian memory
only.

> P.P.P.S.: Yes, I read Documentation/IO-mapping.txt couple of times.
> P.P.P.P.S.: It (IO-mapping.txt) is a bit outdated, isn't it?
> unsigned long signature = *(unsigned int*)0xC0000;
> (at the end of doc) never worked in 2.1, isn't it?
> P.P.P.P.P.S.: While I'm talking about ioremap, is there any chance that
> ioremap_nocache or __ioremap will exist on non-ia32? Otherwise I have
> to use #ifdef __i386__ ... Can I achieve same (similar) effect by MTRR
> on non-Intel or non-PII (non-P6)? I do not believe that every PC hardware
> around can handle this correctly without _nocache.

On SPARC64 particularly, ioremap is a nop, as all RAM and all I/O memory is
mapped all the time at PAGE_OFFSET+PA, in case of I/O memory non-cacheable
with side-effects.
So don't care about sparc64 very much in decisions about ioremap (and about
sparc32 neither, as I guess you cannot have a matroxfb in a JavaStation).

Cheers,
Jakub
___________________________________________________________________
Jakub Jelinek | jj@sunsite.mff.cuni.cz | http://sunsite.mff.cuni.cz
Administrator of SunSITE Czech Republic, MFF, Charles University
___________________________________________________________________
Ultralinux - first 64bit OS to take full power of the UltraSparc
Linux version 2.1.127 on a sparc64 machine (498.80 BogoMips).
___________________________________________________________________

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