Re: How to write portable MMIO code?

Kai Germaschewski
2002-04-30T16:19:19Z

On Tue, 30 Apr 2002, José Fonseca wrote:

> I'm currently trying to get the Mach64 DRI driver to run on PowerPC. It's
> mostly working but there are some strange behaviors (DMA works, MMIO not
> really unless you make long waits when submiting, etc.). This is most
> likely related with the MMIO programming macros in the kernel module.
>
> My question is: How to code MMIO to be portable across all platforms,
> i.e., taking in consideration the endian format and memory caches?
>
> I've search thorougly the answer to this question but found
> incomplete/contraditory answers:
>
> - should one use readl/writel or dereference the address directly?

Use readl/writel, that's why they exist.

> - is the use of readl/writel macros suficient to account for endian
> correctness or it's also needed to use the cpu_to_le32/le32_to_cpu macros?

They'll give you little endian always (i.e. they byteswap on big endian
archs), so normally (register accesses) you shouldn't need additional
swapping.

> - should one in general (i.e., assuming the worst case) do wmb() on
> writes, and mb() on reads?

I don't think mb() will help you. You're probably experiencing PCI posting
problems - when a writel() has executed, that doesn't necessarily mean
that the transaction has actually happened it may (and will) be buffered
for a potentially long time.

However, PCI won't reorder reads vs. writes, so you when you want to be
sure that a write() actually reached the hardware, do a dummy read()
afterwards, that'll flush the write buffer.

--Kai