[ELDK] Non cacheable memory request

[Douglas, Jim (Jim)]

> From: eldk-bounces at lists.denx.de [mailto:eldk-bounces at lists.denx.de]
On
> Behalf Of Fariya
> 
> I am working on the PPC 405EX processor with a peripheral attached to
the
> EBC. I've requested for I/O memory. How do I ensure that the memory
range
> that I requested is non-cacheable? I've to work with non-cacheable
memory.
>  I've requested for memory using request_mem_region( ) & mapped it
using
> ioremap( ) . Which functions should I use to ensure the
non-cacheability
> of
> the requested memory region?
[Jim Douglas] 
I do not know about 405EX, but try kmalloc(..., GFP_DMA | GFP_KERNEL)
or
dma_alloc_coherent(...)

> 
> Between any consecutive writes or any consecutive reads, if I
introduce a
> delay of 500msec, the read/write is completing.  Does this have to do
> anything with caching? What could be the  reason for this?
[Jim Douglas] 
Maybe compiler or CPU reordering/optimisation.  This is what memory
barriers are for.  

Conventional kernel practise is to avoid directly writing to IO memory,
but to use macros like ioread8() etc, instead.  Watch out, though,
because some of them are little-endian.  For ppc, 'volatile' is also
used a lot.  

See Documentation/memory-barriers.txt
See also Documentation/volatile-considered-harmful.txt

--jim