Re: _PAGE_PCD bit in DMAable memory

[fc scsi <scsi_fc_group@yahoo.com>]

Hi,

Thanks for the reply and the information. Actually,
the code will have to run on Intel Xscale
processor(ARM based). Since I don't have Linux running
on the board as of now, I was trying out things on my
i386 machine to get non-cacheable memory. That was
when I thought that GFP_DMA should give me
non-cacheable memory(_PAGE_PCD on i386 should be set
for such a memory, and L_PTE_CACHEABLE should be clear
on Xscale by this analogy). But, as you have explained
_PAGE_PCD need not be set for non-cacheable memory for
i386. I am new to Xscale architecture and don't know
if pci_alloc_consistent() will give me non-cacheable
memory or not. Can anyone who has worked on Xscale
confirm this.

Another curiousity for me is, if
pci_alloc_consistent() gives non-cacheable memory for
Xscale(arm) then will the L_PTE_CACHEABLE bit set in
it or not?

Anyway, thanks Roland for the previous answer. It was
a great help.

Regards.


--- Roland Dreier <roland@topspin.com> wrote:
>     fc> Hi, I am trying to get DMAable memory on
> i386
>     fc> (kmalloc(GFP_DMA,)) but seems _PAGE_PCD is
> not set on the
>     fc> pages that i get back the memory from. Am I
> getting the
>     fc> correct results? If yes, then is it not that
> GFP_DMA should
>     fc> give me non-cacheable memory (equivalent to
> setting _PAGE_PCD
>     fc> along with _PAGE_PWT on i386). Can anyone
> confirm for me that
>     fc> GFP_DMA will *always* give me non-cacheable
> and contiguous
>     fc> memory.
> 
> On i386, the GFP_DMA flag controls _where_ the
> memory will be
> allocated, namely in the low 16 MB.  This is really
> a relic of the
> days of 24-bit ISA DMA.
> 
> On i386, the memory does not have to be
> non-cacheable, since in the PC
> architecture the bus controller will maintain
> consistency by snooping
> the CPU.  However, to be portable, your code should
> use
> pci_alloc_consistent() to get memory for DMAing. 
> This will do the
> right thing on all platforms, including making sure
> that the memory is
> non-cacheable on architectures where the bus
> controller does not snoop.
> 
> (By the way, kmalloc() will always return contiguous
> memory, but you
> should still use pci_alloc_consistent for DMA
> memory)
> 
>  - Roland



	
		
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