(allocating non-cachable memory) (or More on the i82596)

[Justin (Gus) Hurwitz]

On Fri, 29 Jun 2001, Matt Porter wrote:

> If you are on broken 6xx/7xx/82xx implementation then you can use the
> "nobats" kernel argument to map memory with only PTEs.  Then the
> consistent_alloc() stuff that Dan has brought over from arch/arm
> will work fine.  The implementation is simply to get_free_pages(),
> virt_to_phys(), ioremap(), and finally throw away the original
> cacheable mappings.
>
> Hrm, I see this stuff Dan's been talking about isn't in linuxppc_devel
> (bad Dan).  If we all nag him perhaps it will get there faster. :)
>
> In the meantime, look at the arch/arm's implementation to get an
> idea of how it optimizes software coherency with the knowledge of
> how you are using the memory region.

I've been poking around in the arch/arm code, impatiently contemplating
bringing the code over the my 2_4_devel tree on my own until it's been
done by someone more powerful. I looks like the big this done in the
consistent_alloc function is the calling of SetPageReserved for each page
allocated- this is a macro that sets the PG_reserved bit for the page,
which looks like it is a linux kernel thing, and not a hardware thig. Am I
missing something here, or will this actually prevent caching of those
pages? Or is the real magic done in the ioremap call, and the PG_reserved
bit tells the kernel not to touch the page (though it can still access the
data)? I presume the latter is the case, but I'd rather be sure before I
muck with this and add make my debugging job even harder.

Thanks
--Gus


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