Non-cacheable memory

Daris A Nevil dnevil at
Wed Aug 8 04:34:09 EST 2001

Gus, Tom,

Here is the procedure I used to allocate pages and mark them as non-cached in
the 2.2.5 kernel (SNMC's QSLinux):

    mem_addr = __get_free_page(GFP_KERNEL);
    pte = va_to_pe(&init_task, mem_addr);
    pte_val(*pte) |= _PAGE_NO_CACHE;
    flush_tlb_page(current->mm_mmap, mem_addr);

I used these pages for SCC buffer descriptors, so they were never returned to
the system. I'm not sure if this is the preferred way, but it certainly works.
I have not tried this in the 2.4 kernel yet.  The functions __get_free_page()
and flush_tlb_page() do exist in 2.4.4, but I could not locate va_to_pe()  (I
think that was a macro in 2.2.5).

You may also want to look at the file linux/asm/pgalloc.h, esp. the functions
get_pgd_slow() and get_pgd_fast().

Daris Nevil

Tom Roberts wrote:

> "Justin (Gus) Hurwitz" wrote:
> > Does the current 2_4_devel tree have support for allocating regions of
> > non-cachable memory? And, if so, what is the proper (or preferred) way of
> > doing this?
> What I did in 2.2.15 was to edit the main startup routine to call my driver
> directly so it could "eat up" a region of memory before Linux started its
> memory management. I then used a BAT to reference it uncached. Yes, this
> is ugly, and requires care in making sure the BAT effective address is
> otherwise unused by the kernel, but it works. This is on a MPC7400, and
> other PowerPC CPUs might not have BATs available....

Daris A Nevil
Simple Network Magic Corporation
401 Kentucky Lane
McKinney, TX 75069
dnevil at

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