Subject: trouble mmapping dma buffer

From: Steve Rossi <[email-masked]>
Date: Tue, 18 Jun 2002 07:58:27 -0500

I wrote a driver, which used to work under the stable 2.4.16 kernel, but
since I've moved up to the 2.4.19-pre7 development kernel it has stopped
working. The driver basically allocates a dma buffer (64K) using
consistent_alloc and it marks all of the pages in the kernel's mapping
of this buffer reserved (calling mem_map_reserve for each page). A user
space application calls mmap on the driver to get direct access to that
64K DMA buffer. The mmap routine in the driver sets VM_RESERVED in
vma->vm_flags then uses remap_page_range to map the physical address of
the DMA buffer to the user space virtual memory area, one page at a
time. It also marks each page _PAGE_NO_CACHE and _PAGE_GUARDED in the
user space mapping.
This all worked just fine under 2.4.16, but now under 2.4.19-pre7 when I
run the application that mmaps the device I get the following:

swap_dup: Bad swap file entry 00000004

repeated 16 times (note - that's how many pages are in my dma buffer)
followed by:

swap_free: Bad swap file entry 00000004

also repeated 16 times.
When the application exists and calls munmap, I get
swap_free: Bad swap file entry 00000004
another 16 times.
I'm running on an 8xx systems with 32MB of RAM. I have no swap space.
The DMA buffer typically gets allocated around physical address
0x19A0000, up near the top of the memory. I'm guessing that maybe the
kernel thinks that the mmaped pages are swapped out?? But why? Has
anything changed between 2.4.16 and 2.4.19-pre7 that could account for
this. Is there a problem with using remap_page_range to map RAM? I would
appreciate any help!

Thanks!
Steve

--
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Steven K. Rossi                     [email-masked]
Senior Staff Engineer
Multimedia Communications Research Laboratory
Motorola Labs
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