Re: Problem with DMA buffer (in 2.2.15)

Christoph Baumann (
Thu, 22 Feb 2001 17:24:58 +0100

I messed up my subscription to this list yesterday so any answer to my question
didn't reach me (I looked through the archive up to 20:50 yesterday).
Now as I read my question again I decided to add some more details.

>But when reading the buffer isn't allocated
This is of course rubbish. I meant the buffer isn't initialized.

Some more details about what I'm doing:
I have a PCI board with 1M of RAM and a PLX9080 on it (and some more chips
which don't matter here). So far I have established normal communication with
the board (to program the PLX etc.). Now I want to write larger junks of data
to the RAM. So I startet to resolve the physical addresses of the user space
data to generate a chain list. Everything works fine for writing to the RAM.
The problem is reading. If I allocate a new buffer for reading back it isn't
of course not yet mapped to physical memory. The "quick and dirty"(tm) hack
was writing a zero to each buffer element to get it mapped. The better version
is to do this in steps of PAGE_SIZE. What I'm looking for is a kernel routine
to force the mapping of previous unmapped pages. Browsing through the source
in mm/ I found make_pages_present(). Could this be the solution? I hadn't the
time to try it out yet.


* Christoph Baumann                                      *
* Kirchhoff-Institut für Physik - Uni Heidelberg         *
* Mail:                    *
* Phone: ++49-6221-54-4329                               *

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