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On Tue, 29 May 2001 19:30:06 GMT, in fa.linux.kernel you wrote:

> said:
>> After using the 'map_user_kiobuf', I observed the followiing:
>> 1. 'kiobuf->maplist[0]->virtual' contains a different virtual address than
>> the user space buffer address
>> 2. But these two addresses are mapped as when i write something using the
>> address 'kiobuf->maplist[0]->virtual' inside the kernel, I see the same
>> data in the user space buffer in my application.
>> 3. I use the 'virt_to_phys' operation to the virtual address
>> 'kiobuf->maplist[0]->virtual' to get the physical address.
>> 4. I use this physical address for DMA operations.
>> Now, using this information I do a DMA from card to system memory. What I
>> have noticed is that DMA happens somewhere else in the system memory. I am
>> not able to execute most of the commands (ls, chmod, cat, clear etc) after
>> my user program exits.
>> Am I doing the steps 3 and 4 above right?
>After calling map_user_kiobuf(), I believe you should be locking the pages
>in memory by calling lock_kiovec(). Otherwise, nothing prevents them from
>being paged out again.
>Also, the pages may be in high memory and not directly accessible. You should
>use kmap() before touching them from the kernel rather than just using
>page->virtual, and obviously kunmap() afterwards.
>The PCI DMA interface is more complex than simply using virt_to_phys() too,
>if you want to deal correctly with the highmem case. I'm sure others will
>give you the details or pointers to the documentation.


I have a couple of user DMA drivers up and running, but in light of your
comment, I am not so sure.

I see the following w/map_user_kiobuf() in memory.c (2.4.4)


* Force in an entire range of pages from the current process's user VA,
* and pin them in physical memory.
#define dprintk(x...)
int map_user_kiobuf(int rw, struct kiobuf *iobuf, unsigned long va, size_t


What is the difference between pinning and locking? I had the impression
that locking had more to do with ownership.


Bill Waddington
Bainbridge Island, WA, USA
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