memory position of numpy arrays

David Cournapeau david at ar.media.kyoto-u.ac.jp
Tue Oct 24 21:28:38 CDT 2006


Andrew Straw wrote:
> David Cournapeau wrote:
>   
>> I don't know anything about your device, but a driver directly accessing 
>> a memory buffer from a userland program sounds like a bug to me. 
>>     
> David, DMA memory (yes, I know thats an example of RAS Syndrome,
> apologies) allows hardware to fill a chunk of RAM and then hand it over
> to a userspace program. In my experience, RAM used for this purpose must
> be pre-allocated, usually in a ring-buffer type arrangement. So this is
> normal operating procedure for something like a frame grabber and not a
> bug at all.
>   
What I understood from former emails was that the user is allocating a 
memory buffer, and that it gives this memory buffer to the hardware. In 
this sense, I don't see how it is possible to avoid kernel panic or 
equivalents. If on the contrary, the driver gives you the memory buffer, 
then, ok, by eg a mmap-like call, you can access directly the device 
memory, but within a range fixed by the driver, which is valid if the 
driver is not buggy.

That's why I don't understand the paging problem and why allocating 
anything from C or python would change anything (I think windows can 
page out kernel, contrary to linux, but I doubt it can page out DMA 
areas), because the user does not allocate anything in this scenario.

But again, this is just what I would think from "common sense", and I 
have never done any system programming, so I may just miss something,

David

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