Re: mm question

[volodya@mindspring.com]

On Mon, 10 Dec 2001, Alan Cox wrote:

> > Right, but then my card refuses to dma into anything with address smaller
> > than 04000000.
> 
> What was your board designer on when they decided to bar DMA below 64Mb ?

Not mine (unfortunately ?). The card in question is ATI All-in-Wonder
Radeon - and I am trying to get video capture working. Unfortunately the
documentation is quite terse - for example the dma table register
description mentions its width, offset and "No description is provided".

Additionally I have to tiptoe my way around DRI driver which also uses
busmastering.

At the moment I have a working driver with one bug: DMA transfer just does
not happen into pages with address less than 0x4000000 which is the same
value as physical address of the ring buffer.

I suspect that this is somehow connected with the amount of available 
"AGP addressable" memory which is ~64meg less than the total amount on my
machine. However, looking around in AGP driver or AGP specs does not seem
to indicate any restriction of the sort and, moreover, I do not need AGP
for this DMA transfer (it is PCI only).

> 
> > amount) but this would place a big load on the system during buffer
> > allocation.
> 
> And might never terminate

I thought of establishing a timeout and giving up after a while. The
buffer is allocated once for each open on /dev/videoX so it is not too
critical - though I would not want to cause and OOM condition.

> 
> > I was hoping for something more elegant, but I am not adverse to writing
> > my own get_free_page_from_range().
> 
> Thats not a trivial task.
> 

Better than giving up.. Unfortunately looking around in
linux/Documentation and drivers did not yield much in terms of
explanation. I know I can use mem_map_reserve to reserve a page but I
don't know how to get page struct from a physical address nor which lock
to use when messing with this.

                     thanks

                         Vladimir Dergachev