Messages in this thread | | | Date | Thu, 9 Sep 2010 21:38:39 -0400 | Subject | ### DMA_ALLOC_COHERENT question | From | "steve spano" <> |
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Hi Folks,
I probably have a simple question and I hope someone can point me in the right direction.
I have a 2.6.30.2 kernel running with a custom device driver for a multi-channel sound card via PCI-Express. Interrupts,DMA, everything is working fine.
We allocate a very small 64KB buffer for our DMA actions using dma_alloc_coherent.
Tripped across an issue with larger amounts of memory on the motherboard. This is an x86 ATOM motherboard.
At 256MB of ram, dma_alloc_coherent gets us an address within the first 128MB of ram, near the upper 1/2 of the 256MB actually.
Then at 1GB, we get an address near the upper 1/2 again (512MB) Then at 2GB, we get an address near the upper 1/2 again (1GB)
This causes a problem on the sound card board because we need to then map in nearly all 2GB of address space across the PCI bar registers so we can access our tiny 64KB dma address range sitting up near the 1GB boundary.
This seems like we are doing something not right?
Ideally, we would get a non-cached block of ram that we can DMA to and the processor can access. This block would ideally always be located near the base of ram (maybe right after the kernel?). That way we can open a 128MB window into the system ram from PCI-Express and always be sure to be able to access our DMA buffer.
Can someone provide some insight in how to do this? Is there a differnet dma allocation procedure we should use? Should we just put an "unsigned char []" of 64k in the driver? Can we make that also non cache? Or do we have to hack the kernel allocation process?
Im sure there is something simple we can do.
Thanks again
Steve Spano FLE
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