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#1 2006-05-13 23:14:52

HH
Developer
Registered: 2006-02-15
Last visit: 2007-03-12
Posts: 103

DMA question

Are DMA operations on the adm5120 limited to the first 16 MB of memory?  The Linux source code says yes, but I'm wondering if that is really true.  The adm5120 PCI interface has 32 address/data lines and there is no ISA bus, which is where the 16 MB limitation usually comes from.

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#2 2006-05-15 07:27:45

sergioag
Developer
Peru
From: Lima
Registered: 2006-02-16
Last visit: 2011-08-15
Posts: 166
Website

Re: DMA question

I bet it doesn't. Is the comment found on a platform-independent part? If so, that probably is a stale comment when that code was PC-specific. If not, then we should check the ADM5120 reference manual.

Regards,

Sergio

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#3 2006-05-15 08:41:24

HH
Developer
Registered: 2006-02-15
Last visit: 2007-03-12
Posts: 103

Re: DMA question

include/asm-mips/dma.h sets MAX_DMA_ADDRESS to a 16 MB limit, but the comments in that file say the values therein only really apply to the ISA bus.  Some of the USB and Switch registers, such as the switch register at offset 0xD0 ("Send High Base Address"), have a "Descriptor Base Address" field that's only 24 bits.  That suggests there is a 16 MB address limit, but they only point to descriptor blocks, which themselves have address DWORDs.  There is nothing written that those addresses have to be below 16 MB.  Maybe they are implying that they must also be below 16 MB.  I'm not sure.

I thought it might have some bearing on the USB problems but if most devices only have 16 MB total RAM then it wouldn't matter one way or the other.

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