Re: Alpha: virt_to_bus/GFP_DMA problem

5116@telia.com
Mon, 13 Dec 1999 08:15:13 +0100 (CET)

On 10 Dec, Alan Cox wrote:
>> Not necessarily. I have ISA PnP cards that work just fine.
>> They must therefore be decoding all 32 bits.
>
> Thats not just implausible its impossible. There are only 24 address wires
> on an ISA bus. It sounds like the ISA bus DMA is being remapped by your
> chipset to a 16Mb window somewhere intelligent. If your board is mapping
> the ISA bus to the low 16Mb of RAM thats good.

Alan, I think you are mistaken here.

When a device uses ISA DMA it is the DMA-controller that generates the
addresses. The DMA controller does not need to be connected though the
ISA bus it self.
Actually the 8237 does only know of 16bit addresses, the higher bits of
the addresses are latched in the DMA page registers outside of the DMAC.
Nothing prevents a smart designer from putting 8 extra bits in the
latch. I think that some EISA chip sets had that feature.

This does not help ISA bus-master devices with their own address
generators on-board, however.

/Daniel 

-- 
Daniel Engström - Lilla Fabriken AB - Technical Support
daniel@lillfab.se - Work related.  5116@telia.com - Private.




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