Re: DMA transfers in 2.5.67

[mru@users.sourceforge.net (Måns Rullgård)]

Alan Cox <alan@lxorguk.ukuu.org.uk> writes:

> > It's an Alpha with 768 MB.  Is it the pci_alloc_* functions you are
> > referring to?  I don't think they are used currently. How much memory
> > can these allocate?  I need chunks of up to 1 MB, not necessarily
> > phycically continuous.
> > 
> > What do those functions do that normal memory allocation does not?
> > Apart from setting up sg mappings, that is.
> 
> A normal memory allocation might not be visible from the device, however
> pci_map_sg() deals with such things. What I really meant was are you
> using the pci_ DMA functionality

Those functions are not used at the moment, but I could change that.
The question remains why DMA transfers are so slow.  The memory is
clearly visible from the bus.

Btw, I just noticed that hard disk throughput is much lower with 2.5
than 2.4.  With 2.4.21-pre5 I get ~40 MB/s, but with 2.5.67 the speed
drops to 25-30 MB/s.  Everything according to hdparm.  Is it possible
that DMA is generally slow for some reason?

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