Re: [PATCH]dma rules violation in the option driver

On Tue, 27 Mar 2007, David Brownell wrote:

> > Hmm... A CPU has to access all that data, and reading a cache line
> > at a time surely as better. Although on the other hand, it's something
> > streaming and thus just using the cache space for no good reason.
> 
> Right.  "Potentially".  Displacing hot cachelines for use-once data
> will in various cases be a lose.

It would be nice to see somewhere a more explicit and detailed discussion 
of the tradeoffs.  Under what conditions is it better to allocate DMA 
memory rather than just using kmalloc() and calling dma_map_single()?


> > OK... I admit I must do something for usbmon working better when
> > DMA is pre-allocated, especially on PPC (where it simply gives
> > up immediately and returns 'D' tag). But it's somewhat difficult.
> > There's no portable API which I could use.
> 
> That is, when the application manages DMA adressing?  The driver "should"
> provide both the DMA and regular address, since not every underlying
> controller wil use the DMA address for all transfers.

That's not quite true.  The driver is obliged to provide a regular address 
if the controller doesn't use DMA, but it isn't obliged if the controller 
does use DMA.  And in fact it frequently happens with usb-storage; the 
underlying SCSI drivers provide a scatter-gather list with page addresses 
that are suitable for DMA but may not lie in kernel-accessible memory.

Alan Stern


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