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

On Wed, 28 Mar 2007 11:25:54 -0400 (EDT), Alan Stern <[email protected]> wrote:

> > 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.

Right. What's more, I found that the transfer_buffer may not even be
NULL in such cases.

The pre-allocation of buffers and passing DMA address up and down
the stack caused a number of complications, which neither network
stack (w/ skbuff) nor block stack (w/ request) experience. For one
thing, conceptually, upper level driver has to query the DMA bits.
We hid this check inside the allocation helper, but it's still there.
Another complication is the special-casing of transfers to the root hub.

Since both network and block I/O are high performance subsystems,
I always regarded all performance arguments for usb_buffer_alloc
as rubbish. The poor granularity of DMA mapping is real, but it's
a different issue.

The usbmon just brought the issues to the forefront, that's all.

-- Pete

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