Re: [RFC] URBs and buffer management

Am Dienstag, 5. Juni 2007 schrieb Pete Zaitcev:
> On Mon, 04 Jun 2007 10:38:44 +0200, Marcel Holtmann <[email protected]> wrote:
> 
> >  static void urb_destroy(struct kref *kref)
> >  {
> >  	struct urb *urb = to_urb(kref);
> > +
> > +	if (urb->transfer_flags & URB_FREE_BUFFER)
> > +		usb_buffer_free(urb->dev, urb->transfer_buffer_length,
> > +				urb->transfer_buffer, urb->transfer_dma);
> > +
> >  	kfree(urb);
> >  }
> 
> This looks good to me, although it's useless for the vast majority
> of drivers, because they use kmalloc, or should use kmalloc, but
> use usb_buffer_alloc due to confusion. I'm actually wondering now
> if your driver is one of the latter...

OK, we need to discuss this.

>From Documentation/usb/dma.txt about usb_buffer_alloc():

  The memory buffer returned is "dma-coherent"; sometimes you might need to
  force a consistent memory access ordering by using memory barriers.  It's
  not using a streaming DMA mapping, so it's good for small transfers on
  systems where the I/O would otherwise tie up an IOMMU mapping.  (See
  Documentation/DMA-mapping.txt for definitions of "coherent" and "streaming"

void *hcd_buffer_alloc(
	struct usb_bus 	*bus,
	size_t			size,
	gfp_t			mem_flags,
	dma_addr_t		*dma
)
{
	struct usb_hcd		*hcd = bus_to_hcd(bus);
	int 			i;

	/* some USB hosts just use PIO */
	if (!bus->controller->dma_mask) {
		*dma = ~(dma_addr_t) 0;
		return kmalloc(size, mem_flags);
	}

	for (i = 0; i < HCD_BUFFER_POOLS; i++) {
		if (size <= pool_max [i])
			return dma_pool_alloc(hcd->pool [i], mem_flags, dma);
	}
	return dma_alloc_coherent(hcd->self.controller, size, dma, 0);
}

This takes reasonably care of granularity.

So what exactly is the problem? Is the documentation wrong?

	Regards
		Oliver

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