Re: [PATCH][1/2] usb: dma bounce buffer support V3

On Jan 22, 2008 3:25 PM, Andrew Morton <[email protected]> wrote:
>
> On Tue, 22 Jan 2008 15:12:53 +0900 "Magnus Damm" <[email protected]> wrote:
>
> > On Jan 22, 2008 2:01 PM, Andrew Morton <[email protected]> wrote:
> > > On Tue, 22 Jan 2008 12:20:10 +0900 Magnus Damm <[email protected]> wrote:
> > >
> > > > usb: dma bounce buffer support V3
> > > >
> > > > This patch adds dma bounce buffer support to the usb core. These buffers
> > > > can be enabled with the HCD_LOCAL_MEM flag, and they make sure that all data
> > > > passed to the host controller is allocated using dma_alloc_coherent().
> > >
> > > It is unobvious (to me) why this code uses put_unaligned() and
> > > get_unaligned() in the places where it does.  So I'll assume that it will
> > > be unobvious to other reviewers and readers.
> >
> > The HCD_LOCAL_MEM dma bounce buffer code is using unaligned functions
> > to temporarily save a pointer at the end of the dma data buffer.
> > hcd_alloc_coherent() saves the pointer which is needed by
> > hcd_free_coherent(). The unaligned functions are used because the size
> > of the buffer may be uneven.
> >
> > It's of course possible to round up the size instead and use aligned
> > access if you prefer that. The memory area used for dma may be limited
> > though so I felt it was natural to optimize for small memory footprint
> > over access speed.
>
> that wasn't a patch ;)

Gotcha. Sorry about the attachment - I have no idea how to reply
inline and not mangle patches using gmail...

/ magnus

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--- 0005/drivers/usb/core/hcd.c
+++ work/drivers/usb/core/hcd.c	2008-01-22 15:40:32.000000000 +0900
@@ -1125,6 +1125,14 @@ static int hcd_alloc_coherent(struct usb
 	if (!vaddr)
 		return -ENOMEM;
 
+	/*
+	 * Store the virtual address of the buffer at the end
+	 * of the allocated dma buffer. The size of the buffer
+	 * may be uneven so use unaligned functions instead
+	 * of just rounding up. It makes sense to optimize for
+	 * memory footprint over access speed since the amount
+	 * of memory available for dma may be limited.
+	 */
 	put_unaligned((unsigned long)*vaddr_handle,
 		      (unsigned long *)(vaddr + size));