Re: [PATCH] USB: use stack allocation for struct usb_ctrlrequest

Wu Fengguang
2008-12-10T14:23:51Z

On Wed, Dec 10, 2008 at 04:07:14PM +0200, Gilad Ben-Yossef wrote:
> Wu Fengguang wrote:
>
> > Hi Laurent,
> >
> > On Wed, Dec 10, 2008 at 11:40:09AM +0200, Laurent Pinchart wrote:
> >
> >> Hi Wu,
> >>
> >> On Wednesday 10 December 2008, Wu Fengguang wrote:
> >>
> >>> sizeof(struct usb_ctrlrequest) = 8, which is as small as the *dt pointer
> >>> in a 64bit system.
> >>>
> >> The usb_ctrlrequest pointer is passed down to the hardware and must point to
> >> DMA-able memory. For this reason you can't use the stack and must kmalloc()
> >> the structure.
> >>
> >
> > Ah thanks for the background. Does GFP_NOIO guarantee that?
> >
> No, GFP_NOIO means - do not generate block IO operations (e.g. move
> pages to swap, sync dirty buffers to permanent storage etc.) in order to
> fulfill this allocation.
>
> The reason for this flag here is presumably that such block IO
> operations may very cause USB transaction of the very same kind we're
> trying to service now, which can easily get us to a loop.

Right.

> > e.g. what if the memory is allocated from ZONE_HIGHMEM?
> >
> In many cases there is no problem to DMA high memory. If you happen to
> be working with a device that does have problems with full 32 bit
> addresses then GFP_DMA would be the right flag, not GFP_NOIO.

For 64bit systems, we can easily go beyond 4GB physical memory.
So at least we should add GFP_DMA32 in addition to GFP_NOIO?

Thanks,
Fengguang