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

[Wu Fengguang]

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

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