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

Wu Fengguang
2008-12-10T13:08:42Z

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?
e.g. what if the memory is allocated from ZONE_HIGHMEM?

Thanks,
Fengguang

>
> > Cc: Greg Kroah-Hartman <gregkh@suse.de>
> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > ---
> > drivers/usb/core/message.c | 27 ++++++++-------------------
> > 1 file changed, 8 insertions(+), 19 deletions(-)
> >
> > --- linux-2.6.orig/drivers/usb/core/message.c
> > +++ linux-2.6/drivers/usb/core/message.c
> > @@ -130,26 +130,15 @@ int usb_control_msg(struct usb_device *d
> > __u8 requesttype, __u16 value, __u16 index, void *data,
> > __u16 size, int timeout)
> > {
> > - struct usb_ctrlrequest *dr;
> > - int ret;
> > -
> > - dr = kmalloc(sizeof(struct usb_ctrlrequest), GFP_NOIO);
> > - if (!dr)
> > - return -ENOMEM;
> > -
> > - dr->bRequestType = requesttype;
> > - dr->bRequest = request;
> > - dr->wValue = cpu_to_le16p(&value);
> > - dr->wIndex = cpu_to_le16p(&index);
> > - dr->wLength = cpu_to_le16p(&size);
> > + struct usb_ctrlrequest dr = {
> > + .bRequestType = requesttype,
> > + .bRequest = request,
> > + .wValue = cpu_to_le16p(&value),
> > + .wIndex = cpu_to_le16p(&index),
> > + .wLength = cpu_to_le16p(&size),
> > + };
> >
> > - /* dbg("usb_control_msg"); */
> > -
> > - ret = usb_internal_control_msg(dev, pipe, dr, data, size, timeout);
> > -
> > - kfree(dr);
> > -
> > - return ret;
> > + return usb_internal_control_msg(dev, pipe, &dr, data, size, timeout);
> > }
> > EXPORT_SYMBOL_GPL(usb_control_msg);
> >
> > --
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