linux-kernel - Re: [PATCH] USB: use stack allocation for struct usb

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Message-ID: <20081210124308.GB6772@localhost>
Date:	Wed, 10 Dec 2008 20:43:11 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Laurent Pinchart <laurent.pinchart@...net.be>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
Subject: Re: [PATCH] USB: use stack allocation for struct usb_ctrlrequest

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@...e.de>
> > Signed-off-by: Wu Fengguang <fengguang.wu@...el.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);
> >
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-usb" in
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> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
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