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

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Date:	Wed, 10 Dec 2008 10:40:09 +0100
From:	Laurent Pinchart <laurent.pinchart@...net.be>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...e.de>, linux-usb@...r.kernel.org
Subject: Re: [PATCH] USB: use stack allocation for struct usb_ctrlrequest

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.

Best regards,

Laurent Pinchart

> 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);
>
> --
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