linux-kernel - Re: [PATCH v3] staging: gdm724x: Fix DMA from stack

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Date:   Wed, 10 Feb 2021 15:55:42 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Amey Narkhede <ameynarkhede03@...il.com>
Cc:     dan.carpenter@...cle.com, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] staging: gdm724x: Fix DMA from stack

On Wed, Feb 10, 2021 at 07:55:12PM +0530, Amey Narkhede wrote:
> Stack allocated buffers cannot be used for DMA
> on all architectures so allocate hci_packet buffer
> using kmalloc.
> 
> Signed-off-by: Amey Narkhede <ameynarkhede03@...il.com>
> ---
> Changes in v3:
> 	- Remove superfluous buf pointer
> 	- Reduce size of allocation of hci_packet to match number of
> 	bytes used for DMA
> 
>  drivers/staging/gdm724x/gdm_usb.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/staging/gdm724x/gdm_usb.c b/drivers/staging/gdm724x/gdm_usb.c
> index dc4da66c3..80c58a3ef 100644
> --- a/drivers/staging/gdm724x/gdm_usb.c
> +++ b/drivers/staging/gdm724x/gdm_usb.c
> @@ -56,20 +56,24 @@ static int gdm_usb_recv(void *priv_dev,
> 
>  static int request_mac_address(struct lte_udev *udev)
>  {
> -	u8 buf[16] = {0,};
> -	struct hci_packet *hci = (struct hci_packet *)buf;
> +	struct hci_packet *hci;
>  	struct usb_device *usbdev = udev->usbdev;
>  	int actual;
>  	int ret = -1;
> 
> +	hci = kmalloc(5, GFP_KERNEL);

Why "5" and not:
	hci = kmalloc(sizeof(*hci), GFP_KERNEL);
?

thanks,

greg k-h