linux-kernel - Re: [PATCH] usbip: Implement map_urb_for_dma function for vhci to skip dma mapping

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Date:   Tue, 18 Jun 2019 11:30:34 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Suwan Kim <suwan.kim027@...il.com>
cc:     shuah@...nel.org, <valentina.manea.m@...il.com>,
        <gregkh@...uxfoundation.org>, <linux-usb@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usbip: Implement map_urb_for_dma function for vhci to
 skip dma mapping

On Tue, 18 Jun 2019, Suwan Kim wrote:

> vhci doesn’t do dma for remote device. Actually, the real dma
> operation is done by network card driver. So, vhci doesn’t use and
> need dma address of transfer buffer of urb.
> 
> But hcd provides dma mapping function by defualt in usb_hcd_submit_urb()
> and it causes unnecessary dma mapping which will be done again at
> NIC driver and it wastes CPU cycles. So, implement map_urb_for_dma
> function for vhci in order to skip the dma mapping procedure.
> 
> Signed-off-by: Suwan Kim <suwan.kim027@...il.com>
> ---
>  drivers/usb/usbip/vhci_hcd.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/usb/usbip/vhci_hcd.c b/drivers/usb/usbip/vhci_hcd.c
> index 667d9c0ec905..9df4d9e36788 100644
> --- a/drivers/usb/usbip/vhci_hcd.c
> +++ b/drivers/usb/usbip/vhci_hcd.c
> @@ -1287,6 +1287,13 @@ static int vhci_free_streams(struct usb_hcd *hcd, struct usb_device *udev,
>  	return 0;
>  }
>  
> +static int vhci_map_urb_for_dma(struct usb_hcd *hcd, struct urb *urb,
> +			    gfp_t mem_flags)
> +{
> +	dev_dbg(hcd->self.controller, "vhci does not map urb for dma\n");
> +	return 0;
> +}
> +
>  static const struct hc_driver vhci_hc_driver = {
>  	.description	= driver_name,
>  	.product_desc	= driver_desc,
> @@ -1302,6 +1309,7 @@ static const struct hc_driver vhci_hc_driver = {
>  	.urb_dequeue	= vhci_urb_dequeue,
>  
>  	.get_frame_number = vhci_get_frame_number,
> +	.map_urb_for_dma = vhci_map_urb_for_dma,
>  
>  	.hub_status_data = vhci_hub_status,
>  	.hub_control    = vhci_hub_control,

If the goal is to avoid wasting CPU cycles, you probably should have a 
vhci_unmap_urb_for_dma routine as well.

Alan Stern