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

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Message-ID: <20190619081641.GA2387@localhost.localdomain>
Date:   Wed, 19 Jun 2019 17:16:42 +0900
From:   Suwan Kim <suwan.kim027@...il.com>
To:     Alan Stern <stern@...land.harvard.edu>
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, Jun 18, 2019 at 11:30:34AM -0400, Alan Stern wrote:
> 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.

I missed that. Thank you for pointing it out.
I will send v2 including unmap function.

Regards

Suwan Kim