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Date:   Tue, 23 Jul 2019 22:40:12 +0900
From:   Suwan Kim <suwan.kim027@gmail.com>
To:     shuah <shuah@kernel.org>
Cc:     valentina.manea.m@gmail.com, stern@rowland.harvard.edu,
        gregkh@linuxfoundation.org, linux-usb@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/2] usbip: Skip DMA mapping and unmapping for urb at
 vhci
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 <d10b23b3-601d-c2c6-df2e-77b2ed9adc66@kernel.org>
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On Mon, Jul 22, 2019 at 02:26:42PM -0600, shuah wrote:
> Hi Suwan,
> 
> On 7/5/19 10:43 AM, Suwan Kim wrote:
> > vhci doesn’t do DMA for remote device. Actually, the real DMA
> > operation is done by network card driver. vhci just passes virtual
> > address of the buffer to the network stack, so vhci doesn’t use and
> > need dma address of the buffer of the URB.
> > 
> > When it comes to supporting SG for vhci, it is useful to use native
> > SG list (urb->num_sgs) instead of mapped SG list because DMA mapping
> > fnuction can adjust the number of SG list (urb->num_mapped_sgs).
> > 
> > But HCD provides DMA mapping and unmapping function by default.
> > Moreover, it causes unnecessary DMA mapping and unmapping which
> > will be done again at the NIC driver and it wastes CPU cycles.
> > So, implement map_urb_for_dma and unmap_urb_for_dma function for
> > vhci in order to skip the DMA mapping and unmapping procedure.
> > 
> > To support SG, vhci_map_urb_for_dma() sets URB_DMA_MAP_SG flag in
> > urb->transfer_flags if URB has SG list and this flag will tell the
> > stub driver to use SG list.
> > 
> > Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
> > ---
> >   drivers/usb/usbip/vhci_hcd.c | 19 +++++++++++++++++++
> >   1 file changed, 19 insertions(+)
> > 
> > diff --git a/drivers/usb/usbip/vhci_hcd.c b/drivers/usb/usbip/vhci_hcd.c
> > index 000ab7225717..14fc6d9f4e6a 100644
> > --- a/drivers/usb/usbip/vhci_hcd.c
> > +++ b/drivers/usb/usbip/vhci_hcd.c
> > @@ -1288,6 +1288,22 @@ 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");
> > +
> > +	if (urb->num_sgs)
> > +		urb->transfer_flags |= URB_DMA_MAP_SG;
> > +
> 
> Shouldn't this be part of patch 2. The debug message saying "no map"
> and setting flag doesn't make sense.

I think you are right. Setting flag should be in patch 2. Thank you
for pointing out :)
I will remove unnecessary debug messages and move setting flag to
patch 2.

> > +       return 0;
> 
> This should be a tab and no spaces btw. chekpatch isn't happy.
> 
> > +}
> > +
> > +static void vhci_unmap_urb_for_dma(struct usb_hcd *hcd, struct urb *urb)
> > +{
> > +       dev_dbg(hcd->self.controller, "vhci does not unmap urb for dma\n");
> 
> This should be a tab and no spaces btw. chekpatch isn't happy.
> 
> 
> WARNING: please, no spaces at the start of a line
> #144: FILE: drivers/usb/usbip/vhci_hcd.c:1299:
> +       return 0;$
> 
> WARNING: please, no spaces at the start of a line
> #149: FILE: drivers/usb/usbip/vhci_hcd.c:1304:
> +       dev_dbg(hcd->self.controller, "vhci does not unmap urb for dma\n");$
> 
> total: 0 errors, 2 warnings, 31 lines checked

I'm sorry for my fault. I will check it.

Regards,
Suwan Kim