linux-kernel - Re: [PATCH v4 1/2] usbip: Skip DMA mapping and unmapping for urb at vhci

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Date:   Wed, 7 Aug 2019 00:32:41 +0900
From:   Suwan Kim <suwan.kim027@...il.com>
To:     shuah <shuah@...nel.org>
Cc:     valentina.manea.m@...il.com, gregkh@...uxfoundation.org,
        stern@...land.harvard.edu, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] usbip: Skip DMA mapping and unmapping for urb at
 vhci

On Tue, Aug 06, 2019 at 09:11:30AM -0600, shuah wrote:
> On 8/6/19 6:31 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.
> > 
> > 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.
> > 
> > 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).
> > And vhci_map_urb_for_dma() prevents isoc pipe from using SG as
> > hcd_map_urb_for_dma() does.
> > 
> > Signed-off-by: Suwan Kim <suwan.kim027@...il.com>
> > ---
> > v3 - v4:
> > - Replace WARN_ON() with pr_err() in the error path.
> > 
> > v2 - v3
> > - Move setting URB_DMA_MAP_SG flag to the patch 2.
> > - Prevent isoc pipe from using SG buffer.
> > 
> > v1 - v2
> > - Add setting URB_DMA_MAP_SG flag in urb->transfer_flags to tell
> > stub driver to use SG buffer.
> > ---
> >   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..429e4e989f38 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)
> > +{
> > +	if (usb_endpoint_xfer_isoc(&urb->ep->desc) && urb->num_sgs) {
> > +		pr_err("SG is not supported for isochronous transfer\n");
> 
> Any reason to not use dev_err()?

Because some codes in vhci_hcd.c use pr_err().There is no other
reason. However, dev_err() seems more appropriate than pr_err().
I will replace pr_err() with dev_err(urb->dev->dev, "SG is ...")
Is it ok?

Regards
Suwan Kim