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    SubjectRe: [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@gmail.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

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