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Date:   Wed, 7 Aug 2019 00:52:38 +0900
From:   Suwan Kim <suwan.kim027@gmail.com>
To:     shuah <shuah@kernel.org>
Cc:     valentina.manea.m@gmail.com, gregkh@linuxfoundation.org,
        stern@rowland.harvard.edu, linux-usb@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 1/2] usbip: Skip DMA mapping and unmapping for urb at
 vhci
Message-ID: <20190806155238.GC3738@localhost.localdomain>
References: <20190806123154.23798-1-suwan.kim027@gmail.com>
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 <c3485edd-f7e3-95a7-38db-acda371575a2@kernel.org>
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 <b5c6c9e5-0855-c055-6bb5-47dbc497f606@kernel.org>
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On Tue, Aug 06, 2019 at 09:38:54AM -0600, shuah wrote:
> On 8/6/19 9:32 AM, Suwan Kim wrote:
> > 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?
> > 
> 
> Please. This way we will have the device information. pr_err()
> won't us that. In general I prefer dev_* if dev is available in
> the code path, which is the case here.

Ok. I will resend v5.

Regards
Suwan Kim