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

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Message-ID: <c3485edd-f7e3-95a7-38db-acda371575a2@kernel.org>
Date:   Tue, 6 Aug 2019 09:11:30 -0600
From:   shuah <shuah@...nel.org>
To:     Suwan Kim <suwan.kim027@...il.com>, valentina.manea.m@...il.com,
        gregkh@...uxfoundation.org, stern@...land.harvard.edu
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        shuah <shuah@...nel.org>
Subject: Re: [PATCH v4 1/2] usbip: Skip DMA mapping and unmapping for urb at
 vhci

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()?

Looks good otherwise.

thanks,
-- Shuah