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

[shuah]

Hi Suwan,

On 6/21/19 11:45 AM, Suwan Kim wrote:
> vhci doesnât do dma for remote device. Actually, the real dma
> operation is done by network card driver. So, vhci doesnât use and
> need dma address of transfer buffer of urb.
>
> When vhci supports SG, it is useful to use native SG list instead
> of mapped SG list because dma mapping fnuction can adjust the
> number of SG list that is urb->num_mapped_sgs.
>
> But hcd provides dma mapping and unmapping function by defualt.

Typo "defualt"

> 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.

How did you verify that unnecessary dma map/unmap are happening?
How many CPU cycles did you manage to reduce with this change?

thanks,
-- Shuah