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

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Date:   Fri, 28 Jun 2019 18:11:54 -0600
From:   shuah <shuah@...nel.org>
To:     Suwan Kim <suwan.kim027@...il.com>, stern@...land.harvard.edu,
        valentina.manea.m@...il.com, gregkh@...uxfoundation.org
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        shuah <shuah@...nel.org>
Subject: Re: [PATCH 1/2] usbip: Skip DMA mapping and unmapping for urb at vhci

Hi Suwan,

On 6/21/19 11:45 AM, Suwan Kim wrote:
> vhci doesn’t do dma for remote device. Actually, the real dma
> operation is done by network card driver. So, vhci doesn’t 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