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Subject: Re: [PATCH v3 0/2] usbip: Implement SG support
To:     Suwan Kim <suwan.kim027@gmail.com>, valentina.manea.m@gmail.com,
        gregkh@linuxfoundation.org, stern@rowland.harvard.edu
Cc:     linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
        shuah <shuah@kernel.org>
References: <20190802173651.22247-1-suwan.kim027@gmail.com>
From:   shuah <shuah@kernel.org>
Message-ID: <1b350c7c-823a-21a8-fc2f-d12dd4b4818d@kernel.org>
Date:   Fri, 2 Aug 2019 16:44:40 -0600
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Hi Suwan,

On 8/2/19 11:36 AM, Suwan Kim wrote:
> There are bugs on vhci with usb 3.0 storage device. Currently, vhci
> doesn't supported SG, so USB storage driver on vhci breaks SG list
> into multiple URBs and it causes error that a transfer got terminated
> too early because the transfer length for one of the URBs was not
> divisible by the maxpacket size.
>   
> In this patch, vhci supports SG regardless of whether the server's
> host controller supports SG or not, because stub driver splits SG
> list into several URBs if the server's host controller doesn't
> support SG.
>   
> To support SG, vhci_map_urb_for_dma() sets URB_DMA_MAP_SG flag in
> urb->transfer_flags if URB has SG list and this flag will tell stub
> driver to use SG list.
>   
> vhci sends each SG list entry to stub driver. Then, stub driver sees
> the total length of the buffer and allocates SG table and pages
> according to the total buffer length calling sgl_alloc(). After stub
> driver receives completed URB, it again sends each SG list entry to
> vhci.
>   
> If the server's host controller doesn't support SG, stub driver
> breaks a single SG request into several URBs and submits them to
> the server's host controller. When all the split URBs are completed,
> stub driver reassembles the URBs into a single return command and
> sends it to vhci.
> 
> Alan fixed vhci bug with the USB 3.0 storage device by modifying
> USB storage driver.
> ("usb-storage: Set virt_boundary_mask to avoid SG overflows")
> But the fundamental solution of it is to add SG support to vhci.
> 
> This patch works well with the USB 3.0 storage devices without Alan's
> patch, and we can revert Alan's patch if it causes some troubles.
> 

Why just 3.0? Please test with lowspeed and VUDC to make sure there are
no regressions.

thanks,
-- Shuah