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Date:   Tue, 27 Aug 2019 02:04:50 +0900
From:   Suwan Kim <suwan.kim027@gmail.com>
To:     shuah@kernel.org, valentina.manea.m@gmail.com,
        gregkh@linuxfoundation.org, stern@rowland.harvard.edu
Cc:     linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v7] usbip: Implement SG support to vhci-hcd and stub
 driver
Message-ID: <20190826170450.GA2314@localhost.localdomain>
References: <20190826150007.12187-1-suwan.kim027@gmail.com>
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On Tue, Aug 27, 2019 at 12:00:07AM +0900, Suwan Kim wrote:
> There are bugs on vhci with usb 3.0 storage device. In USB, each SG
> list entry buffer should be divisible by the bulk max packet size.
> But with native SG support, this problem doesn't matter because the
> SG buffer is treated as contiguous buffer. But without native SG
> support, USB storage driver breaks SG list into several URBs and the
> error occurs because of a buffer size of URB that cannot be divided
> by the bulk max packet size. The error situation is as follows.
> 
> When USB Storage driver requests 31.5 KB data and has SG list which
> has 3584 bytes buffer followed by 7 4096 bytes buffer for some
> reason. USB Storage driver splits this SG list into several URBs
> because VHCI doesn't support SG and sends them separately. So the
> first URB buffer size is 3584 bytes. When receiving data from device,
> USB 3.0 device sends data packet of 1024 bytes size because the max
> packet size of BULK pipe is 1024 bytes. So device sends 4096 bytes.
> But the first URB buffer has only 3584 bytes buffer size. So host
> controller terminates the transfer even though there is more data to
> receive. So, vhci needs to support SG transfer to prevent this error.
> 
> 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 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. After receiving urb from stub driver, vhci clear URB_DMA_MAP_SG
> flag to avoid dma unmapping in HCD.
 
I have a mistake not modifying this commit log according to the
change of the code. I will resend v8.

Regards
Suwan Kim