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Subject: Re: [PATCH v3 2/2] usbip: Implement SG support to vhci-hcd and stub
 driver
To:     Suwan Kim <suwan.kim027@gmail.com>
Cc:     valentina.manea.m@gmail.com, gregkh@linuxfoundation.org,
        stern@rowland.harvard.edu, linux-usb@vger.kernel.org,
        linux-kernel@vger.kernel.org, shuah <shuah@kernel.org>
References: <20190802173651.22247-1-suwan.kim027@gmail.com>
 <20190802173651.22247-3-suwan.kim027@gmail.com>
 <43f32889-0117-cd94-2494-23ab29be6ca8@kernel.org>
 <20190805080451.GB8904@localhost.localdomain>
From:   shuah <shuah@kernel.org>
Message-ID: <9490fd7e-b5e7-21fb-4094-ab9c9e0e0aa5@kernel.org>
Date:   Mon, 5 Aug 2019 09:07:24 -0600
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On 8/5/19 2:04 AM, Suwan Kim wrote:
> On Fri, Aug 02, 2019 at 04:41:52PM -0600, shuah wrote:
>> Hi Suwan,
>>
>> On 8/2/19 11:36 AM, Suwan Kim wrote:
>>> There are bugs on vhci with usb 3.0 storage device.
>>
>> This sentence doesn't make sense to me. What bugs? Ca you eloborate?
> 
> Alan's patch description "usb-storage: Set virt_boundary_mask to
> avoid SG overflows" elaborates the problem. In USB, each SG list
> entry buffer should be divisible by the bulk maxpacket 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, storage driver breaks SG list into several URBs and each
> URB is sparate transfer.
> 
> Let's assume that driver request 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 send 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.
> 
> Please also reference the mail thread
> https://marc.info/?t=155449209600003&r=1&w=2
> https://marc.info/?l=linux-usb&m=155482317722223&w=2
> 

I am asking you to elaborate and add it to the commit log instead of
just saying "There are bugs on vhci with usb 3.0 storage device."

thanks,
-- Shuah