Date:   Tue, 6 Aug 2019 11:27:36 -0600
From:   shuah <shuah@...nel.org>
To:     Suwan Kim <suwan.kim027@...il.com>
Cc:     valentina.manea.m@...il.com, gregkh@...uxfoundation.org,
        stern@...land.harvard.edu, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, shuah <shuah@...nel.org>
Subject: Re: [PATCH v4 2/2] usbip: Implement SG support to vhci-hcd and stub
 driver

On 8/6/19 9:48 AM, Suwan Kim wrote:
> On Tue, Aug 06, 2019 at 09:13:54AM -0600, shuah wrote:
>> On 8/6/19 6:31 AM, 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_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.
>>>
>>> Moreover, in the situation where vhci supports SG, but stub driver
>>> does not, or vice versa, usbip works normally. Because there is no
>>> protocol modification, there is no problem in communication between
>>> server and client even if the one has a kernel without SG support.
>>>
>>> In the case of vhci supports SG and stub driver doesn't, because
>>> vhci sends only the total length of the buffer to stub driver as
>>> it did before the patch applied, stub driver only needs to allocate
>>> the required length of buffers regardless of whether vhci supports
>>> SG or not.
>>>
>>> If stub driver needs to send data buffer to vhci because of IN pipe,
>>> stub driver also sends only total length of buffer as metadata and
>>> then sends real data as vhci does. Then vhci receive data from stub
>>> driver and store it to the corresponding buffer of SG list entry.
>>>
>>> In the case of stub driver that supports SG, buffer is allocated by
>>> sgl_alloc(). However, stub driver that does not support SG allocates
>>> buffer using only kmalloc(). Therefore, if vhci supports SG and stub
>>> driver doesn't, stub driver has to allocate buffer with kmalloc() as
>>> much as the total length of SG buffer which is quite huge when vhci
>>> sends SG request, so it has big overhead in buffer allocation.
>>>
>>> And for the case of stub driver supports SG and vhci doesn't, since
>>> the USB storage driver checks that vhci doesn't support SG and sends
>>> the request to stub driver by splitting the SG list into multiple
>>> URBs, stub driver allocates a buffer with kmalloc() as it did before
>>> this patch.
>>>
>>> VUDC also works well with this patch. Tests are done with two USB
>>> gadget created by CONFIGFS USB gadget. Both use the BULK pipe.
>>>
>>>           1. Serial gadget
>>>           2. Mass storage gadget
>>>
>>>    * Serial gadget test
>>>
>>> Serial gadget on the host sends and receives data using cat command
>>> on the /dev/ttyGS<N>. The client uses minicom to communicate with
>>> the serial gadget.
>>>
>>>    * Mass storage gadget test
>>>
>>> After connecting the gadget with vhci, use "dd" to test read and
>>> write operation on the client side.
>>>
>>> Read  - dd if=/dev/sd<N> iflag=direct of=/dev/null bs=1G count=1
>>> Write - dd if=<my file path> iflag=direct of=/dev/sd<N> bs=1G count=1
>>>
>>
>> Thanks for the test results.
>>
>> Were you able to test with USB lowspeed devices?
> 
> I tested USB3 super-speed device and USB2 high-speed device.
> But I don't have full/low speed device that uses BULK transfer.
> In USB spec, low-speed device doesn't support BULK transfer.
> 

I think I mentioned earlier that the reason to test with lowspeed
is make sure there are no regressions due this change. You aren't
testing BULK transfer, you are looking for any regressions.

> Do I add test description about the USB3 super-speed and USB2
> high-speed device to the commit log?
> 

Please do.

thanks,
-- Shuah

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