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Subject: Re: [External] Re: [PATCH v5 5/8] usb: mausb_host: Introduce PAL
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To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org,
        linux-usb@vger.kernel.org, mausb-host-devel@displaylink.com
References: <Pine.LNX.4.44L0.2004301112390.27217-100000@netrider.rowland.org>
From:   Vladimir Stankovic <vladimir.stankovic@displaylink.com>
Message-ID: <32cc80c0-c2ff-440c-7505-e848aba782d8@displaylink.com>
Date:   Thu, 30 Apr 2020 17:34:48 +0200
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On 30.4.20. 17:18, Alan Stern wrote:
> On Thu, 30 Apr 2020, Vladimir Stankovic wrote:
>=20
>> On 26.4.20. 22:56, Alan Stern wrote:
>>> On Sun, 26 Apr 2020, Vladimir Stankovic wrote:
>>>
>>>> On 26.4.20. 16:31, Alan Stern wrote:
>>>>> On Sun, 26 Apr 2020, Vladimir Stankovic wrote:
>>>>>
>>>>>> On 26.4.20. 02:32, Alan Stern wrote:
>>>>>>> On Sat, 25 Apr 2020 vladimir.stankovic@displaylink.com wrote:
>>>>>>>
>>>>>>>> Protocol adaptation layer (PAL) implementation has been added to
>>>>>>>> introduce MA-USB structures and logic.
>>>>>>>>
>>>>>>>> Signed-off-by: Vladimir Stankovic <vladimir.stankovic@displaylink.=
com>
>>>>>>>
>>>>>>> ...
>>>>>>>
>>>>>>>> +=09/*
>>>>>>>> +=09 * Masking URB_SHORT_NOT_OK flag as SCSI driver is adding it w=
here it
>>>>>>>> +=09 * should not, so it is breaking the USB drive on the linux
>>>>>>>> +=09 */
>>>>>>>> +=09urb->transfer_flags &=3D ~URB_SHORT_NOT_OK;
>=20
> ...
>=20
>>> Okay.  What SCSI driver does the comment refer to?  Is it something=20
>>> internal to DisplayLink or is it part of the regular Linux kernel?
>>>
>>> Alan Stern
>>>
>> Hi,
>>
>> Comment was related to the SCSI driver that's part of regular Linux kern=
el -
>> once the remote USB flash drive gets enumerated by host, it would appear=
 as
>> directly attached to host and then handled by the kernel.
>>
>> With current implementation, following messages are being logged:
>>
>> scsi 3:0:0:0: Direct-Access     Kingston DataTraveler 3.0 PMAP PQ: 0 ANS=
I: 6
>> sd 3:0:0:0: Attached scsi generic sg2 type 0
>>
>> after which the flash drive is usable/accessible from host side.
>=20
> More context please.  Without the log messages preceding this one we=20
> can't tell whether the device is using the usb-storage driver or the=20
> uas driver.
>=20
> Also, what makes you think the driver is setting the SHORT_NOT_OK flag
> at the wrong time?  In fact, how can there be a wrong time? =20
> SHORT_NOT_OK is a valid flag to use with any control or bulk URB.
>=20
> Alan Stern
>=20
The comment is clearly wrong - as mentioned earlier, this fix was added in =
early
development phase and I guess that implementer was not clear on how the par=
ticular
flag was added. Investigation is ongoing around proper fix for this.

Anyhow, it is a usb-storage driver related to this - here is usb-related lo=
g snippet:

usb 3-1.1.2: new high-speed USB device number 5 using mausb_host_hcd_dev
usb 3-1.1.2: New USB device found, idVendor=3D0951, idProduct=3D1666
usb 3-1.1.2: New USB device strings: Mfr=3D1, Product=3D2, SerialNumber=3D3
usb 3-1.1.2: Product: DataTraveler 3.0
usb 3-1.1.2: Manufacturer: Kingston
usb 3-1.1.2: SerialNumber: 0C9D9210E304E311095E087A
usb-storage 3-1.1.2:1.0: USB Mass Storage device detected
scsi host3: usb-storage 3-1.1.2:1.0
scsi 3:0:0:0: Direct-Access     Kingston DataTraveler 3.0 PMAP PQ: 0 ANSI: =
6
Attached scsi generic sg2 type 0
[sdb] 30277632 512-byte logical blocks: (15.5 GB/14.4 GiB)

As can be seen, USB flash attached to remote device is properly enumerated =
via
MA USB. Without the fix, usb driver is not able to read USB descriptors, en=
ding
up in USB storage not being accessible.

--=20
Regards,
Vladimir.