linux-kernel - Re: [External] Re: [PATCH v5 5/8] usb: mausb

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Date:   Sun, 26 Apr 2020 16:45:29 +0200
From:   Vladimir Stankovic <vladimir.stankovic@...playlink.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        linux-usb@...r.kernel.org, mausb-host-devel@...playlink.com
Subject: Re: [External] Re: [PATCH v5 5/8] usb: mausb_host: Introduce PAL
 processing

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@...playlink.com wrote:
>>>
>>>> Protocol adaptation layer (PAL) implementation has been added to
>>>> introduce MA-USB structures and logic.
>>>>
>>>> Signed-off-by: Vladimir Stankovic <vladimir.stankovic@...playlink.com>
>>>
>>> ...
>>>
>>>> +	/*
>>>> +	 * Masking URB_SHORT_NOT_OK flag as SCSI driver is adding it where it
>>>> +	 * should not, so it is breaking the USB drive on the linux
>>>> +	 */
>>>> +	urb->transfer_flags &= ~URB_SHORT_NOT_OK;
>>>
>>> Removing the SHORT_NOT_OK flag is _not_ a valid thing to do.  It will 
>>> cause drivers to malfunction.
>>>
>>> Can you please explain this comment?
>>>
>>> 	What SCSI driver?
>>>
>>> 	When is the flag being added?
>>>
>>> 	How does it break USB drives?
>>>
>>> 	Why haven't you already reported this problem to the 
>>> 	appropriate maintainers?
>>>
>>> Alan Stern
>>>
>>
>> Hi,
>>
>> Issue that removal of SHORT_NOT_OK flag addressed is linked to particular
>> set of Kingston USB 3.0 flash drives (super speed) - other USB flash drives
>> haven't had this flag set. Without this "fix", those Kingston flash drives
>> are not being enumerated properly.
> 
> Please explain in detail how the enumeration of these Kingston flash
> drives fails.  Or if such an explanation has already been posted,
> please provide a link to it.

Will reproduce the issue once again (w/o the fix) and run through the events.
Issue has been noticed during early development, and addressed right away.
> 
>> This particular line was added in the early stage of development, during
>> enumeration process implementation. The reason why it remained in the code
>> since is because we haven't noticed any side-effects, even with various
>> USB devices being attached to remote MA-USB device, including flash drives,
>> cameras, wireless mice, etc.
> 
> Come to think of it, the SHORT_NOT_OK flag is mainly used with HCDs
> that don't have scatter-gather support.  Since your mausb driver does
> support scatter-gather, you most likely won't encounter any problems 
> unless you go looking for them specifically.
> 
>> The problem has been reported, and is actively being investigated.
> 
> Where was the problem reported (URL to a mailing list archive)?  Who is
> investigating it?

Ticket has been submitted to DisplayLink's internal issue-tracking system
and is being investigated by mausb-host-devel team.
> 
>> As soon as it gets addressed properly (w/o global negation of the flag),
>> a new patch will be pushed.
> 
> Thank you.
> 
> Alan Stern
> 


-- 
Regards,
Vladimir.