linux-kernel - Re: [PATCH] cdc-acm: some enhancement on acm delayed write

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Date:	Fri, 11 Apr 2014 06:51:26 +0800
From:	Xiao Jin <jin.xiao@...el.com>
To:	Oliver Neukum <oneukum@...e.de>
CC:	jhovold@...il.com, gnomes@...rguk.ukuu.org.uk,
	gregkh@...uxfoundation.org, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org, david.a.cohen@...ux.intel.com,
	yanmin.zhang@...el.com
Subject: Re: [PATCH] cdc-acm: some enhancement on acm delayed write

Hi, Oliver,

On 04/10/2014 04:02 PM, Oliver Neukum wrote:
> On Wed, 2014-04-09 at 22:57 +0800, Xiao Jin wrote:
>> Thanks all for the review. We meet with the problems when developing
>> product. I would like to explain my understanding.
>>
>> On 04/08/2014 11:05 AM, Xiao Jin wrote:
>>>
>>> We find two problems on acm tty write delayed mechanism.
>>> (1) When acm resume, the delayed wb will be started. But now
>>> only one write can be saved during acm suspend. More acm write
>>> may be abandoned.
>>
>> The scenario usually happened when user space write series AT after acm
>> suspend. If acm accept the first AT, what's the reason for acm to refuse
>> the second AT? If write return 0, user space will try repeatedly until
>> resume. It looks simpler that acm accept all the data and sent out urb
>> when resume.
>
> No. We cannot accept an arbitrary amount of data. It would let any
> user OOM the system. There will have to be an arbitrary limit.
> The simplest limit is 1 urb. And that is because we said that we
> are ready to accept data.
>

We apply cdc-acm for modem AT data. I can find other usb modem driver 
usb_wwan_write use list to accept more data when suspend, maybe usbnet 
is the same. Do you have any more reason for me to understand why 
cdc-acm accept only one?

>>> (2) acm tty port ASYNCB_INITIALIZED flag will be cleared when
>>> close. If acm resume callback run after ASYNCB_INITIALIZED flag
>>> cleared, there will have no chance for delayed write to start.
>>> That lead to acm_wb.use can't be cleared. If user space open
>>> acm tty again and try to setd, tty will be blocked in
>>> tty_wait_until_sent for ever.
>>>
>>
>> We see tty write and close concurrently after acm suspend in this case.
>> It looks no method to avoid it from tty layer. acm_tty_write and
>
> There is a delay user space can set.
>
>> acm_resume call after acm_port_shutdown. It looks any action in
>> acm_port_shutdown can't solve the problem. As acm has accepted the user
>> space data, we can only find a way to send out urb. I feel anyway to
>> discard the data looks like a lie to user space.
>>
>> In my understanding acm should accept data as much as possible, and send
>> out urb as soon as possible. What do you think of?
>
> There's certainly no problem with sending out the data. Yet
> simply resuming the device in shutdown() should do the job.
>

We see tty write and close concurrently, we have debug log to show that 
acm_tty_write and acm_resume is called after acm_port_shutdown, I don't 
understand "resuming the device in shutdown() should do the job".

> 	Regards
> 		Oliver
>
>

My understanding may be superficial, please correct me if anything 
wrong. Thanks.

Br, Jin
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