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

[Xiao Jin]

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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