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

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Message-ID: <1396964322.25633.21.camel@linux-fkkt.site>
Date:	Tue, 08 Apr 2014 15:38:42 +0200
From:	Oliver Neukum <oneukum@...e.de>
To:	Johan Hovold <jhovold@...il.com>
Cc:	Xiao Jin <jin.xiao@...el.com>, gregkh@...uxfoundation.org,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	david.a.cohen@...ux.intel.com, yanmin.zhang@...el.com,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] cdc-acm: some enhancement on acm delayed write

On Tue, 2014-04-08 at 15:17 +0200, Johan Hovold wrote:
> [ +CC: Alan ]
> 
> On Tue, Apr 08, 2014 at 12:33:31PM +0200, Oliver Neukum wrote:
> > On Tue, 2014-04-08 at 09:33 +0200, Johan Hovold wrote:
> > > On Tue, Apr 08, 2014 at 11:05:20AM +0800, Xiao Jin wrote:
> > 
> > > > (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.
> > > >
> > > > This patch have two modification.
> > > > (1) use list_head to save the write acm_wb during acm suspend.
> > > > It can ensure no acm write abandoned.
> > > > (2) enable flush buffer callback when acm tty close. acm delayed
> > > > wb will start before acm port shutdown callback, it make sure
> > > > the delayed acm_wb.use to be cleared. The patch also clear
> > > > acm_wb.use and acm.transmitting when port shutdown.
> > > 
> > > This is not the right way to do this. See below.
> > 
> > If I see this correctly, then ASYNCB_INITIALIZED is cleared in
> > tty_port_close() we is called from acm_tty_close(). Thus it should
> > be enough to make sure that the device is resumed at the beginning
> > of acm_tty_close() and acm_resume() will do the job automatically.
> > What do you think?
> 
> But the device should already be about to be resumed, right? If the port

Yes.

> is closed fast enough that resume hasn't had time to run before
> shutdown is called I think the right thing to do is simply to discard
> the delayed bytes (in shutdown).

I think if we said we have transmitted then we should do so.

	Regards
		Oliver


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