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Subject: Re: Serial-Core: USB-Serial port current issues.
From: Paul Fulghum <paulkf@microgate.com>
To: Greg KH <gregkh@suse.de>
Cc: "Luiz Fernando N. Capitulino" <lcapitulino@mandriva.com.br>,
       Russell King <rmk+lkml@arm.linux.org.uk>,
       Pete Zaitcev <zaitcev@redhat.com>, alan@lxorguk.ukuu.org.uk,
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Date: Mon, 26 Jun 2006 19:49:09 -0500
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On Mon, 2006-06-26 at 15:26 -0700, Greg KH wrote:
> On Fri, Jun 23, 2006 at 02:28:42PM -0300, Luiz Fernando N. Capitulino wrote:
> > On Thu, 22 Jun 2006 09:29:40 +0100
> > Russell King <rmk+lkml@arm.linux.org.uk> wrote:
> > 
> > | 
> > | Consider this scenario with what you're proposing:
> > | 
> > | 	thread				irq
> > | 
> > | 	take mutex
> > | 	get_mctrl
> > | 					cts changes state
> > | 					take port lock
> > | 					mctrl state read
> > | 					tty->hw_stopped changed state
> > | 					release port lock
> > | 	releaes mutex
> > | 	take port lock
> > | 	update tty->hw_stopped
> > | 	release port lock
> > | 
> > | Now, tty->hw_stopped does not reflect the hardware state, which will be
> > | buggy and can cause a loss of transmission.
> > | 
> > | I'm not sure what to suggest on this one since for USB drivers you do
> > | need to be able to sleep in this method... but for UARTs you must not.

What about this ugly fragment?
(assuming get_mctrl not called from IRQ)

  take mutex
  take port lock
again:
  save local copy of icount
  release port lock
  get_mctrl
  take port lock
  if (icount changed)
    goto again
  update tty->hw_stopped
  release port lock
  release mutex

--
Paul