Re: Serial-Core: USB-Serial port current issues.

[Luiz Fernando N. Capitulino]

On Thu, 22 Jun 2006 09:29:40 +0100
Russell King <rmk+lkml@xxxxxxxxxxxxxxxx> wrote:

| On Wed, Jun 21, 2006 at 06:15:13PM -0300, Luiz Fernando N. Capitulino wrote:
|
| > | With get_mctrl(), the situation is slightly more complicated, because
| > | we need to atomically update tty->hw_stopped in some circumstances
| > | (that may also be modified from irq context.)  Therefore, to give
| > | the driver a consistent locking picture, the spinlock is _always_
| > | held.
| > 
| >  Is it too bad (wrong?) to only protect the tty->hw_stopped update
| > by the spinlock? Then the call to get_mctrl() could be protected by
| > a mutex, or is it messy?
| 
| 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.

 Neither do I. :((

 I thought we could move the 'tty->hw_stopped' update to a workqueue
but it has the same problem you explained above...

 Greg, any suggestions?

-- 
Luiz Fernando N. Capitulino
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