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

[Greg KH <gregkh@suse.de>]

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:
> 
> | 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?

Nope, sorry, I don't know what to suggest :(

greg k-h
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