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