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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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