On Sun, Jul 01, 2012 at 07:43:17AM +0000, tilman wrote:
> Hello
>
> many of the usb serial drivers are using the code fragement below to fill
> received data into the read queue of the attached tty.
>
> read_callback:
> ...
> tty = tty_port_tty_get(&port->port);
> if (tty != NULL && urb->actual_length > 0) {
> tty_insert_flip_string(tty, data, urb->actual_length);
> tty_flip_buffer_push(tty);
> }
> tty_kref_put(tty);
> ...
>
> Typically, tty->low_latency is set to 1 in the open function.
This is simply not a correct statement -- no usb serial driver should be
setting low_latency in open and only the fairly recently added metro_usb
driver currently does. This should be fixed.
> Now, I read in the man page of tty_flip_buffer_push that it is not to be called
> from an interrupt context if tty->low_latency=1. I understand however that the
> callback handler runs in an interrupt context.
Correct. That is why low_latency should generally not be set.
> In a test driver I wrote, tty_flip_buffer_push(tty) makes the kernal crash if
> tty->low_latency=1. If I remove this line, the kernal does not crash. And I have
> a second driver that does not crash if tty->low_latency=1. It uses
> tty_flip_buffer_push in the read_callback handler.
>
> Can somebody please shed some light under what circumstance tty_flip_buffer_push
> can be used from a callback handler ?
With changes made to the tty layer a fair while back there should no
longer be any need to use the low_latency flag as the tty layer provides
plenty of buffering these days.
Simply leave it unset and call tty_flip_buffer_push in your completion
handler.
Johan
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