linux-kernel - Re: [PATCH] USB: ftdi_sio: Remove tty->low

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Date:	Tue, 29 Sep 2009 16:55:14 +0200
From:	Johan Hovold <jhovold@...il.com>
To:	Michael Trimarchi <trimarchi@...dalf.sssup.it>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Johan Hovold <jhovold@...il.com>,
	Oliver Neukum <oliver@...kum.org>,
	Greg Kroah-Hartman <gregkh@...e.de>, linux-usb@...r.kernel.org,
	Alan Cox <alan@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: ftdi_sio: Remove tty->low_latency.

On Fri, Sep 25, 2009 at 07:46:51PM +0200, Michael Trimarchi wrote:
> Johan Hovold wrote:
> > On Thu, Sep 24, 2009 at 08:21:07PM +0100, Alan Cox wrote:
> >> ftdi_sio is correct with low_latency set as it uses a work queue to
> >> process the packets received.
> >>     
> > AFAICT it only uses the work queue if tty_buffer_request_room fails to
> > allocate enough space. This being the exception, the completion
> > handler normally processes the packets in interrupt context and this is
> > where I get my lockdep traces (and it happens every time I hit the echo
> > or throttle paths).
> >   
> Using two urb, double buffering and schedule a tasklet to complete the
> the reading phase. The usb will use the other free urb during receiving 
> process.
> I think remove tty_latency is not a good fix.

What do you say, Alan? Should ftdi_sio be rewritten so that it actually
defers all processing, or should low_latency go?

As it stands today ftdi_sio does indeed call tty_flip_buffer_push from
interrupt context with low_latency set and that is obviously incorrect,
right?

Regards,
Johan

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