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

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Date:	Sat, 3 Oct 2009 16:05:22 +0200
From:	Johan Hovold <jhovold@...il.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Johan Hovold <jhovold@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Michael Trimarchi <trimarchi@...dalf.sssup.it>,
	Oliver Neukum <oliver@...kum.org>, linux-usb@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: ftdi_sio: Remove tty->low_latency.

On Sat, Oct 03, 2009 at 02:18:07PM +0100, Alan Cox wrote:
> On Sat, 3 Oct 2009 13:42:29 +0200
> Johan Hovold <jhovold@...il.com> wrote:
> > On Fri, Oct 02, 2009 at 05:33:08PM +0100, Alan Cox wrote:
> > And, obviously, this doesn't solve the problem of tty_flip_buffer_push
> > being called from interrupt context (but I assume that was never the
> > intention).
> 
> Calling tty_flip_buffer_push from an interrupt is perfectly acceptable
> providing tty->low_latency isn't set: which it isn't.

Of course -- the "with low latency set" part fell out (or, was implicit
;-) ). Your patch, however, still has low_latency set when it calls
tty_flip_buffer_push and that's the problem.

> > -	spin_lock_irqsave(&priv->rx_lock, flags);
> > -	priv->rx_flags &= ~(THROTTLED | ACTUALLY_THROTTLED);
> > -	spin_unlock_irqrestore(&priv->rx_lock, flags);
> > +	spin_lock_irqsave(&port->lock, flags);
> > +	port->throttled = 0;
> > +	spin_unlock_irqrestore(&port->lock, flags);
> 
> If you only have a single bit use the set_bit/clear_bit/test_and_xxx_bit
> stuff as it's faster on most boxes

The generic driver uses two fields in usb_serial_port for
throttled/trottle_req, whereas ftdi_sio, whiteheat, aircable, cypress
and perhaps a couple more have private a flag field for THROTTLED and
ACTUALLY_THROTTLED.

How about unifying them to all use a single flag field (with two flags)
in usb_serial_port?

> > +	 * The per character mucking around with sysrq path it too slow, so
> > +	 * shortcircuit it in the 99.9999999% of cases where the USB serial is
> > +	 * not a console anyway.
> > +	 */
> > +	ch = packet + 2;
> > +	len -= 2;
> > +	if (!port->console || !port->sysrq)
> 
> You need && flag == TTY_NORMAL ?

You tell me. :-) Are we interested in them unless port->console is set?

> Definitely a move in the right direction

Thanks,
Johan

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