linux-kernel - Re: [PATCH v2 43/44] tty/metag

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Date:	Mon, 7 Jan 2013 11:30:09 +0000
From:	James Hogan <james.hogan@...tec.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	<linux-arch@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	"Arnd Bergmann" <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2 43/44] tty/metag_da: Add metag DA TTY driver

Hi Alan,

On 04/01/13 17:00, Alan Cox wrote:
>> Agreed and done. I didn't realise the TTY layer was so prone to
>> providing lots of itty bitty fragments (even with a single 4k write from
>> userland). The thing that really hurt was not the allocations but that
>> each one was written out to the DA separately each incurring it's own
>> large latency. Using a write buffer makes it a lot faster :)
> 
> Think about typing a command and the echo - you get a lot of very small
> writes !

Yes. The case I tried was a large write (strace cat /etc/somefile >
/dev/ttyDA2) and strace showed a single large write, but it got to the
driver in chunks of a dozen or so characters. Is that expected to happen?

>>>> +	channel_driver->init_termios = tty_std_termios;
>>>> +	channel_driver->init_termios.c_cflag =
>>>> +	    B38400 | CS8 | CREAD | HUPCL | CLOCAL;
>>>> +	channel_driver->flags = TTY_DRIVER_REAL_RAW;
>>>
>>> Need to set the speed flags
>>
>> Do you mean c_ispeed, and c_ospeed? These are set in tty_std_termios,
>> and they aren't meaningful in the context of this driver anyway, so are
>> they still necessary?
> 
> You update c_cflag to B38400 so you should set c_ispeed/c_ospeed
> accordingly. For a virtual interface it really only exists so that
> applications have an answer. You want a higher speed like 38400 so that
> apps don't try and do clever stuff for low speed links, that is all
> really.

Okay, assuming no objections I'll do the following to avoid specifying
an arbitrary speed in the first place (since tty_std_termios sets
c_cflag and c_{i,o}speed to 38400 anyway):

@@ -613,8 +613,7 @@ static int __init dashtty_init(void)
 	channel_driver->type = TTY_DRIVER_TYPE_SERIAL;
 	channel_driver->subtype = SERIAL_TYPE_NORMAL;
 	channel_driver->init_termios = tty_std_termios;
-	channel_driver->init_termios.c_cflag =
-	    B38400 | CS8 | CREAD | HUPCL | CLOCAL;
+	channel_driver->init_termios.c_cflag |= CLOCAL;
 	channel_driver->flags = TTY_DRIVER_REAL_RAW;

 	tty_set_operations(channel_driver, &dashtty_ops);

Thanks
James

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