linux-kernel - Re: How important is it that tty_write

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Message-ID: <20110223225728.GR2924@thunk.org>
Date:	Wed, 23 Feb 2011 17:57:28 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Timur Tabi <timur@...escale.com>
Cc:	Greg KH <gregkh@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: How important is it that tty_write_room doesn't lie?

On Wed, Feb 23, 2011 at 02:48:18PM -0600, Timur Tabi wrote:
> Greg KH wrote:
> > I think that people don't normally hit this as the console code isn't
> > used as a tty and a console at the same time, right?
> 
> That's another thing I never understood.  It's rare for a driver to
> support both the console and tty layers.  The serial core driver
> does that, but I can't find any other examples.  I would think that
> a driver would support both interfaces, because both are needed.
> Simplistically, printk --> console, and printf --> tty.  When would
> ever want user-space support but not kernel support?

When you are supporting a large bank of modems?  A number of the
serial cards were originally created to support 32, 64, 128 serial
ports per PCI board, specifically to drive modems, in the bad old days
of modems.  :-)

And I suspect many modern serial devices are just 8250/16550A based,
so they get the console and userspace serial for free.  A large number
of the more exotic boards were created because the 16550A uart isn't
ideal if you are driving vast number of modems at the same time.

> The FIFO can vary, but it's probably at least 2KB it size.  At
> least, we hope to able to set it to that size in the field.
> Currently, we set it to 4KB.

Wow, the FIFO has gotten a lot larger than I ever remember them
getting even when people were doing 460kbps.  I'm guessing this is
because you're trying to defer interrupts for power saving reasons,
yes?  I was used to seeing FIFO sizes more in the 32-128 bytes, tops.  :-)

      	    	    	   	      - Ted
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