linux-kernel - Re: [PATCH] hvc_console: returning 0 from put

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Message-ID: <20091016153330.GB11838@loki.buserror.net>
Date:	Fri, 16 Oct 2009 10:33:30 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	linuxppc-dev@...abs.org, brueckner@...ux.vnet.ibm.com,
	Timur Tabi <timur@...escale.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] hvc_console: returning 0 from put_chars is not an error

On Fri, Oct 16, 2009 at 03:46:45PM +1100, Benjamin Herrenschmidt wrote:
> On Thu, 2009-10-15 at 13:57 -0500, Scott Wood wrote:
> > I'd say the dropping approach is quite undesirable (significant 
> > potential for output loss unless the buffer is huge), unless there's 
> > simply no way to safely spin.  Hopefully there are no such backends, but 
> > if there are perhaps we can have them return some special code to 
> > indicate that.
> 
> Should never spin.

Why is a hypervisor console different than a serial port in this regard?

> Best is to keep a copy in the upper layer of the pending data and throttle
> (not accept further data from tty layer) until we have managed to flush
> out that "pending" buffer.

The data isn't coming from the tty layer -- we're talking about printk.  How
do you throttle that without spinning?

I agree that it shouldn't spin when handling tty I/O.

-Scott
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