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

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Message-Id: <1255668405.19032.13.camel@pasglop>
Date:	Fri, 16 Oct 2009 15:46:45 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Scott Wood <scottwood@...escale.com>
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 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. 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.

> > If we just busy loop, it actually does not matter how we let hvc_console react 
> > on 0, as long as we adopt all backends to use that interface consistent.
> > 
> > On the other hand, backends might want to do special magic on congestion so I 
> > personally tend to let the backend loop instead of hvc_console. But I am really  
> > not sure.
> 
> Doing it in the backend requires the backend to know whether it's being 
> called for printk or for user I/O.  In the latter case, we don't want to 
> spin, but rather wait for an IRQ (or poll with a timer if there's no IRQ). 

Ben.


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