linux-kernel - Re: [PATCH] hvc_console: fix dropping of characters when output byte channel is full

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Date:	Tue, 14 Sep 2010 12:17:21 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Timur Tabi <timur@...escale.com>
Cc:	linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org,
	benh@...nel.crashing.org, kumar.gala@...escale.com,
	amit.shah@...hat.com
Subject: Re: [PATCH] hvc_console: fix dropping of characters when output
 byte channel is full

On Fri, 20 Aug 2010 13:45:21 -0500
Timur Tabi <timur@...escale.com> wrote:

> hvc_console_print() calls the HVC client driver's put_chars() callback
> to write some characters to the console.  If the callback returns 0, that
> indicates that no characters were written (perhaps the output buffer is
> full), but hvc_console_print() treats that as an error and discards the
> rest of the buffer.
> 
> So change hvc_console_print() to just loop and call put_chars() again if it
> returns a 0 return code.

Seems rather dangerous.  The upper layer will sit there chewing 100%
CPU for as long as the lower layer is congested.

> This change makes hvc_console_print() behave more like hvc_push(), which
> does check for a 0 return code and re-schedules itself.

Yes, hvc_push() reschedules.  It doesn't sit in a tight loop burning
electrons!

Can we do something safer&smarter here?
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