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 14:22:36 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Timur Tabi <timur@...escale.com>, amit.shah@...hat.com,
	linuxppc-dev@...abs.org, kumar.gala@...escale.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hvc_console: fix dropping of characters when output
 byte channel is full

On Tue, 14 Sep 2010 12:17:21 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> 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 is just for printk(), not user output.  This is exactly what
printk() has always done for real serial ports.

> > 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.

hvc_push() is not relevant to kernel console output.

hvc_console_write() currently does not reschedule anything.  It just
drops characters when busy.

-Scott

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