linux-kernel - Re: [PATCH] fbcon: use default if cursor blink interval is not valid

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Date:	Thu, 19 May 2016 17:25:36 -0500 (CDT)
From:	Scot Doyle <lkml14@...tdoyle.com>
To:	David Daney <ddaney@...iumnetworks.com>
cc:	Tomi Valkeinen <tomi.valkeinen@...com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	David Daney <david.daney@...ium.com>,
	Ming Lei <ming.lei@...onical.com>,
	Dann Frazier <dann.frazier@...onical.com>,
	Jeremy Kerr <jk@...abs.org>,
	Peter Hurley <peter@...leysoftware.com>,
	Pavel Machek <pavel@....cz>, Jonathan Liu <net147@...il.com>,
	Alistair Popple <alistair@...ple.id.au>,
	Jean-Philippe Brucker <jean-philippe.brucker@....com>,
	"Chintakuntla, Radha" <Radha.Chintakuntla@...iumnetworks.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>, David Airlie <airlied@...ux.ie>,
	ddaney.cavm@...il.com, Scot Doyle <lkml14@...tdoyle.com>,
	dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] fbcon: use default if cursor blink interval is not
 valid

On Thu, 19 May 2016, David Daney wrote:
> On 05/18/2016 09:21 PM, Scot Doyle wrote:
> > Two current [1] and three previous [2] systems locked during boot
> > because the cursor flash timer was set using an ops->cur_blink_jiffies
> > value of 0. Previous patches attempted to solve the problem by moving
> > variable initialization earlier in the setup sequence [2].
> > 
> > Use the normal cursor blink default interval of 200 ms if
> > ops->cur_blink_jiffies is not in the range specified in commit
> > bd63364caa8d. Since invalid values are not used, specific system
> > initialization timings should not cause lockups.
> > 
> 
> This patch just papers over the problem that you yourself introduced in commit
> bd63364caa8d ("vt: add cursor blink interval escape sequence").
> 
> As you know, I have a patch that fixes the problem at the source:
> https://lkml.org/lkml/2016/5/17/455
> 
> I don't like the idea of silently ignoring bad values passed in from other
> code (drivers/tty/vt/vt.c), and much less doing the check for bad values each
> time the timer expires rather than just once, where the bad value is first
> introduced.
> 
> I think it would be preferable to WARN() at the site the bad value is
> introduced, so that we can easily find the real source of the problem.
> Initialize cur_blink_jiffies to a sane default value, then if something
> attempts to set it to a value that would cause soft lockup, WARN and refuse to
> change it.

I agree this approach would be cleaner and am willing to give it a try
by submitting an alternative patch and ack'ing yours. Thanks for taking 
the time to critique my proposal.