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

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Date:	Thu, 19 May 2016 08:59:12 -0700
From:	David Daney <ddaney@...iumnetworks.com>
To:	Scot Doyle <lkml14@...tdoyle.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>,
	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>, <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 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.

Also there is a stylistic issue...


> [1] https://bugs.launchpad.net/bugs/1574814
> [2] see commits: 2a17d7e80f1d, f235f664a8af, a1e533ec07d5
>
> Signed-off-by: Scot Doyle <lkml14@...tdoyle.com>
> Cc: <stable@...r.kernel.org> [v4.2]
> ---
>   drivers/video/console/fbcon.c | 17 +++++++++++++----
>   1 file changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/video/console/fbcon.c b/drivers/video/console/fbcon.c
> index 6e92917..da61d87 100644
> --- a/drivers/video/console/fbcon.c
> +++ b/drivers/video/console/fbcon.c
> @@ -396,13 +396,23 @@ static void fb_flashcursor(struct work_struct *work)
>   	console_unlock();
>   }
>
> +static int cursor_blink_jiffies(int candidate)
> +{
> +	if (candidate >= msecs_to_jiffies(50) &&
> +	    candidate <= msecs_to_jiffies(USHRT_MAX))
> +		return candidate;
> +	else
> +		return HZ / 5;

You use msecs_to_jiffies() is several places, then here open code the 
division.  Please use  msecs_to_jiffies(), that is it's intended job.


> +}
> +
>   static void cursor_timer_handler(unsigned long dev_addr)
>   {
>   	struct fb_info *info = (struct fb_info *) dev_addr;
>   	struct fbcon_ops *ops = info->fbcon_par;
>
>   	queue_work(system_power_efficient_wq, &info->queue);
> -	mod_timer(&ops->cursor_timer, jiffies + ops->cur_blink_jiffies);
> +	mod_timer(&ops->cursor_timer, jiffies +
> +	    cursor_blink_jiffies(ops->cur_blink_jiffies));
>   }
>
>   static void fbcon_add_cursor_timer(struct fb_info *info)
> @@ -417,7 +427,8 @@ static void fbcon_add_cursor_timer(struct fb_info *info)
>
>   		init_timer(&ops->cursor_timer);
>   		ops->cursor_timer.function = cursor_timer_handler;
> -		ops->cursor_timer.expires = jiffies + ops->cur_blink_jiffies;
> +		ops->cursor_timer.expires = jiffies +
> +		    cursor_blink_jiffies(ops->cur_blink_jiffies);
>   		ops->cursor_timer.data = (unsigned long ) info;
>   		add_timer(&ops->cursor_timer);
>   		ops->flags |= FBCON_FLAGS_CURSOR_TIMER;
> @@ -709,7 +720,6 @@ static int con2fb_acquire_newinfo(struct vc_data *vc, struct fb_info *info,
>   	}
>
>   	if (!err) {
> -		ops->cur_blink_jiffies = HZ / 5;
>   		info->fbcon_par = ops;
>
>   		if (vc)
> @@ -957,7 +967,6 @@ static const char *fbcon_startup(void)
>   	ops->currcon = -1;
>   	ops->graphics = 1;
>   	ops->cur_rotate = -1;
> -	ops->cur_blink_jiffies = HZ / 5;
>   	info->fbcon_par = ops;
>   	p->con_rotate = initial_rotation;
>   	set_blitting_type(vc, info);
>