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Date:   Sat, 25 May 2019 19:00:54 +0200
From:   Sam Ravnborg <sam@ravnborg.org>
To:     Daniel Vetter <daniel.vetter@ffwll.ch>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        =?utf-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= <mirq-linux@rere.qmqm.pl>,
        Hans de Goede <hdegoede@redhat.com>,
        Mikulas Patocka <mpatocka@redhat.com>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Peter Rosin <peda@axentia.se>
Subject: Re: [PATCH 25/33] fbmem: pull fbcon_fb_blanked out of fb_blank
Message-ID: <20190525170054.GA9076@ravnborg.org>
References: <20190524085354.27411-1-daniel.vetter@ffwll.ch>
 <20190524085354.27411-26-daniel.vetter@ffwll.ch>
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Hi Daniel.

On Fri, May 24, 2019 at 10:53:46AM +0200, Daniel Vetter wrote:
> There's a callchain of:
> 
> fbcon_fb_blaned -> do_(un)blank_screen -> consw->con_blank
           ^^^^^^
Spelling error - as this is a callchain it would be good to have it
fixed.

Patch itself looks fine.

	Sam

> 	-> fbcon_blank -> fb_blank
> 
> Things don't go horribly wrong because the BKL console_lock safes the
> day, but that's about it. And the seeming recursion is broken in 2
> ways:
> - Starting from the fbdev ioctl we set FBINFO_MISC_USEREVENT, which
>   tells the fbcon_blank code to not call fb_blank. This was required
>   to not deadlock when recursing on the fb_notifier_chain mutex.
> - Starting from the con_blank hook we're getting saved by the
>   console_blanked checks in do_blank/unblank_screen. Or at least
>   that's my theory.
> 
> Anyway, recursion isn't awesome, so let's stop it. Breaking the
> recursion avoids the need to be in the FBINFO_MISC_USEREVENT critical
> section, so lets move it out of that too.
> 
> The astute reader will notice that fb_blank seems to require
> lock_fb_info(), which the fbcon code seems to ignore. I have no idea
> how to fix that problem, so let's keep ignoring it.
> 
> v2: I forgot the sysfs blanking code.
> 
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
> Cc: "Michał Mirosław" <mirq-linux@rere.qmqm.pl>
> Cc: Peter Rosin <peda@axentia.se>
> Cc: Hans de Goede <hdegoede@redhat.com>
> Cc: Mikulas Patocka <mpatocka@redhat.com>
> Cc: Rob Clark <robdclark@gmail.com>
> ---
>  drivers/video/fbdev/core/fbmem.c   | 4 +++-
>  drivers/video/fbdev/core/fbsysfs.c | 8 ++++++--
>  2 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 9366fbe99a58..d6713dce9e31 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1068,7 +1068,6 @@ fb_blank(struct fb_info *info, int blank)
>  	event.data = &blank;
>  
>  	early_ret = fb_notifier_call_chain(FB_EARLY_EVENT_BLANK, &event);
> -	fbcon_fb_blanked(info, blank);
>  
>  	if (info->fbops->fb_blank)
>   		ret = info->fbops->fb_blank(blank, info);
> @@ -1198,6 +1197,9 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
>  		info->flags |= FBINFO_MISC_USEREVENT;
>  		ret = fb_blank(info, arg);
>  		info->flags &= ~FBINFO_MISC_USEREVENT;
> +
> +		/* might again call into fb_blank */
> +		fbcon_fb_blanked(info, arg);
>  		unlock_fb_info(info);
>  		console_unlock();
>  		break;
> diff --git a/drivers/video/fbdev/core/fbsysfs.c b/drivers/video/fbdev/core/fbsysfs.c
> index 5f329278e55f..252d4f52d2a5 100644
> --- a/drivers/video/fbdev/core/fbsysfs.c
> +++ b/drivers/video/fbdev/core/fbsysfs.c
> @@ -18,6 +18,7 @@
>  #include <linux/kernel.h>
>  #include <linux/slab.h>
>  #include <linux/fb.h>
> +#include <linux/fbcon.h>
>  #include <linux/console.h>
>  #include <linux/module.h>
>  
> @@ -305,12 +306,15 @@ static ssize_t store_blank(struct device *device,
>  {
>  	struct fb_info *fb_info = dev_get_drvdata(device);
>  	char *last = NULL;
> -	int err;
> +	int err, arg;
>  
> +	arg = simple_strtoul(buf, &last, 0);
>  	console_lock();
>  	fb_info->flags |= FBINFO_MISC_USEREVENT;
> -	err = fb_blank(fb_info, simple_strtoul(buf, &last, 0));
> +	err = fb_blank(fb_info, arg);
>  	fb_info->flags &= ~FBINFO_MISC_USEREVENT;
> +	/* might again call into fb_blank */
> +	fbcon_fb_blanked(fb_info, arg);
>  	console_unlock();
>  	if (err < 0)
>  		return err;
> -- 
> 2.20.1
> 
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