From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcin Slusarz Subject: Re: [PATCH RESEND] vgacon: remember scrollback buffer on console switch Date: Sun, 26 Oct 2008 00:43:01 +0200 Message-ID: <20081025224247.GA25497@joi> References: <20081025195814.GD10932@joi> <20081025134615.36f7285f.akpm@linux-foundation.org> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <20081025134615.36f7285f.akpm@linux-foundation.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Andrew Morton Cc: LKML , Antonino Daplas , Krzysztof Helt , linux-fbdev-devel@lists.sourceforge.net On Sat, Oct 25, 2008 at 01:46:15PM -0700, Andrew Morton wrote: > On Sat, 25 Oct 2008 21:58:19 +0200 Marcin Slusarz wrote: > > > Add support for persistent console history, surviving > > console switches. It allocates new scrollback buffer only when > > user switches console for the first time. > > > > Signed-off-by: Marcin Slusarz > > Cc: Antonino Daplas > > Cc: Krzysztof Helt > > Cc: Andrew Morton > > Cc: linux-fbdev-devel@lists.sourceforge.net > > --- > > drivers/video/console/Kconfig | 11 ++++++ > > drivers/video/console/vgacon.c | 75 +++++++++++++++++++++++++++++++++++++-- > > 2 files changed, 82 insertions(+), 4 deletions(-) > > > > diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig > > index 2f50a80..09e3d98 100644 > > --- a/drivers/video/console/Kconfig > > +++ b/drivers/video/console/Kconfig > > @@ -43,6 +43,17 @@ config VGACON_SOFT_SCROLLBACK_SIZE > > buffer. Each 64KB will give you approximately 16 80x25 > > screenfuls of scrollback buffer > > > > +config VGACON_REMEMBER_SCROLLBACK > > + bool "Remember scrollback buffer on console switch" > > + depends on VGACON_SOFT_SCROLLBACK > > + default y > > + help > > + Say 'Y' here if you want the scrollback buffer to be remembered > > + on console switch and restored when you switch back. > > + > > + Note: every VGA console will use its own buffer, but it will be > > + allocated only when you switch to this console for the first time. > > I'd question the value in adding the config option. Why not make the > feature unconditionally present? > > > > > +#define SCROLLBACK_SIZE (CONFIG_VGACON_SOFT_SCROLLBACK_SIZE * 1024) > > + > > ... > > > > +#ifdef CONFIG_VGACON_REMEMBER_SCROLLBACK > > +static struct vgacon_scrollback_info { > > + void *data; > > + int cnt; > > + int tail; > > + int cur; > > + int rows; > > + int size; > > +} vgacon_scrollbacks[MAX_NR_CONSOLES]; > > Perhaps you were concerned about memory consumption? Yes. I could imagine scenario where this memory would be considered "wasted". > If so, it would be much much better to make this feature switchable at > runtime (module parameter/boot option or a /proc or /sys knob). /sys knob seems to be the most flexible option. /sys/class/vtconsole/vtconX/persistent_history? 0/1 > > +static int vgacon_last_vc_num; > > We have lots of global state here with no apparent locking protecting > it. Possibly there's some higher-level lock which provides > seralisation? If so, the addition of a comment explaining all > this would be good. I checked it and this code is called under console_sem. vgacon_switch_scrollback <- vgacon_switch <- con_switch <- redraw_screen <- switch_screen <- complete_change_console <- 1) vt_ioctl (calls acquire_console_sem before complete_change_console) 2) change_console <- console_callback (calls acquire_console_sem before change_console) Thanks for a review! PS: why DECLARE_MUTEX _defines_ _semaphore_? there are only 8 uses of this macro so it's not a big problem to rename it to e.g. DEFINE_SEMAPHORE (I can provide a patch) Marcin