Date:   Tue, 10 Jan 2017 18:31:57 +0100
From:   Manuel Schölling <manuel.schoelling@....de>
To:     Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Cc:     gregkh@...uxfoundation.org, jslaby@...e.com, kilobyte@...band.pl,
        lkml14@...tdoyle.com, rdunlap@...radead.org, shorne@...il.com,
        andrey_utkin@...tmail.com, akpm@...ux-foundation.org,
        paul.burton@...tec.com, daniel.vetter@...ll.ch, tj@...nel.org,
        hdegoede@...hat.com, linux-kernel@...r.kernel.org,
        linux-fbdev@...r.kernel.org
Subject: Re: [PATCH v8 3/3] console: Add persistent scrollback buffers for
 all VGA consoles

Hi Bartlomiej,

First of all thank you for you feedback!

On Tue, 2017-01-10 at 17:44 +0100, Bartlomiej Zolnierkiewicz wrote:
> On Tuesday, January 10, 2017 05:22:22 PM Bartlomiej Zolnierkiewicz
> wrote:
> > 
> > Hi,
> > 
> > The patchset generally looks fine to me but I have a question
> > regarding new VGACON_SOFT_SCROLLBACK_PERSISTENT config option.
> > 
> > Since the code size impact to support the persistent scrollback
> > feature is minimal wouldn't it be better to always include it?
> > 
> > The feature would be disabled by default and could be enabled
> > by using the new kernel command line parameter (you may also add
> > a new config option for enabling it by default if desired).
> 
> Something like:
> 
> #ifdef VGACON_SOFT_SCROLLBACK_PERSISTENT_ENABLE_BY_DEFAULT
> static bool scrollback_persistent = 1;
> #else
> static bool scrollback_persistent;
> #endif
> 
> module_param_named(scrollback_persistent, scrollback_persistent,
> bool, 0);
> MODULE_PARM_DESC(scrollback_persistent, "Enable persistent scrollback
> feature");
> 
> and then use scrollback_persistent variable in
> vgacon_scrollback_switch()
> to control the actual behavior instead of ifdefs.
Sounds pretty good to me. You are right: The code size impact is rather
small. I will apply your suggestions and send you another patch as soon
as I find the time to implement it (should be any time this week).

Thanks again!

Manuel



> Best regards,
> --
> Bartlomiej Zolnierkiewicz
> Samsung R&D Institute Poland
> Samsung Electronics
> 
> > Best regards,
> > --
> > Bartlomiej Zolnierkiewicz
> > Samsung R&D Institute Poland
> > Samsung Electronics
> > 
> > On Thursday, January 05, 2017 12:33:20 PM Manuel Schölling wrote:
> > > Add a scrollback buffers for each VGA console. The benefit is
> > > that
> > > the scrollback history is not flushed when switching between
> > > consoles
> > > but is persistent.
> > > The buffers are allocated on demand when a new console is opened.
> > > 
> > > This breaks tools like clear_console that rely on flushing the
> > > scrollback history by switching back and forth between consoles
> > > which is why this feature is disabled by default.
> > > Use the escape sequence \e[3J instead for flushing the buffer.
> > > 
> > > Signed-off-by: Manuel Schölling <manuel.schoelling@....de>
> > > Reviewed-by: Andrey Utkin <andrey_utkin@...tmail.com>
> > > Tested-by: Andrey Utkin <andrey_utkin@...tmail.com>
> > > Tested-by: Adam Borowski <kilobyte@...band.pl>
> > > ---
> > >  drivers/video/console/Kconfig  |  25 +++++++-
> > >  drivers/video/console/vgacon.c | 142 ++++++++++++++++++++++++++-
> > > --------------
> > >  2 files changed, 111 insertions(+), 56 deletions(-)
> > > 
> > > diff --git a/drivers/video/console/Kconfig
> > > b/drivers/video/console/Kconfig
> > > index c3f1fb9ee820..f500e58f7636 100644
> > > --- a/drivers/video/console/Kconfig
> > > +++ b/drivers/video/console/Kconfig
> > > @@ -43,9 +43,28 @@ config VGACON_SOFT_SCROLLBACK_SIZE
> > >         range 1 1024
> > >         default "64"
> > >         help
> > > -         Enter the amount of System RAM to allocate for the
> > > scrollback
> > > -	 buffer.  Each 64KB will give you approximately 16 80x25
> > > -	 screenfuls of scrollback buffer
> > > +	  Enter the amount of System RAM to allocate for
> > > scrollback
> > > +	  buffers of VGA consoles. Each 64KB will give you
> > > approximately
> > > +	  16 80x25 screenfuls of scrollback buffer.
> > > +
> > > +config VGACON_SOFT_SCROLLBACK_PERSISTENT
> > > +	bool "Persistent Scrollback History for each console"
> > > +	depends on VGACON_SOFT_SCROLLBACK
> > > +	default n
> > > +	help
> > > +	  Say Y here if the scrollback history should persist
> > > when switching
> > > +	  between consoles. Otherwise, the scrollback history
> > > will be flushed
> > > +	  each time the console is switched.
> > > +
> > > +	  This feature might break your tool of choice to flush
> > > the scrollback
> > > +	  buffer, e.g. clear(1) will work fine but Debian's
> > > clear_console(1)
> > > +	  will be broken, which might cause security issues.
> > > +	  You can use the escape sequence \e[3J instead if this
> > > feature is
> > > +	  activated.
> > > +
> > > +	  Note that a buffer of VGACON_SOFT_SCROLLBACK_SIZE is
> > > taken for each
> > > +	  created tty device.
> > > +	  So if you use a RAM-constrained system, say N here.
> > >  
> > >  config MDA_CONSOLE
> > >  	depends on !M68K && !PARISC && ISA
> > > diff --git a/drivers/video/console/vgacon.c
> > > b/drivers/video/console/vgacon.c
> > > index 9a7c2bbc5326..ca23d222e029 100644
> > > --- a/drivers/video/console/vgacon.c
> > > +++ b/drivers/video/console/vgacon.c
> > > @@ -162,7 +162,7 @@ static inline void vga_set_mem_top(struct
> > > vc_data *c)
> > >  
> > >  #ifdef CONFIG_VGACON_SOFT_SCROLLBACK
> > >  /* software scrollback */
> > > -static struct vgacon_scrollback_info {
> > > +struct vgacon_scrollback_info {
> > >  	void *data;
> > >  	int tail;
> > >  	int size;
> > > @@ -171,74 +171,110 @@ static struct vgacon_scrollback_info {
> > >  	int cur;
> > >  	int save;
> > >  	int restore;
> > > -} vgacon_scrollback;
> > > +};
> > > +
> > > +static struct vgacon_scrollback_info *vgacon_scrollback_cur;
> > > +#ifdef CONFIG_VGACON_SOFT_SCROLLBACK_PERSISTENT
> > > +static struct vgacon_scrollback_info
> > > vgacon_scrollbacks[MAX_NR_CONSOLES];
> > > +#else
> > > +static struct vgacon_scrollback_info vgacon_scrollbacks[1];
> > > +#endif
> > >  
> > > -static void vgacon_scrollback_reset(size_t reset_size)
> > > +static void vgacon_scrollback_reset(int vc_num, size_t
> > > reset_size)
> > >  {
> > > -	if (vgacon_scrollback.data && reset_size > 0)
> > > -		memset(vgacon_scrollback.data, 0, reset_size);
> > > +	struct vgacon_scrollback_info *scrollback =
> > > &vgacon_scrollbacks[vc_num];
> > > +
> > > +	if (scrollback->data && reset_size > 0)
> > > +		memset(scrollback->data, 0, reset_size);
> > >  
> > > -	vgacon_scrollback.cnt  = 0;
> > > -	vgacon_scrollback.tail = 0;
> > > -	vgacon_scrollback.cur  = 0;
> > > +	scrollback->cnt  = 0;
> > > +	scrollback->tail = 0;
> > > +	scrollback->cur  = 0;
> > >  }
> > >  
> > > -static void vgacon_scrollback_init(int pitch)
> > > +static void vgacon_scrollback_init(int vc_num)
> > >  {
> > > -	int rows = CONFIG_VGACON_SOFT_SCROLLBACK_SIZE *
> > > 1024/pitch;
> > > -
> > > -	if (vgacon_scrollback.data) {
> > > -		vgacon_scrollback.cnt  = 0;
> > > -		vgacon_scrollback.tail = 0;
> > > -		vgacon_scrollback.cur  = 0;
> > > -		vgacon_scrollback.rows = rows - 1;
> > > -		vgacon_scrollback.size = rows * pitch;
> > > +	int pitch = vga_video_num_columns * 2;
> > > +	size_t size = CONFIG_VGACON_SOFT_SCROLLBACK_SIZE * 1024;
> > > +	int rows = size / pitch;
> > > +	void *data;
> > > +
> > > +	data = kmalloc_array(CONFIG_VGACON_SOFT_SCROLLBACK_SIZE,
> > > 1024,
> > > +			     GFP_NOWAIT);
> > > +
> > > +	vgacon_scrollbacks[vc_num].data = data;
> > > +	vgacon_scrollback_cur = &vgacon_scrollbacks[vc_num];
> > > +
> > > +	vgacon_scrollback_cur->rows = rows - 1;
> > > +	vgacon_scrollback_cur->size = rows * pitch;
> > > +
> > > +	vgacon_scrollback_reset(vc_num, size);
> > > +}
> > > +
> > > +static void vgacon_scrollback_switch(int vc_num)
> > > +{
> > > +#ifndef CONFIG_VGACON_SOFT_SCROLLBACK_PERSISTENT
> > > +	vc_num = 0;
> > > +#endif
> > > +
> > > +	if (!vgacon_scrollbacks[vc_num].data) {
> > > +		vgacon_scrollback_init(vc_num);
> > > +	} else {
> > > +#ifdef CONFIG_VGACON_SOFT_SCROLLBACK_PERSISTENT
> > > +		vgacon_scrollback_cur =
> > > &vgacon_scrollbacks[vc_num];
> > > +#else
> > > +		size_t size = CONFIG_VGACON_SOFT_SCROLLBACK_SIZE
> > > * 1024;
> > > +
> > > +		vgacon_scrollback_reset(vc_num, size);
> > > +#endif
> > >  	}
> > >  }
> > >  
> > >  static void vgacon_scrollback_startup(void)
> > >  {
> > > -	vgacon_scrollback.data =
> > > kcalloc(CONFIG_VGACON_SOFT_SCROLLBACK_SIZE,
> > > -		1024, GFP_NOWAIT);
> > > -	vgacon_scrollback_init(vga_video_num_columns * 2);
> > > +	vgacon_scrollback_cur = &vgacon_scrollbacks[0];
> > > +	vgacon_scrollback_init(0);
> > >  }
> > >  
> > >  static void vgacon_scrollback_update(struct vc_data *c, int t,
> > > int count)
> > >  {
> > >  	void *p;
> > >  
> > > -	if (!vgacon_scrollback.size || c->vc_num != fg_console)
> > > +	if (!vgacon_scrollback_cur->data ||
> > > !vgacon_scrollback_cur->size ||
> > > +	    c->vc_num != fg_console)
> > >  		return;
> > >  
> > >  	p = (void *) (c->vc_origin + t * c->vc_size_row);
> > >  
> > >  	while (count--) {
> > > -		scr_memcpyw(vgacon_scrollback.data +
> > > vgacon_scrollback.tail,
> > > +		scr_memcpyw(vgacon_scrollback_cur->data +
> > > +			    vgacon_scrollback_cur->tail,
> > >  			    p, c->vc_size_row);
> > > -		vgacon_scrollback.cnt++;
> > > +
> > > +		vgacon_scrollback_cur->cnt++;
> > >  		p += c->vc_size_row;
> > > -		vgacon_scrollback.tail += c->vc_size_row;
> > > +		vgacon_scrollback_cur->tail += c->vc_size_row;
> > >  
> > > -		if (vgacon_scrollback.tail >=
> > > vgacon_scrollback.size)
> > > -			vgacon_scrollback.tail = 0;
> > > +		if (vgacon_scrollback_cur->tail >=
> > > vgacon_scrollback_cur->size)
> > > +			vgacon_scrollback_cur->tail = 0;
> > >  
> > > -		if (vgacon_scrollback.cnt >
> > > vgacon_scrollback.rows)
> > > -			vgacon_scrollback.cnt =
> > > vgacon_scrollback.rows;
> > > +		if (vgacon_scrollback_cur->cnt >
> > > vgacon_scrollback_cur->rows)
> > > +			vgacon_scrollback_cur->cnt =
> > > vgacon_scrollback_cur->rows;
> > >  
> > > -		vgacon_scrollback.cur = vgacon_scrollback.cnt;
> > > +		vgacon_scrollback_cur->cur =
> > > vgacon_scrollback_cur->cnt;
> > >  	}
> > >  }
> > >  
> > >  static void vgacon_restore_screen(struct vc_data *c)
> > >  {
> > > -	vgacon_scrollback.save = 0;
> > > +	vgacon_scrollback_cur->save = 0;
> > >  
> > > -	if (!vga_is_gfx && !vgacon_scrollback.restore) {
> > > +	if (!vga_is_gfx && !vgacon_scrollback_cur->restore) {
> > >  		scr_memcpyw((u16 *) c->vc_origin, (u16 *) c-
> > > >vc_screenbuf,
> > >  			    c->vc_screenbuf_size > vga_vram_size
> > > ?
> > >  			    vga_vram_size : c-
> > > >vc_screenbuf_size);
> > > -		vgacon_scrollback.restore = 1;
> > > -		vgacon_scrollback.cur = vgacon_scrollback.cnt;
> > > +		vgacon_scrollback_cur->restore = 1;
> > > +		vgacon_scrollback_cur->cur =
> > > vgacon_scrollback_cur->cnt;
> > >  	}
> > >  }
> > >  
> > > @@ -252,41 +288,41 @@ static void vgacon_scrolldelta(struct
> > > vc_data *c, int lines)
> > >  		return;
> > >  	}
> > >  
> > > -	if (!vgacon_scrollback.data)
> > > +	if (!vgacon_scrollback_cur->data)
> > >  		return;
> > >  
> > > -	if (!vgacon_scrollback.save) {
> > > +	if (!vgacon_scrollback_cur->save) {
> > >  		vgacon_cursor(c, CM_ERASE);
> > >  		vgacon_save_screen(c);
> > > -		vgacon_scrollback.save = 1;
> > > +		vgacon_scrollback_cur->save = 1;
> > >  	}
> > >  
> > > -	vgacon_scrollback.restore = 0;
> > > -	start = vgacon_scrollback.cur + lines;
> > > +	vgacon_scrollback_cur->restore = 0;
> > > +	start = vgacon_scrollback_cur->cur + lines;
> > >  	end = start + abs(lines);
> > >  
> > >  	if (start < 0)
> > >  		start = 0;
> > >  
> > > -	if (start > vgacon_scrollback.cnt)
> > > -		start = vgacon_scrollback.cnt;
> > > +	if (start > vgacon_scrollback_cur->cnt)
> > > +		start = vgacon_scrollback_cur->cnt;
> > >  
> > >  	if (end < 0)
> > >  		end = 0;
> > >  
> > > -	if (end > vgacon_scrollback.cnt)
> > > -		end = vgacon_scrollback.cnt;
> > > +	if (end > vgacon_scrollback_cur->cnt)
> > > +		end = vgacon_scrollback_cur->cnt;
> > >  
> > > -	vgacon_scrollback.cur = start;
> > > +	vgacon_scrollback_cur->cur = start;
> > >  	count = end - start;
> > > -	soff = vgacon_scrollback.tail - ((vgacon_scrollback.cnt
> > > - end) *
> > > -					 c->vc_size_row);
> > > +	soff = vgacon_scrollback_cur->tail -
> > > +		((vgacon_scrollback_cur->cnt - end) * c-
> > > >vc_size_row);
> > >  	soff -= count * c->vc_size_row;
> > >  
> > >  	if (soff < 0)
> > > -		soff += vgacon_scrollback.size;
> > > +		soff += vgacon_scrollback_cur->size;
> > >  
> > > -	count = vgacon_scrollback.cnt - start;
> > > +	count = vgacon_scrollback_cur->cnt - start;
> > >  
> > >  	if (count > c->vc_rows)
> > >  		count = c->vc_rows;
> > > @@ -300,13 +336,13 @@ static void vgacon_scrolldelta(struct
> > > vc_data *c, int lines)
> > >  
> > >  		count *= c->vc_size_row;
> > >  		/* how much memory to end of buffer left? */
> > > -		copysize = min(count, vgacon_scrollback.size -
> > > soff);
> > > -		scr_memcpyw(d, vgacon_scrollback.data + soff,
> > > copysize);
> > > +		copysize = min(count, vgacon_scrollback_cur-
> > > >size - soff);
> > > +		scr_memcpyw(d, vgacon_scrollback_cur->data +
> > > soff, copysize);
> > >  		d += copysize;
> > >  		count -= copysize;
> > >  
> > >  		if (count) {
> > > -			scr_memcpyw(d, vgacon_scrollback.data,
> > > count);
> > > +			scr_memcpyw(d, vgacon_scrollback_cur-
> > > >data, count);
> > >  			d += count;
> > >  		}
> > >  
> > > @@ -320,13 +356,13 @@ static void vgacon_flush_scrollback(struct
> > > vc_data *c)
> > >  {
> > >  	size_t size = CONFIG_VGACON_SOFT_SCROLLBACK_SIZE * 1024;
> > >  
> > > -	if (c->vc_num == fg_console)
> > > -		vgacon_scrollback_reset(size);
> > > +	vgacon_scrollback_reset(c->vc_num, size);
> > >  }
> > >  #else
> > >  #define vgacon_scrollback_startup(...) do { } while (0)
> > >  #define vgacon_scrollback_init(...)    do { } while (0)
> > >  #define vgacon_scrollback_update(...)  do { } while (0)
> > > +#define vgacon_scrollback_switch(...)  do { } while (0)
> > >  
> > >  static void vgacon_restore_screen(struct vc_data *c)
> > >  {
> > > @@ -805,7 +841,7 @@ static int vgacon_switch(struct vc_data *c)
> > >  			vgacon_doresize(c, c->vc_cols, c-
> > > >vc_rows);
> > >  	}
> > >  
> > > -	vgacon_scrollback_init(c->vc_size_row);
> > > +	vgacon_scrollback_switch(c->vc_num);
> > >  	return 0;		/* Redrawing not needed */
> > >  }
> 
> 

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