Re: [PATCH v8 3/3] console: Add persistent scrollback buffers for all VGA consoles

[Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>]

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).

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@gmx.de>
> Reviewed-by: Andrey Utkin <andrey_utkin@fastmail.com>
> Tested-by: Andrey Utkin <andrey_utkin@fastmail.com>
> Tested-by: Adam Borowski <kilobyte@angband.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 */
>  }