Re: [Linux-fbdev-devel] [RFC] double buffering for atmel_lcdfb

[Geert U. <ge...@li...>]

On Wed, 6 Aug 2008, Stanislaw Gruszka wrote:
> I try to implement double buffering for Atmel LCD display on AT91SAM9263 based 
> board. I could not find any document about panning display on linux 
> framebuffer, so I have some question about it and generic question about 
> double buffer design. 
> 
> 1) Is FBIOPAN_DISPLAY ioctl right way to implement double buffering?

Yes.

> 2) What is the difference between ypanstep and ywrapstep, which one should be 
> used for double buffering.

Ypanstep is the allowed step increment for panning vertically.
E.g. for a screen with 200 lines and y.offset = 50, it will display
lines 50-249 of the frame buffer.

Ywrapstep is the allowed step increment for wrapping the screen
vertically.
E.g. for a screen with 200 lines and y.offset = 50, it will display
lines 50-199 followed by lines 0-49 of the frame buffer.

> 3) Does double buffer stuff should be turned on using module parameter, 
> compile time parameter, board specific parameter (flag from platform device).
> As it consume 2x memory I don't thing anyone want it by default.

No, let userspace ask for it when it specifies a large
fb_var_screeninfo.yres_virtual, if possible. If you have to allocate all
frame buffer memory at initialization time, you can use a board specific
parameter or a kernel command line parameter.

> diff --git a/drivers/video/atmel_lcdfb.c b/drivers/video/atmel_lcdfb.c
> index b004036..d0b2bed 100644
> --- a/drivers/video/atmel_lcdfb.c
> +++ b/drivers/video/atmel_lcdfb.c
> @@ -7,7 +7,6 @@
>   * License.  See the file COPYING in the main directory of this archive for
>   * more details.
>   */
> -
>  #include <linux/kernel.h>
>  #include <linux/platform_device.h>
>  #include <linux/dma-mapping.h>
> @@ -41,6 +40,8 @@
>  #if defined(CONFIG_ARCH_AT91)
>  #define	ATMEL_LCDFB_FBINFO_DEFAULT	FBINFO_DEFAULT
>  
> +static int double_buffering = 1;
> +
>  static inline void atmel_lcdfb_update_dma2d(struct atmel_lcdfb_info *sinfo,
>  					struct fb_var_screeninfo *var)
>  {
> @@ -789,7 +790,13 @@ static int __init atmel_lcdfb_probe(struct 
> platform_device *pdev)
>  		 * Don't clear the framebuffer -- someone may have set
>  		 * up a splash image.
>  		 */
> +		double_buffering = 0;
> + 		dev_info(dev, "dissable double buffering\n");
>  	} else {
> + 		if (double_buffering) {
> + 			info->fix.ypanstep = info->var.yres_virtual;

I would set it to 1, if there's no hardware limitation preventing this.

> + 			info->var.yres_virtual *= 2;

This is done at initialization time, hence it cannot be changed later by
using the ioctls?

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@li...

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds