Re: [PATCH resend 2/5] video: fbdev: ssd1307fb: Simplify ssd1307fb_update_display()

[Sam Ravnborg <sam@ravnborg.org>]

Hi Geert,
On Wed, Jul 14, 2021 at 04:58:01PM +0200, Geert Uytterhoeven wrote:
> Simplify the nested loops to handle conversion from linear frame buffer
> to ssd1307 page layout:
>   1. Move last page handling one level up, as the value of "m" is the
>      same inside a page,
>   2. array->data[] is filled linearly, so there is no need to
>      recalculate array_idx over and over again; a simple increment is
>      sufficient.
> 
> Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
> ---
>  drivers/video/fbdev/ssd1307fb.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/video/fbdev/ssd1307fb.c b/drivers/video/fbdev/ssd1307fb.c
> index e6b6263e3bef847f..6d7bd025bca1a175 100644
> --- a/drivers/video/fbdev/ssd1307fb.c
> +++ b/drivers/video/fbdev/ssd1307fb.c
> @@ -158,6 +158,7 @@ static int ssd1307fb_update_display(struct ssd1307fb_par *par)
>  	u8 *vmem = par->info->screen_buffer;
>  	unsigned int line_length = par->info->fix.line_length;
>  	unsigned int pages = DIV_ROUND_UP(par->height, 8);
> +	u32 array_idx = 0;
>  	int ret, i, j, k;
>  
>  	array = ssd1307fb_alloc_array(par->width * pages, SSD1307FB_DATA);
> @@ -194,19 +195,21 @@ static int ssd1307fb_update_display(struct ssd1307fb_par *par)
>  	 */
>  
>  	for (i = 0; i < pages; i++) {
> +		int m = 8;
> +
> +		/* Last page may be partial */
> +		if (i + 1 == pages && par->height % 8)
> +			m = par->height % 8;
>  		for (j = 0; j < par->width; j++) {
> -			int m = 8;
> -			u32 array_idx = i * par->width + j;
> -			array->data[array_idx] = 0;
> -			/* Last page may be partial */
> -			if (i + 1 == pages && par->height % 8)
> -				m = par->height % 8;
> +			u8 data = 0;
> +
>  			for (k = 0; k < m; k++) {
If the last page is partial then m will be less than 8 for all bytes in
j = 0..par-width - but m should only be less than 8 for the last
iteration of the loop.

Do I miss something or is the code buggy?
>  				u8 byte = vmem[(8 * i + k) * line_length +
>  					       j / 8];
>  				u8 bit = (byte >> (j % 8)) & 1;
> -				array->data[array_idx] |= bit << k;
> +				data |= bit << k;
>  			}
> +			array->data[array_idx++] = data;
>  		}
>  	}

	Sam