[U-Boot] [PATCH 15/17] sunxi: Ippo_q8h defconfigs: Enable the LCD panel found on these tablets.

[Siarhei Siamashka <siarhei.siamashka@gmail.com>]

On Wed, 31 Dec 2014 12:22:17 +0100
Hans de Goede <hdegoede@redhat.com> wrote:

> On 30-12-14 13:17, Siarhei Siamashka wrote:
> > On Tue, 30 Dec 2014 11:26:51 +0100
> > Hans de Goede <hdegoede@redhat.com> wrote:
> >
> >> Currently basically only lcd_if = 0 and lcd_frm = 1 are supported, it
> >> should be possible to add support for other lcd_frm = x values easily,
> >> so if you encounter those let me know, lcd_if != 0 is going to be much
> >> harder to support and currently is not on my schedule.
> >
> > It's all in the orange part of the table at the bottom. The lcd_frm = 0
> > seems to be relatively common. The links to FEX files for each device
> > are also there in the table and can be used to confirm the details.
> >
> > The http://linux-sunxi.org/Wexler_TAB_7200 tablet with its fex file
> > https://github.com/linux-sunxi/sunxi-boards/blob/master/sys_config/a20/wexler_tab_7200.fex
> > is one of the examples.
> 
> Ok, so I've looked this up in the linux-sunxi code again to freshen my
> memory, and grepping that code gives this:
> 
> drivers/video/sunxi/disp/ebios_lcdc_tve.h
> 51:     LCDC_FRM_RGB888 = 0,
> 52:     LCDC_FRM_RGB666 = 1,
> 53:     LCDC_FRM_RGB656 = 2,
> 
> All 3 of which are already supported (but other then LCDC_FRM_RGB666
> untested) in the u-boot lcd code :
> 
>      LCDC_FRM_RGB888 -> depth:24
>      LCDC_FRM_RGB666 -> depth:18
>      LCDC_FRM_RGB656 -> depth:17
> 
> So this results in the following translation:
> 
>      lcd_frm = 0  -> depth:24
>      lcd_frm = 1  -> depth:18
>      lcd_frm = 2  -> depth:17

In fact the 'lcd_frm' FEX option only controls dithering. The color
depth is controlled by the 'lcd_cpu_if' FEX option. These color
depth settings end up in the TCON0_CPU_IF_REG register, and your
LCD patches currently unconditionally set it to 18-bit depth
anyway.

There is also a wiki page, describing FEX settings with a lot of
useful information there:
    http://linux-sunxi.org/Fex_Guide#lcd.5B0.2F1.5D_configuration

As for the dithering settings, you can try to compare the results of
having "depth:0" vs. "depth:18" in CONFIG_VIDEO_LCD_MODE and using
the following simple test program.

With "depth:0", the picture looks correct, but blocky (6 bits per
color component is not great for gradients without dithering).
With "depth:18", the picture looks smooth.

/******** Display a gradient picture ************/
#include <stdint.h>
#include <stdio.h>
#include <fcntl.h>
#include <linux/fb.h>
#include <sys/ioctl.h>
#include <sys/mman.h>

int main()
{
    int fd, x, y;
    uint32_t *fb;
    struct fb_fix_screeninfo finfo;
    struct fb_var_screeninfo vinfo;

    if ((fd = open("/dev/fb0", O_RDWR)) == -1) {
        printf("Can't open /dev/fb0\n");
        return 1;
    }

    if (ioctl(fd, FBIOGET_FSCREENINFO, &finfo)) {
        printf("FBIOGET_FSCREENINFO failed\n");
        return 1;
    }

    if (ioctl(fd, FBIOGET_VSCREENINFO, &vinfo)) {
        printf("FBIOGET_VSCREENINFO failed\n");
        return 1;
    }

    if (vinfo.bits_per_pixel != 32) {
        printf("Only 32bpp framebuffer is supported\n");
        return 1;
    }

    fb = mmap(0, finfo.smem_len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (fb == (void *)-1) {
        printf("mmap failed\n");
        return 1;
    }

    for (y = 0; y < vinfo.yres; y++)
        for (x = 0; x < vinfo.xres; x++)
            fb[y * vinfo.xres + x] = (255 * x / vinfo.xres) * 0x000100 +
                                     (255 * y / vinfo.yres) * 0x010001;

    return 0;
}
/************************************************/

So "lcd_frm = 0" is not expected to cause any problems and is
already supported. But naming and meaning of the options in the
CONFIG_VIDEO_LCD_MODE string could be improved.

I just wonder why there are so many FEX files with lcd_frm = 0. It
does not seem to be a great idea, unless there is some good reason.

-- 
Best regards,
Siarhei Siamashka