Add support for Sitronix ST7571 LCD controller

Sitronix ST7571 is a 4bit gray scale dot matrix LCD controller. The controller has a SPI, I2C and 8bit parallel interface, this driver is for the I2C interface only.

Reviewed-by Thomas Zimmermann <[email protected]>

Reviewed-by Javier Martinez Canillas <[email protected]>

Signed-off-by Marcus Folkesson <[email protected]>

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Sitronix ST7571 is a 4bit gray scale dot matrix LCD controller. The controller has a SPI, I2C and 8bit parallel interface, this driver is for the I2C interface only.

Reviewed-by Thomas Zimmermann <[email protected]>

Reviewed-by Javier Martinez Canillas <[email protected]>

Signed-off-by Marcus Folkesson <[email protected]>

drivers/gpu/drm/tiny/st7571-i2c.c new file +35 -0

 
+static int st7571_fb_update_rect_grayscale(struct drm_framebuffer *fb, struct drm_rect *rect)
+{
+       struct st7571_device *st7571 = drm_to_st7571(fb->dev);
+       u32 format = fb->format->format;
+       char *row = st7571->row;
+       int x1;
+       int x2;
+
+       /* Align y to display page boundaries */
+       rect->y1 = round_down(rect->y1, ST7571_PAGE_HEIGHT);
+       rect->y2 = min_t(unsigned int, round_up(rect->y2, ST7571_PAGE_HEIGHT), st7571->nlines);
+
+       switch (format) {
+       case DRM_FORMAT_XRGB8888:
+               /* Threated as monochrome (R1) */
+               fallthrough;
+       case DRM_FORMAT_R1:
+               x1 = rect->x1;
+               x2 = rect->x2;
+               break;
+       case DRM_FORMAT_R2:
+               x1 = rect->x1 * 2;
+               x2 = rect->x2 * 2;
+               break;
+       }
+
+       for (int y = rect->y1; y < rect->y2; y += ST7571_PAGE_HEIGHT) {
+               for (int x = x1; x < x2; x++)
+                       row[x] = st7571_transform_xy(st7571->hwbuf, x, y);
+
+               st7571_set_position(st7571, rect->x1, y);
+
+               /* TODO: Investige why we can't write multiple bytes at once */
+               for (int x = x1; x < x2; x++) {
+                       regmap_bulk_write(st7571->regmap, ST7571_DATA_MODE, row + x, 1);

Hi Marcus,

drivers/gpu/drm/tiny/st7571-i2c.c line 388

Quoted 9 lines

drivers/gpu/drm/tiny/st7571-i2c.c new file +9 -0

+                       /*
+                        * As the display supports grayscale, all pixels must be written as two bits
+                        * even if the format is monochrome.
+                        *
+                        * The bit values maps to the following grayscale:
+                        * 0 0 = White
+                        * 0 1 = Light gray
+                        * 1 0 = Dark gray
+                        * 1 1 = Black

That is not R2, but D2?
include/uapi/drm/drm_fourcc.h:

    /* 2 bpp Red (direct relationship between channel value and brightness) */
    #define DRM_FORMAT_R2             fourcc_code('R', '2', ' ', ' ')

/* [7:0] R0:R1:R2:R3 2:2:2:2 four pixels/byte */

    /* 2 bpp Darkness (inverse relationship between channel value and
brightness) */
    #define DRM_FORMAT_D2             fourcc_code('D', '2', ' ', ' ')

/* [7:0] D0:D1:D2:D3 2:2:2:2 four pixels/byte */

So the driver actually supports D1 and D2, and XRGB8888 should be inverted while converting to monochrome (and grayscale, which is not yet implemented).

drivers/gpu/drm/tiny/st7571-i2c.c line 388

Quoted 9 lines

drivers/gpu/drm/tiny/st7571-i2c.c new file +9 -0

+                       /*
+                        * As the display supports grayscale, all pixels must be written as two bits
+                        * even if the format is monochrome.
+                        *
+                        * The bit values maps to the following grayscale:
+                        * 0 0 = White
+                        * 0 1 = Light gray
+                        * 1 0 = Dark gray
+                        * 1 1 = Black

That is not R2, but D2?
include/uapi/drm/drm_fourcc.h:

    /* 2 bpp Red (direct relationship between channel value and brightness) */
    #define DRM_FORMAT_R2             fourcc_code('R', '2', ' ', ' ')

/* [7:0] R0:R1:R2:R3 2:2:2:2 four pixels/byte */

    /* 2 bpp Darkness (inverse relationship between channel value and
brightness) */
    #define DRM_FORMAT_D2             fourcc_code('D', '2', ' ', ' ')

/* [7:0] D0:D1:D2:D3 2:2:2:2 four pixels/byte */

So the driver actually supports D1 and D2, and XRGB8888 should be inverted while converting to monochrome (and grayscale, which is not yet implemented).

On Thu, Apr 24, 2025 at 10:38:33AM +0200, Geert Uytterhoeven wrote:

[...]

Quoted 17 lines

drivers/gpu/drm/tiny/st7571-i2c.c line 388

Quoted 9 lines

drivers/gpu/drm/tiny/st7571-i2c.c new file +9 -0

+                       /*
+                        * As the display supports grayscale, all pixels must be written as two bits
+                        * even if the format is monochrome.
+                        *
+                        * The bit values maps to the following grayscale:
+                        * 0 0 = White
+                        * 0 1 = Light gray
+                        * 1 0 = Dark gray
+                        * 1 1 = Black

That is not R2, but D2?
include/uapi/drm/drm_fourcc.h:

    /* 2 bpp Red (direct relationship between channel value and brightness) */
    #define DRM_FORMAT_R2             fourcc_code('R', '2', ' ', ' ')

/* [7:0] R0:R1:R2:R3 2:2:2:2 four pixels/byte */

Quoted 8 lines

    /* 2 bpp Darkness (inverse relationship between channel value and
brightness) */
    #define DRM_FORMAT_D2             fourcc_code('D', '2', ' ', ' ')

/* [7:0] D0:D1:D2:D3 2:2:2:2 four pixels/byte */

So the driver actually supports D1 and D2, and XRGB8888 should be inverted while converting to monochrome (and grayscale, which is not yet implemented).

The display supports "reverse" grayscale, so the mapping becomes 1 1 = White
1 0 = Light gray
0 1 = Dark gray
0 0 = Black
instead.

So I will probably add support for D1 and D2 formats and invert the pixels for the R1, R2 and XRGB8888 formats.

Could that work or are there any side effects that I should be aware of?

Hi Marcus,

Quoted 37 lines · Marcus Folkesson

On Thu, Apr 24, 2025 at 10:38:33AM +0200, Geert Uytterhoeven wrote:

[...]

Quoted 17 lines

drivers/gpu/drm/tiny/st7571-i2c.c line 388

Quoted 9 lines

drivers/gpu/drm/tiny/st7571-i2c.c new file +9 -0

+                       /*
+                        * As the display supports grayscale, all pixels must be written as two bits
+                        * even if the format is monochrome.
+                        *
+                        * The bit values maps to the following grayscale:
+                        * 0 0 = White
+                        * 0 1 = Light gray
+                        * 1 0 = Dark gray
+                        * 1 1 = Black

That is not R2, but D2?
include/uapi/drm/drm_fourcc.h:

    /* 2 bpp Red (direct relationship between channel value and brightness) */
    #define DRM_FORMAT_R2             fourcc_code('R', '2', ' ', ' ')

/* [7:0] R0:R1:R2:R3 2:2:2:2 four pixels/byte */

Quoted 8 lines

    /* 2 bpp Darkness (inverse relationship between channel value and
brightness) */
    #define DRM_FORMAT_D2             fourcc_code('D', '2', ' ', ' ')

/* [7:0] D0:D1:D2:D3 2:2:2:2 four pixels/byte */

So the driver actually supports D1 and D2, and XRGB8888 should be inverted while converting to monochrome (and grayscale, which is not yet implemented).

The display supports "reverse" grayscale, so the mapping becomes 1 1 = White
1 0 = Light gray
0 1 = Dark gray
0 0 = Black
instead.

Quoted 4 lines

So I will probably add support for D1 and D2 formats and invert the pixels for the R1, R2 and XRGB8888 formats.

Could that work or are there any side effects that I should be aware of?

That should work fine.

Note that you do not have to support R1 and R2, as they are non-native. AFAIK XRGB8888 is the only format all drivers must support.

Gr{oetje,eeting}s,

This series add support for the ST7571 LCD Controller.
It is a 4 gray scale dot matrix LCD controller that supports several interfaces such as SPI, I2C and a 8bit parallell port. The controlelr supports both monochrome and grayscale displays.

This driver only supports the I2C interface, but all common parts could easily be put into a common file to be used with other interfaces. I only have I2C to test with.

The device is a little defiant, it tends to NAK some commands, but all commands takes effect, hence the I2C_M_IGNORE_NAK flag.

Signed-off-by Marcus Folkesson <[email protected]>

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