Re: [PATCH] atmel_lcdfb: support 16bit BGR:565 mode, remove

Peter Korsgaard wrote:

Because it is arguable wrong as far as I understand the HW.
There's two parts here:

- 1: Format of framebuffer memory
- 2: Wiring of LCD (RGB/BGR order and number of bits)

quoted

From the datasheet, the following framebuffer formats are supported:

1, 2, 4, 8 bits per pixel (palletized), 16, 24 bits per pixel
(non-palletized) for TFT.

So it doesn't really support RGB555 mode. The controller reads up to
32bit of framebuffer data and outputs 24bit on the LCD pins. You CAN
wire up a RGB555 panel by just skipping the LSB green of a RGB565
wiring, but that is independent of the framebufffer format. The
controller/driver doesn't do any RGB/BGR swapping, so the RBG/BGR wiring
settings are just used as a software hint (in FBIOGET_VSCREENINFO) about
the meaning of the individual bits of a pixel in the framebuffer.

Similar you can connect a 12bit 4:4:4 panel by just connecting it to the
MSB LCD pins.

If you're connecting an RGB555 or RGB444 panel, don't you want the
software using the framebuffer to know this so it will dither appropriately?

E.g. gradients look "bandy" if drawn in RGB565 then green's LSB is dropped.

-- Jamie