A way to get a 16 bit framebuffer - RGB565

[alexfreed58]

Hi,

I realize that V3s is not H2 or H3, but I hope the question is generic.  Is it possible to make the FB 16 bit? In mainline Linux? I'm working with a Lichee Pi board. The depth is 18 bits but the fb in Linux is 32 bit. I'd love to get it down to 16 bits to save 1/2 of the video memory and just 2 color bits are a small price to pay. I understand that Linux inherits the FB from u-boot.

Edited April 20, 2021 by alexfreed58

[Werner]

Moved to Common issues / peer to peer technical support

[jernej]

7 hours ago, alexfreed58 said:

I understand that Linux inherits the FB from u-boot.

That may or may not be true. Linux has proper V3s display driver for a long time. However, U-Boot may still hand over its FB to Linux. In that case U-Boot FB is used for a very short time and it's replaced with Linux FB during boot. Now, I'm not sure if U-Boot FB memory is actually released or just stays reserved. Best way to check would be to disable display driver in U-Boot (via U-Boot config and rebuild) and compare free memory.

 

I don't know how to force 16-bit buffer. There is probably some kind of kernel parameter for that.

[alexfreed58]

I have built u-boot with different parameters in the .config file for several LCD panels: 800x480 and 800x600. That becomes the size of the reported uboot screen  and also the Linux kernel inherits the same size. But always 32 bit, even if I put the depth=16 rather than the standard depth=18 into the configuration.

My problem is not with the memory allocated internally for the FB: it's the application in the user space that needs many off the screen windows (pixel buffers, really) that have to match the ARGB FB format. 

 

Anybody?

 

[jernej]

Ah, you didn't add panel description to DT for Linux. So yes, Linux uses U-Boot FB. 32-bit colours are hardcoded in U-Boot here: https://source.denx.de/u-boot/u-boot/-/blob/master/drivers/video/sunxi/sunxi_de2.c#L255 Fortunately for you, I added handling 16-bit colours (if you change constant), but I can't guarantee it will work. I only tested it in U-Boot for H3 and not in Linux.

 

EDIT: Apparently you have to change also this line: https://source.denx.de/u-boot/u-boot/-/blob/master/drivers/video/sunxi/sunxi_de2.c#L405