On Mon, Apr 26, 2021 at 02:18:05PM +0200, Thomas Zimmermann wrote:

Hi

Am 20.04.21 um 10:46 schrieb Daniel Vetter:

quoted

On Mon, Apr 19, 2021 at 10:00:56AM +0200, Geert Uytterhoeven wrote:

quoted

Hi Thomas,

On Fri, Apr 16, 2021 at 11:00 AM Thomas Zimmermann [off-list ref] wrote:

quoted

This patchset adds support for simple-framebuffer platform devices and
a handover mechanism for native drivers to take-over control of the
hardware.

The new driver, called simpledrm, binds to a simple-frambuffer platform
device. The kernel's boot code creates such devices for firmware-provided
framebuffers, such as EFI-GOP or VESA. Typically the BIOS, UEFI or boot
loader sets up the framebuffers. Description via device tree is also an
option.

I guess this can be used as a replacement for offb, too...

quoted

Patches 4 to 8 add the simpledrm driver. It's build on simple DRM helpers
and SHMEM. It supports 16-bit, 24-bit and 32-bit RGB framebuffers. During

.... if support for 8-bit frame buffers would be added?

Offb doesn't seem to be tied to the simple-framebuffer support. So adding a
new driver or extending the simple-framebuffer code would be required. Not a
big deal, though. Patch 3 of this patchset adds the ability to create
generic drivers within DRM.

quoted

Is that 8-bit greyscale or 8-bit indexed with 256 entry palette? Former
shouldn't be a big thing, but the latter is only really supported by the
overall drm ecosystem in theory. Most userspace assumes that xrgb8888
works, and we keep that illusion up by emulating it in kernel for hw which
just doesn't support it. But reformatting xrgb8888 to c8 is tricky at
best. The uapis are all there for setting the palette, and C8 is a defined
format even with atomic kms interface, but really there's not much
userspace for it. In other words, it would work as well as current offb
would, but that's at least that.

I think we can just use a shadow palette in the drm driver: If the drm
framebuffer is in C8, use the userspace's palette. If the drm framebuffer is
in XRGB, use a palette that represents RGB332. The driver would do
on-the-fly conversion; just like cirrus does.

Hm yeah rgb332 palette sounds like a reasonable idea. Could even have that
palette defined/generated in format conversion helpers, and then an
xrgb8888->rgb332 converter.

Lower palettes probably stop making sense as rgb, maybe there we just do
greyscale or something like that for the xrgb8888 emulation.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch