[PATCH 1/3] RFT: drm/pl111: Support grayscale

[Linus Walleij]

On Tue, Jul 23, 2019 at 11:07 PM Daniel Vetter <daniel at ffwll.ch> wrote:
> On Tue, Jul 23, 2019 at 7:25 PM Adam Jackson <ajax at redhat.com> wrote:
> >
> > On Tue, 2019-07-23 at 15:37 +0200, Linus Walleij wrote:
> > > Migrating the TI nspire calculators to use the PL111 driver for
> > > framebuffer requires grayscale support for the elder panel
> > > which uses 8bit grayscale only.
> > >
> > > DRM does not support 8bit grayscale framebuffers in memory,
> > > but by defining the bus format to be MEDIA_BUS_FMT_Y8_1X8 we
> > > can get the hardware to turn on a grayscaling feature and
> > > convert the RGB framebuffer to grayscale for us.
> >
> > What's wrong with DRM_FORMAT_R8? Yes the hardware is not really
> > "redscale", but it's still a single color channel and there's not
> > really any ambiguity.
>
> Yeah, I think with a comment or an aliasing #define to _Y8 (or both)
> this is good to go.

Is there something really wrong with just biting the bullet and do this:

/* 8 bpp grayscale */
#define DRM_FORMAT_Y8 fourcc_code('Y', '8', ' ', ' ') /* [7:0] Y */

It's quite an embarrasement for my OCD tendencies to talk about
black-and-white TV as if it was 256 Shades of Red (good title
for a novel by the way).

I don't know how these FOURCC things work, possibly new
fourcc:s can only be defined by some especially enlightened
cabal of standardization?

(It beats me how it can not already exist in that case.)

> You probably still want to expose the rgb format since too much
> userspace just assumes that xrgb8888 works. Same reason why the
> tinydrm drivers do the sw conversion.

Yes this is what we do on PL111 now, we just run it through
the hardware grayscaler.

This hardware graciously supports reading black-white and
grayscale bitmaps with 1 (monochrome), 2, 4 and 8 bits per
pixel which would be Y1, Y2, Y4 and Y8. But we only have
hardware supporting Y8 at least on the other side so
I don't see any need for the others ATM.

I suspect the Y1 etc could be useful for people doing not
only Hercules video drivers (hah!) but also for ePaper
displays of say, some random Kindle.

Yours,
Linus Walleij