Re: [PATCH 0/6] Support for LEGO MINDSTORMS EV3 LCD display

[David Lechner]

On 08/01/2017 01:08 PM, Noralf TrÃnnes wrote:
> (cc: Daniel Vetter)
>
>
> Den 01.08.2017 18.51, skrev David Lechner:
> > On 07/30/2017 12:14 PM, Noralf TrÃnnes wrote:
> > > Den 29.07.2017 21.40, skrev David Lechner:
> > > > On 07/29/2017 02:17 PM, David Lechner wrote:
> > > > > The goal of this series is to get the built-in LCD of the LEGO 
> > > > > MINDSTORMS EV3
> > > > > working. But, most of the content here is building up the 
> > > > > infrastructure to do
> > > > > that.
> > > >
> > > > Some general comments/questions:
> > > >
> > > > I have noticed that DRM doesn't really have support for monochrome 
> > > > displays. I'm guessing that is because no one really uses them anymore?
> > >
> > > The repaper driver was the first monochrome drm driver and I chose to
> > > present it to userspace as XRGB8888 and convert it to monochrome.
> > > The reason for this is that everything, libraries, apps, plymouth (boot
> > > splash, no rgb565) supports it. I didn't see any point in adding a new
> > > monochrome drm format that didn't have, or probably wouldn't get
> > > userspace support (by libraries at least). The application of course
> > > needs to know this to get a good result.
> > >
> > > tinydrm_xrgb8888_to_gray8() does the conversion:
> > > https://cgit.freedesktop.org/drm/drm-misc/commit/drivers/gpu/drm/tinydrm?id=379ea9a1a59a5a32c8db6f164e80a3fd00cb3781 
> > >
> > >
> > > > The LEGO EV3 display is just an LCD (not the backlit kind). It has 
> > > > two modes of operation. It can to 2bbp grayscale or it can do 1bpp 
> > > > monochrome. The grayscale isn't the best (looks splotchy in places), 
> > > > so it would be nice to be able to choose between these two modes. 
> > > > How would I implement something like that?
> > >
> > > Do you expect anyone to use grayscale if it doesn't look good?
> > > Maybe better to add it later if there's a demand for it.
> >
> > I don't expect many people to use it at all. The people that do will 
> > be hackers like me that want to see what it is capable of, so I think 
> > I will keep it grayscale by default.
>
> It looks like the best way to satisfy your needs is to add specific 
> formats.
>
> Video for Linux have these:
>
> include/uapi/linux/videodev2.h
>
> /* Grey formats */
> #define V4L2_PIX_FMT_GREY    v4l2_fourcc('G', 'R', 'E', 'Y') /*  8 
> Greyscale     */
> #define V4L2_PIX_FMT_Y4      v4l2_fourcc('Y', '0', '4', ' ') /*  4 
> Greyscale     */
> #define V4L2_PIX_FMT_Y6      v4l2_fourcc('Y', '0', '6', ' ') /*  6 
> Greyscale     */
> #define V4L2_PIX_FMT_Y10     v4l2_fourcc('Y', '1', '0', ' ') /* 10 
> Greyscale     */
> #define V4L2_PIX_FMT_Y12     v4l2_fourcc('Y', '1', '2', ' ') /* 12 
> Greyscale     */
> #define V4L2_PIX_FMT_Y16     v4l2_fourcc('Y', '1', '6', ' ') /* 16 
> Greyscale     */
> #define V4L2_PIX_FMT_Y16_BE  v4l2_fourcc_be('Y', '1', '6', ' ') /* 16 
> Greyscale BE  */
>
>
> Maybe we can add formats like these:
>
> include/uapi/drm/drm_fourcc.h
>
> #define DRM_FORMAT_MONO        fourcc_code('Y', '0', '1', ' ') /* [0] 
> Monochrome */
> or
> #define DRM_FORMAT_Y1        fourcc_code('Y', '0', '1', ' ') /* [0] 
> Monochrome */
>
> #define DRM_FORMAT_Y2        fourcc_code('Y', '0', '2', ' ') /* [1:0] 
> Greyscale */
>
> Daniel, what do you think?

2 of the first 3 tinydrm drivers are monochrome (and I would like to 
write a driver for the Seeed/Grove I2C OLED displays which are also 
monochrome), so I think the 1bpp modes would definitely be useful. I 
think there is enough userspace code still around that knows about 
FB_VISUAL_MONO01 and FB_VISUAL_MONO10 that could use these.

I don't think a Y02 mode would be useful though. There is pretty much 
nothing out there (that I could find) that uses such a mode.

A Y08 mode for 8bpp grayscale would be useful though. This is another 
more commonly used format.

> > > > Also, how can I indicate to userspace that this display really is 
> > > > monochrome/grayscale since the reported color depth 16bpp?
> > >
> > > There isn't unless we add formats for it.
> > > Since this display is in a Lego piece, doesn't it have custom userspace
> > > that already know it's monochrome?
> >
> > The official LEGO software is like this, yes. But I am working on a 
> > project that supports other LEGO compatible devices that have color 
> > screens, so the same software needs to be able to detect at runtime 
> > which type of screen it is using. Currently I have a plain fbdev 
> > driver that provides FB_VISUAL_MONO10 for the EV3 display and so the 
> > software knows when it has a monochrome screen and when it doesn't. 
> > But since plain fbdev drivers can't be accepted into mainline, I need 
> > to find a different way to detect what type of screen this is so that 
> > my graphics library can treat it differently.
>
> You can tell userspace about the preferred bitdepth:
> drm->mode_config.preferred_depth
>
>
> Noralf.