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SubjectRe: [PATCH 0/6] Support for LEGO MINDSTORMS EV3 LCD display
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(cc: Daniel Vetter)


Den 01.08.2017 18.51, skrev David Lechner:
> On 07/30/2017 12:14 PM, Noralf Trønnes 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?

>>
>>> 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.

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