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SubjectRe: [PATCH 0/6] Support for LEGO MINDSTORMS EV3 LCD display
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On 08/01/2017 01:08 PM, Noralf Trønnes wrote:
> (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?

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.
>

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