AM62A7: Transparency with framebuffer device

Sathiya Narayanan

Part Number: AM62A7

Tool/software:

Hi,

I wanted to stream a transparent GUI frame along with some text over the video, and I don't want to use CPU for overlaying the GUI.

Referring to previous thread, this can be achieved by VIDL pipeline of the display subsystem (tidss).
The VIDL pipeline is exposed as a framebuffer device, in which GUI can be drawn.

The issue is, though the framebuffer device takes 4bytes (32 bit) of data, only 3 bytes are given to the display.
The format it is taking is BGRx where 4th byte is ignored

root@am62axx-evm:~# fbset --info

mode "1920x1080"
    geometry 1920 1080 1920 1080 32
    timings 0 0 0 0 0 0 0
    accel true
    rgba 8/16,8/8,8/0,0/0
endmode

Frame buffer device information:
    Name        : tidssdrmfb
    Address     : (nil)
    Size        : 8294400
    Type        : PACKED PIXELS
    Visual      : TRUECOLOR
    XPanStep    : 1
    YPanStep    : 1
    YWrapStep   : 0
    LineLength  : 7680
    Accelerator : No

But the tidss can support transparency with few formats line RA24, BA24, etc.
How can I configure my framebuffer device for BA24 format? Do I need to change it in driver or the linux itself?

11 months ago

0 Krunal Bhargav34 11 months ago

TI__Mastermind 44965 points

Hi,

On the output side, the format will be 24bits and on the input side, it's up to your userpsace application to configure alpha channel or the input buffers. In TIDSS, we also have alpha blending through which you set set the transparency of two planes. I don't have gstreamer example but here is a test binary that we used to test it: https://software-dl.ti.com/processor-sdk-linux/esd/AM62X/10_00_07_04/exports/docs/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.html?highlight=tidss

Regards,
Krunal

0 Sathiya Narayanan 11 months ago in reply to Krunal Bhargav34

Intellectual 790 points

Hi krunal,

Where in the TIDSS driver, I can configure the framebuffer for BA24 format? Which has 24 bits output and 6 bit for each channel.

Let's say TIDSS is taking 24 Bits out of 32 bits. That is good to go. The issue here is the TIDSS will not align this 24 bits to 6 bit BGRA. Instead it is configured for 8 bit BGR. How can I tell TIDSS to process these 24 bits as 6 bit BGRA.

In other cases like kmssink, tidss is automatically taking alpha values also.

Krunal Bhargav34 said:
I don't have gstreamer example but here is a test binary that we used to test it: https://software-dl.ti.com/processor-sdk-linux/esd/AM62X/10_00_07_04/exports/docs/linux/Foundational_Components/Kernel/Kernel_Drivers/Display/DSS7.html?highlight=tidss

Here in this example, global alpha blending is shown, not pixel wise part.

0 Divyansh Mittal 11 months ago in reply to Sathiya Narayanan

TI__Genius 10720 points

Hi,

Looking into it internally, please expect a reply in 2 days.

0 Divyansh Mittal 11 months ago in reply to Divyansh Mittal

TI__Genius 10720 points

Hi,

Configuring BA24 format is more of a userspace DRM application.

I recommend looking at the following line in this header file (https://github.com/torvalds/linux/blob/21266b8df5224c4f677acf9f353eecc9094731f0/include/uapi/drm/drm_fourcc.h#L201C1-L201C103):

#define DRM_FORMAT_BGRA8888	fourcc_code('B', 'A', '2', '4') /* [31:0] B:G:R:A 8:8:8:8 little endian */

Also have a look at the highlighted section below from AM62A TRM:

Above image mentions how the BGRA32 format will be interpreted by TIDSS.

Regards

Divyansh

+1 Sathiya Narayanan 10 months ago in reply to Divyansh Mittal

Intellectual 790 points

Hi Divyansh,

I can be able to configure framebuffer with transparency forcefully.

Sathiya Narayanan said:
Let's say TIDSS is taking 24 Bits out of 32 bits. That is good to go. The issue here is the TIDSS will not align this 24 bits to 6 bit BGRA. Instead it is configured for 8 bit BGR. How can I tell TIDSS to process these 24 bits as 6 bit BGRA

I was wrong here. TIDSS will take full 32 bits, and the default format set is BGRx [DRM_FORMAT_XRGB8888, In fourcc: XR24]. I forced the tidss driver to set BGRA [DRM_FORMAT_ARGB8888, In fourcc:AR24] whenever BGRx is requested. Thus, the alpha pixel value is also used in the framebuffer.

The drawback of this method is we can't use BGRx if needed. It is not required for my application anyway.
When I was getting into this issue, I noticed that in the tidss driver code the framebuffer is initialize with generic function drm_fbdev_generic_setup(ddev, 32) which don't give provision of changing configuration. It would be better if framebuffer is initialized with more control on the driver side.

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AM62A7: Transparency with framebuffer device