Why am I unable to use the framebuffer interface?

Question

I am using the rasbperry pi 3. I am working on an embedded project. I wrote some frame buffer initialization code, which seems to work since the color palette rectangle disappears and I get a black screen. However I have been unsuccessful at setting the value of a single pixel so far.

My goal is to get a white screen at the end, but I only get a black screen, I don't even see changes in the individual pixels.

I am using the 8th channel (property tags ARM->VC) to communicate with the mailbox, since it seems channel 1 is unreliable.

This is my structure:

struct temp
{
  int size;
  int request;

  int tag1;
  int buff_size1;
  int val_length1;
  int widthP;
  int heightP;

  int tag2;
  int buff_size2;
  int val_length2;
  int widthV;
  int heightV;

  int tag3;
  int buff_size3;
  int val_length3;
  int depth;

  int tag4;
  int buff_size4;
  int val_length4;
  int fb_ptr;
  int fb_size;   

  int end;
};

Which is statically initialized as follows:

volatile temp t __attribute__ ((aligned (16)))=
{
  .size = sizeof(temp),
  .request = 0,

  .tag1 = SET_PHYSICAL_WIDTH_HEIGHT,
  .buff_size1 = 8,
  .val_length1 = 8,
  .widthP = 1024,
  .heightP = 768,

  .tag2 = SET_VIRTUAL_WIDTH_HEIGHT,
  .buff_size2 = 8,
  .val_length2 = 8,
  .widthV = 1024,
  .heightV = 768,

  .tag3 = SET_DEPTH,
  .buff_size3 = 4,
  .val_length3 = 4,
  .depth = 16,

  .tag4 = ALLOCATE,
  .buff_size4 = 8,
  .val_length4 = 8,
  .fb_ptr = 0,
  .fb_size = 0,

  .end = END,
};

And then I finally do:

void init_display()
{
  write_to_mailbox((uint32_t) &t, (Channel)(PTAG_ARM_TO_VC));

  for(int i=0; i<t.fb_size; i++)
  {
    *(volatile uint32_t *)(t.fb_ptr + i) = 0xFFFF;
  }
}

I have been successful at blinking the ACT led so I think write_to_mailbox() works as intended, which is why I didn't post it here.

Answer 1

Seen your project, I would advice you to work with the OpenMax display system.

Pros :
- It will give you draw and display functions
- It allow you to use GPU instead of CPU
- It is lightweight
- It can manage multiple display layers with transparency

Cons :
- Don't provide user input management
- It lacks documentation
- It lacks community

I had good result with OMX, to replace SDL, being a total newbie in C.

You can find code samples here

Answer 2

Here is another way to achieve the most bare-metal display system i can imagine :
Writing raw pixel data to the frambuffer device :

Prerequites :
- the touchscreen has to be recognized as /dev/fb0.
- the frambuffer resolution has to be defined in /boot/config.txt or via fbset command.

You can aquire raw pixel data by reading /dev/fb0 :
cat /dev/fb0 > image.raw
and display in the same way :
cat image.raw > /dev/fb0

Raw image data can be edited with Gimp, as long as you respect original image format and resolution, or can be computed on-the-fly, using some usual graphic functions.

I use this trick to get a boot splashscreen before plymouth launch, it may be enough for a simple UI.