Thanks for your kind words, i am using FasmArm so no tool chain
.
Theres just no info and the linux kernel is a mind field as it links all over the place.
But there is info if you look very deep, i am going to write a full tut, but i myself still need to fully understand some of its secrets.
Example i setup the screen as below, but there may be a case where you do not need to do this, you may just need to get buffer address form a offset at 0x100 plus ?.
But i have not tested that theory, as i only just got my print function working, i will dump stuff to screen and test for this.
I am making some progress, but slowly.
dewlch67 is a good coder and has done some nice demos.
But the basic info to get stuff to screen is here:
* ************************************************************************ *
* **** BCM2835 Specific Stuff **** *
* ************************************************************************ *
* This is where things get interesting (and specific to the BCM2835).
* Most of this was worked out by reading the Linux source code (mostly
* drivers/video/bcm2708_fb.c and arch/arm/mach-bcm2708/) and experimentation.
*
*
* **** Basic procedure to get stuff on screen ****
* The basic procedure to get a frame buffer is:
* 1) Set up a structure with the frame buffer specification (resolution, etc)
* 2) Tell the GPU about this structure by writing to the mailbox
* 3) Wait by reading from the mailbox for the GPU to modify this structure
* 4) Write to the frame buffer at the pointer we got in stage 3
* Only step 4 is required for subsequent writes to the frame buffer. Currently,
* I do not know how to enable the HDMI output, so this will always operate the
* composite, and not the HDMI.
*
*
* **** Mailbox operations ****
* Read/write operatitons on the mailbox consist of transfering data via a
* 32 bit register. 28 bits of this 32 bit register are the data to be sent
* to the receiver, while the lower 4 bits specify the channel (channel 1 is
* the frame buffer, but there are others).
*
* To send data via the mailbox:
* 1) Wait for space in the mailbox
* 2) Write ((data << 4) || channel) to the write register TODO: Make implementation match
*
* To receive data via the mailbox:
* 1) Wait for the mailbox to be non-empty
* 2) Execute a memory barrier
* 3) Read from the read register
* 4) Check the lowest 4 bits of the read value for the correct channel
* 5) If the channel is not the one we wish to read from (i.e: 1), go to step 1
* 6) Return the read value >> 4 TODO: Make implementation match
* Note: This will not work if we're interested in reading from more than one
* channel as it does not handle the reception of other channels' data
*
*
* **** Memory mapped registers ****
* The bus address for the mailbox memory mapped registers is 0x7E00B880.
* This corresponds to an ARM physical address of 0x2000B880 (the address we
* use from the ARM processor, and hence here). We use three registers from
* the mail box:
* - The read register for mailbox 0 at offset 0x00
* - The status register for mailbox 0 at offset 0x1C
* - The write register for mailbox 0 at offset 0x20 (this is actually the read
* register for mailbox 1).
*
*
* **** Notes ****
* - The address of the frame buffer must be at least a multiple of 16 (in
* order to be accurately transmitted in the 28 bits available in the
* mailbox)
* - The 32 bit value we actually send over the mailbox (including the channel)
* is (ADDRESS | 1) where ADDRESS is the address of the structure. This is
* equivalent to sending as the data (ADDRESS >> 4) (remember we do data << 4)
* - This works if we set vwidth = width, vheight = height, x = 0, y = 0.
* - I haven't managaged to make anything but 24 bit depth work, however the
* Linux source seems to use 16 bit?!
* - Sometimes the procedure described to get stuff on the screen doesn't
* work first time. I've hacked around this by repeating until it does work.
* - The two conditions for successfully acquiring a frame buffer are:
* - The data read from the mailbox (with the 4 least significant bits set
* to zero) is 0 (or 1 including the channel)
* - The pointer in the structure is non-zero after the mailbox read
* - Once we have the frame buffer, we can just write to it. The pixels (in
* 24 bit mode) are RGB ordered by y then x coordinate. The address of a
* subpixel is given by: y * pitch + x * 3 + rgb_channel, where rgb_channel
* is 0 for red, 1 for green, and 2 for blue.
Good luck and any ? just ask.
PS: To get to this stage, i have done over 1000 read/writes to SD card, i do not know how much more my R-PI sd slot can take.
Also a big problem R-PI OSDev's have to over come, is keyboard input.
.