rpi4-osdev/part12-wgt/wsetmode.c

#include "wgt.h"

void vga256(void)
{
    mbox[0] = 35*4; // Length of message in bytes
    mbox[1] = MBOX_REQUEST;

    mbox[2] = MBOX_TAG_SETPHYWH; // Tag identifier
    mbox[3] = 8; // Value size in bytes
    mbox[4] = 8; // Value size in bytes (again!)
    mbox[5] = 1920; // Value(width)
    mbox[6] = 1080; // Value(height)

    mbox[7] = MBOX_TAG_SETVIRTWH;
    mbox[8] = 8;
    mbox[9] = 8;
    mbox[10] = 1920;
    mbox[11] = 1080;

    mbox[12] = MBOX_TAG_SETVIRTOFF;
    mbox[13] = 8;
    mbox[14] = 8;
    mbox[15] = 0; // Value(x)
    mbox[16] = 0; // Value(y)

    mbox[17] = MBOX_TAG_SETDEPTH;
    mbox[18] = 4;
    mbox[19] = 4;
    mbox[20] = 32; // Bits per pixel

    mbox[21] = MBOX_TAG_SETPXLORDR;
    mbox[22] = 4;
    mbox[23] = 4;
    mbox[24] = 1; // RGB

    mbox[25] = MBOX_TAG_GETFB;
    mbox[26] = 8;
    mbox[27] = 8;
    mbox[28] = 4096; // FrameBufferInfo.pointer
    mbox[29] = 0;    // FrameBufferInfo.size

    mbox[30] = MBOX_TAG_GETPITCH;
    mbox[31] = 4;
    mbox[32] = 4;
    mbox[33] = 0; // Bytes per line

    mbox[34] = MBOX_TAG_LAST;

    // Check call is successful and we have a pointer with depth 32
    if (mbox_call(MBOX_CH_PROP) && mbox[20] == 32 && mbox[28] != 0) {
        mbox[28] &= 0x3FFFFFFF;        // Convert GPU address to ARM address
        WGT_SYS.xres = mbox[10];       // Actual physical width
        WGT_SYS.yres = mbox[11];       // Actual physical height
        WGT_SYS.screenwidth = mbox[10];
        WGT_SYS.screenheight = mbox[11];

        abuf = (unsigned int *)((long)mbox[28]);
    }
}