syscalls needed for doom (tell/eof/draw_fb) + minor fixes, compiler shut up etc

src/syscall/screen.c

@@ -0,0 +1,105 @@
+/*
+ * @author  xamidev <xamidev@riseup.net>
+ * @brief   Screen/framebuffer system calls
+ * @license GPL-3.0-only
+ */
+
+// This is not part of POSIX or anything btw
+
+#include <stdint.h>
+#include <kernel.h>
+
+extern struct boot_context boot_ctx;
+
+/*
+ * pack_rbga_to_fb - Convert rgb values to rgba pixel for framebuffer
+ * @r: red value
+ * @g: green value
+ * @b: blue value
+ *
+ * This function uses the mask sizes and shifts of the Limine
+ * framebuffer to convert raw rgb values into usable pixels.
+ *
+ * Return:
+ * <pixel> - usable pixel
+ */
+static uint32_t pack_rgba_to_fb(uint8_t r, uint8_t g, uint8_t b)
+{
+    uint32_t pixel = 0;
+
+    if (boot_ctx.fb->red_mask_size) {
+        pixel |= ((uint32_t)(r >> (8 - boot_ctx.fb->red_mask_size)) << boot_ctx.fb->red_mask_shift);
+    }
+    if (boot_ctx.fb->green_mask_size) {
+        pixel |= ((uint32_t)(g >> (8 - boot_ctx.fb->green_mask_size)) << boot_ctx.fb->green_mask_shift);
+    }
+    if (boot_ctx.fb->blue_mask_size) {
+        pixel |= ((uint32_t)(b >> (8 - boot_ctx.fb->blue_mask_size)) << boot_ctx.fb->blue_mask_shift);
+    }
+
+    return pixel;
+}
+
+/*
+ * sys_draw_fb - Draw a framebuffer subset
+ * @src:      Source frame
+ * @width:    width of the frame
+ * @height:   height of the frame
+ * @channels: how many channels (RGB, RGBA)
+ *
+ * This system call draws the frame @src, having some
+ * @width and @height, in the top right of the Limine
+ * framebuffer. (Used for DOOM)
+ *
+ * Return:
+ * %0 - on success
+ * On error, a negative value with an error code is returned.
+ */
+int sys_draw_fb(const uint8_t* src, int width, int height, int channels)
+{
+    if (!boot_ctx.fb || !src) {
+        return -EINVAL;
+    }
+    if (channels != 4 || width <= 0 || height <= 0) {
+        return -EINVAL;
+    }
+    if (boot_ctx.fb->bpp < 24) {
+        return -EIO;
+    }
+
+    uint32_t* dst = (uint32_t*)boot_ctx.fb->address;
+    uint64_t dst_w = boot_ctx.fb->width;
+    uint64_t dst_h = boot_ctx.fb->height;
+    uint64_t dst_pitch_px = boot_ctx.fb->pitch / 4;
+    uint64_t scale = 2;
+    uint64_t scaled_w = (uint64_t)width * scale;
+    uint64_t scaled_h = (uint64_t)height * scale;
+
+    if (scaled_w > dst_w || scaled_h > dst_h) {
+        return -EINVAL;
+    }
+
+    uint64_t dst_x = dst_w - scaled_w;
+    uint64_t dst_y = 0;
+
+    for (uint64_t y = 0; y < (uint64_t)height; ++y) {
+        const uint8_t* src_row = src + y * (uint64_t)width * 4ULL;
+        uint32_t* dst_row0 = dst + (dst_y + y * scale) * dst_pitch_px + dst_x;
+        uint32_t* dst_row1 = dst_row0 + dst_pitch_px;
+
+        for (uint64_t x = 0; x < (uint64_t)width; ++x) {
+            const uint8_t* p = src_row + x * 4ULL;
+            uint32_t pixel = pack_rgba_to_fb(p[0], p[1], p[2]);
+            uint64_t dx = x * scale;
+
+            // Write 2x2 block. This is because source DOOM frame (320x200)
+            // is quite small.
+            dst_row0[dx + 0] = pixel;
+            dst_row0[dx + 1] = pixel;
+            dst_row1[dx + 0] = pixel;
+            dst_row1[dx + 1] = pixel;
+        }
+    }
+
+    return 0;
+}