From 0e7f27d25dca35cc6c6323dd93479336f4d36ffd Mon Sep 17 00:00:00 2001
From: Matt Taylor <mstaveleytaylor@gmail.com>
Date: Tue, 16 Jul 2019 08:20:53 +0100
Subject: [PATCH] Use volatile accesses in VGA code and make font dependency
 optional (#67)

---
 Cargo.toml                    |  3 ++-
 src/printer/mod.rs            |  3 +++
 src/printer/vga_320x200.rs    | 25 ++++++++++++-------------
 src/printer/vga_text_80x25.rs | 24 ++++++++++++------------
 4 files changed, 29 insertions(+), 26 deletions(-)

diff --git a/Cargo.toml b/Cargo.toml
index d4dd8227..f7fe9fef 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -18,13 +18,14 @@ fixedvec = "0.2.3"
 version = "0.2.4"
 default-features = false
 features = ["unicode"]
+optional = true
 
 [build-dependencies]
 llvm-tools = "0.1"
 
 [features]
 default = []
-vga_320x200 = []
+vga_320x200 = ["font8x8"]
 recursive_page_table = []
 map_physical_memory = []
 
diff --git a/src/printer/mod.rs b/src/printer/mod.rs
index b4ecd7b3..726bb765 100644
--- a/src/printer/mod.rs
+++ b/src/printer/mod.rs
@@ -4,5 +4,8 @@ pub use self::vga_text_80x25::*;
 #[cfg(feature = "vga_320x200")]
 pub use self::vga_320x200::*;
 
+#[cfg(feature = "vga_320x200")]
 mod vga_320x200;
+
+#[cfg(not(feature = "vga_320x200"))]
 mod vga_text_80x25;
diff --git a/src/printer/vga_320x200.rs b/src/printer/vga_320x200.rs
index 22b35fce..fb1e36af 100644
--- a/src/printer/vga_320x200.rs
+++ b/src/printer/vga_320x200.rs
@@ -1,30 +1,28 @@
 use core::fmt::{Result, Write};
-use core::slice;
 use core::sync::atomic::{AtomicUsize, Ordering};
 
 const VGA_BUFFER: *mut u8 = 0xa0000 as *mut _;
 const SCREEN_WIDTH: usize = 320;
 const SCREEN_HEIGHT: usize = 200;
 
-pub static X_POS: AtomicUsize = AtomicUsize::new(1); // must not be 0 so that we don't have a .bss section
-pub static Y_POS: AtomicUsize = AtomicUsize::new(1); // must not be 0 so that we don't have a .bss section
+// must not be 0 so that we don't have a .bss section
+pub static X_POS: AtomicUsize = AtomicUsize::new(1);
+pub static Y_POS: AtomicUsize = AtomicUsize::new(1);
 
 pub struct Printer;
 
 impl Printer {
     pub fn clear_screen(&mut self) {
-        let vga_buffer = Self::vga_buffer();
-        for byte in vga_buffer {
-            *byte = 0x00;
+        for i in 0..(SCREEN_WIDTH * SCREEN_HEIGHT) {
+            unsafe {
+                VGA_BUFFER.offset(i as isize).write_volatile(0);
+            }
         }
+
         X_POS.store(0, Ordering::SeqCst);
         Y_POS.store(0, Ordering::SeqCst);
     }
 
-    fn vga_buffer() -> &'static mut [u8] {
-        unsafe { slice::from_raw_parts_mut(VGA_BUFFER, SCREEN_WIDTH * SCREEN_HEIGHT) }
-    }
-
     fn newline(&mut self) {
         let y_pos = Y_POS.fetch_add(8, Ordering::SeqCst);
         X_POS.store(0, Ordering::SeqCst);
@@ -41,8 +39,6 @@ impl Printer {
             return;
         }
 
-        let vga_buffer = Self::vga_buffer();
-
         let x_pos = X_POS.fetch_add(8, Ordering::SeqCst);
         let y_pos = Y_POS.load(Ordering::SeqCst);
 
@@ -57,7 +53,10 @@ impl Printer {
                             continue;
                         }
                         let color = 0xf;
-                        vga_buffer[(y_pos + y) * SCREEN_WIDTH + x_pos + x] = color;
+                        let idx = (y_pos + y) * SCREEN_WIDTH + x_pos + x;
+                        unsafe {
+                            VGA_BUFFER.offset(idx as isize).write_volatile(color);
+                        }
                     }
                 }
             }
diff --git a/src/printer/vga_text_80x25.rs b/src/printer/vga_text_80x25.rs
index 1202054a..fd689987 100644
--- a/src/printer/vga_text_80x25.rs
+++ b/src/printer/vga_text_80x25.rs
@@ -1,35 +1,35 @@
 use core::fmt::{Result, Write};
-use core::slice;
 use core::sync::atomic::{AtomicUsize, Ordering};
 
 const VGA_BUFFER: *mut u8 = 0xb8000 as *mut _;
 const SCREEN_SIZE: usize = 80 * 25;
 
+// must not be 0 so that we don't have a .bss section
 pub static CURRENT_OFFSET: AtomicUsize = AtomicUsize::new(160);
 
 pub struct Printer;
 
 impl Printer {
     pub fn clear_screen(&mut self) {
-        let vga_buffer = Self::vga_buffer();
-        for byte in vga_buffer {
-            *byte = 0;
+        for i in 0..SCREEN_SIZE {
+            unsafe {
+                VGA_BUFFER.offset(i as isize).write_volatile(0);
+            }
         }
-        CURRENT_OFFSET.store(0, Ordering::Relaxed);
-    }
 
-    fn vga_buffer() -> &'static mut [u8] {
-        unsafe { slice::from_raw_parts_mut(VGA_BUFFER, SCREEN_SIZE * 2) }
+        CURRENT_OFFSET.store(0, Ordering::Relaxed);
     }
 }
 
 impl Write for Printer {
     fn write_str(&mut self, s: &str) -> Result {
-        let vga_buffer = Self::vga_buffer();
         for byte in s.bytes() {
-            let index = CURRENT_OFFSET.fetch_add(2, Ordering::Relaxed);
-            vga_buffer[index] = byte;
-            vga_buffer[index + 1] = 0x4f;
+            let index = CURRENT_OFFSET.fetch_add(2, Ordering::Relaxed) as isize;
+
+            unsafe {
+                VGA_BUFFER.offset(index).write_volatile(byte);
+                VGA_BUFFER.offset(index + 1).write_volatile(0x4f);
+            }
         }
 
         Ok(())