improved scheduling code

kernel.c

@@ -17,12 +17,15 @@ extern void memset(unsigned int, void*, void*);
 
 extern void init()
 {
-    init_ecall_table();
-
     dbgln("Kernel started!", 15);
-
+    // initialize scheduler
+    scheudler_init();
+    // initialize tabel for associating ecall codes with their handlers
+    init_ecall_table();
+    // read supplied binaries, this will call malloc_init with the memory layout
+    // then it will create a new process for each loaded binary
     read_binary_table();
-
+    // give control to the scheudler and start runnign user programs
     scheduler_run_next();
 }
 

kinclude/boot.S

@@ -5,35 +5,32 @@ stack_bottom:
 .space      4096
 stack_top:
 
+// put the startup code in a special section so that the linker can position it at the start of the binary
 .section    .text._start
 
+// tell the linker that init is a function located elsewhere
 .extern     init
 .type       init, @function
 
-.extern     trap_handle
-.type       trap_handle, @function
-
-
 .global     _start
 _start:
+            // setup mie register, enable timer and software interrupts targeting machine mode
+            // mie[7] MTIE = 1 - enable timer interrupts
+            // mie[3] MSIE = 1 - enable software interrupts
+            li      a0, 0x88
+            csrw    CSR_MIE, a0         // write to mie csr
+            // load trap vector address into a0
+            la      a0, trap_vector
+            csrw    CSR_MTVEC, a0       // write to mtvec csr
             // enable interrupts in mstatus
-            // this is the setting loaded:
-            // [07]    MPIE    =   1  - we want to enable interrupts with mret
-            // [03]    MIE     =   0  - we don't want interrupts now
-            // [11:12] MPP     =   0  - we want to return into user mode
-            // all other bits should be zero
+            // mstatus[07] MPIE = 1 - we want to enable interrupts with mret
             li      a0, 0x80
-            csrrw   zero, CSR_MSTATUS, a0         // write to mstatus
-            // setup a0 to hold |trap tbl addr|mode|
-            //              len:| 30          | 2  |
-            la      a0, trap_vector
-            csrrw   zero, CSR_MTVEC, a0         // write a0 into mtvec csr entry
-            // write
+            csrw    CSR_MSTATUS, a0     // write to mstatus csr
 .option push
 .option norelax
             // init sp and gp
             la      sp, stack_top
-            la      gp, _gp
+            la      gp, __global_pointer$
 .option pop
             // clear kernel bss section
             mv      a0, zero
@@ -47,9 +44,13 @@ _start:
             // halt machine after returning from init
             li      t0, -1
             csrw    CSR_HALT, t0
+            // if the halt CSR somehow didn't exit immediately trap execution in this infinite loop
 1:
             j       1b
 
+.extern     trap_handle
+.type       trap_handle, @function
+
 .align 4
 trap_vector:
             // save all registers into the PCB struct

kinclude/ktypes.h

@@ -13,6 +13,7 @@ enum error_code {
     ENOMEM  = 3,    // not enough memory
     ENOBUFS = 4,    // no space left in buffer
     ESRCH   = 5,    // no such process
+    EABORT  = 6
 };
 
 /*

kinclude/sched.c

@@ -29,6 +29,11 @@ void scheduler_run_next()
     scheduler_switch_to(current_process);
 }
 
+void scheudler_init()
+{
+    current_process = processes + PROCESS_COUNT - 1;
+}
+
 // try to return to a process
 void scheduler_try_return_to(ProcessControlBlock* pcb)
 {
@@ -58,47 +63,59 @@ ProcessControlBlock* scheduler_select_free()
     unsigned long long int mtime;
     int timeout_available = false; // note if a timeout is available
 
-    if (current_process == NULL)
-        current_process = processes + PROCESS_COUNT - 1;
-
     while (true) {
         mtime = read_time();
-        ProcessControlBlock* pcb = current_process + 1;
-        if (pcb > processes + PROCESS_COUNT)
-            pcb = processes;
+        // start at the last scheduled process
+        ProcessControlBlock* pcb = current_process;
+
+        // iterate once over the whole list
+        do {
+            // get next pcb
+            pcb++;
+            // wrap around the end of the list
+            if (pcb > processes + PROCESS_COUNT)
+                pcb = processes;
 
-        while (pcb != current_process) {
+            // when we find a process which is ready to be scheduled, return it!
             if (pcb->status == PROC_RDY)
                 return pcb;
 
+            // if it's sleeping, check if it is time to wake it up
             if (pcb->status == PROC_WAIT_SLEEP) {
                 if (pcb->asleep_until < mtime) {
+                    pcb->status = PROC_RDY;
                     return pcb;
                 }
                 timeout_available = true;
             }
 
+            // if it's waiting for another process, check if the process exited
+            // or if is waiting with a timeout, tell it the timeout expired
             if (pcb->status == PROC_WAIT_PROC) {
+                if (pcb->waiting_for_process != NULL &&
+                    pcb->waiting_for_process->status == PROC_DEAD) {
+                    // the requested process exited, so we can set the status code and
+                    pcb->regs[REG_A0] = pcb->waiting_for_process->exit_code;
+                    pcb->regs[REG_A0+1] = 0;
+                    pcb->status = PROC_RDY;
+                    return pcb;
+                }
                 if (pcb->asleep_until != 0) {
                     if (pcb->asleep_until < mtime) {
-                        //TODO: set process return args!
+                        // if the timeout ran out, set an error code
+                        pcb->regs[REG_A0 + 1] = EABORT;
+                        pcb->status = PROC_RDY;
                         return pcb;
                     }
                     timeout_available = true;
                 }
             }
-            pcb++;
-            if (pcb > processes + PROCESS_COUNT)
-                pcb = processes;
-        }
-
-        if (current_process->status == PROC_RDY) {
-            return current_process;
-        }
+        } while (pcb != current_process);
 
+        // when we finished iterating over all processes and no process can be scheduled we have a problem
         if (timeout_available == false) {
-            // either process deadlock or no processes alive.
-            //TODO: handle missing executable thread
+            // either process deadlock without timeout or no processes alive.
+            //TODO: handle deadlocks by killing a process
             dbgln("No thread active!", 17);
             HALT(22);
         }

kinclude/sched.h

@@ -8,6 +8,7 @@
 extern ProcessControlBlock processes[PROCESS_COUNT];
 
 // scheduler methods
+void scheudler_init();
 ProcessControlBlock* scheduler_select_free();
 void set_next_interrupt();
 void __attribute__((noreturn)) scheduler_run_next();