From 075058a958641b3c7067b6ba10c4847ec39382a1 Mon Sep 17 00:00:00 2001
From: xamidev <xamidev@riseup.net>
Date: Wed, 31 Dec 2025 17:42:26 +0100
Subject: [PATCH] PMM: init with freelist

---
 src/mem/paging/pmm.c | 83 +++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 78 insertions(+), 5 deletions(-)

diff --git a/src/mem/paging/pmm.c b/src/mem/paging/pmm.c
index 0f1e956..3ce044b 100644
--- a/src/mem/paging/pmm.c
+++ b/src/mem/paging/pmm.c
@@ -10,8 +10,10 @@ to see which pages are used by kernel/bootloader/mmio/fb etc.
 #include "paging.h"
 #include <limine.h>
 #include <stddef.h>
+#include <stdint.h>
 #include <kernel.h>
 #include "../misc/utils.h"
+#include "pmm.h"
 
 /*
 First we'll have to discover the physical memory layout,
@@ -25,20 +27,42 @@ and use this for the bitmap. The reserved memory will be ignored.
 
 struct usable_memory* usable_mem;
 struct limine_memmap_entry* biggest_entry;
-
+/* 
 uint64_t* bitmap;
+uint64_t* usable_memory_begin;
 
+// Bitmap will be placed at the start of the biggest usable region
+// 1 bitmap page = 4096 bytes = 32768 bits
+// So 1 bitmap page can track up to 32768 pages of memory
 static void pmm_allocate_bitmap(struct limine_hhdm_response* hhdm)
 {
     uint64_t pages = biggest_entry->length / PAGE_SIZE;
-    DEBUG("we need %u pages (bits) that will fit in %u uint64_t", pages, pages/64);
+    uint64_t bitmap_items = (pages+BITS_PER_ROW-1)/BITS_PER_ROW;
+    DEBUG("we need %u pages (bits) that will fit in %u uint64_t", pages, bitmap_items);
 
     bitmap = (uint64_t*)(biggest_entry->base + hhdm->offset);
     DEBUG("bitmap will live at 0x%p", bitmap);
 
+    uint64_t bitmap_bytes = bitmap_items*sizeof(uint64_t);
+
     // All pages are marked free since we're in a USABLE region
-    memset(bitmap, 0x00, pages/64);
-}
+    memset(bitmap, 0x00, bitmap_bytes);
+
+    uint64_t bitmap_pages = (bitmap_bytes + PAGE_SIZE - 1) / PAGE_SIZE;
+
+    // The 1st memory page we can use is at bitmap + bitmap_pages
+    usable_memory_begin = (void*)((uintptr_t)bitmap + bitmap_pages*PAGE_SIZE);
+
+    // Mark bitmap as allocated
+    for (uint64_t i=0; i<bitmap_pages; i++)
+    {
+        uint64_t row = i / BITS_PER_ROW;
+        uint64_t bit = i % BITS_PER_ROW;
+        bitmap[row] |= (1ULL << bit);
+    }
+
+    DEBUG("bitmap occupies %u pages", bitmap_pages);
+} */
 
 static void pmm_find_biggest_usable_region(struct limine_memmap_response* memmap, struct limine_hhdm_response* hhdm)
 {
@@ -68,8 +92,57 @@ static void pmm_find_biggest_usable_region(struct limine_memmap_response* memmap
             biggest_entry->base+offset, biggest_entry->base+biggest_entry->length+offset);
 }
 
+// Offset from Higher Half Direct Map
+uint64_t hhdm_off;
+
+static uintptr_t g_freelist = 0;
+
+#define PHYS_TO_VIRT(x) ((void*)((uintptr_t)(x) + hhdm_off))
+#define VIRT_TO_PHYS(x) ((uintptr_t)(x) - hhdm_off)
+
+// Stole it
+#define ALIGN_UP(x, align)   (((x) + ((align) - 1)) & ~((align) - 1))
+#define ALIGN_DOWN(x, align) ((x) & ~((align) - 1))
+
+uintptr_t pmm_alloc()
+{
+    if (!g_freelist) return 0;
+    uintptr_t addr = g_freelist;
+    g_freelist = *(uintptr_t*) PHYS_TO_VIRT(g_freelist);
+    return addr;
+}
+
+void pmm_free(uintptr_t addr)
+{
+    *(uintptr_t*) PHYS_TO_VIRT(addr) = g_freelist;
+    g_freelist = addr;
+}
+
+static void pmm_init_freelist()
+{
+    // We simply call pmm_free() on each page that is marked USABLE
+    // in our big memory region.
+    uint64_t base = ALIGN_UP(biggest_entry->base, PAGE_SIZE);
+    uint64_t end = ALIGN_DOWN(biggest_entry->base + biggest_entry->length, PAGE_SIZE);
+
+    uint64_t page_count=0;
+    for (uint64_t addr = base; addr < end; addr += PAGE_SIZE)
+    {
+        pmm_free(addr);
+        //DEBUG("page %u lives at phys 0x%p (virt 0x%p)", page_count, addr, PHYS_TO_VIRT(addr));
+        //page_count++;
+    }
+}
+
 void pmm_init(struct limine_memmap_response* memmap, struct limine_hhdm_response* hhdm)
 {
+    hhdm_off = hhdm->offset;
     pmm_find_biggest_usable_region(memmap, hhdm);
-    pmm_allocate_bitmap(hhdm);
+    //pmm_allocate_bitmap(hhdm); too complicated for my small brain
+    
+    // Now we have biggest USABLE region,
+    // so to populate the free list we just iterate through it
+    pmm_init_freelist();
+
+    DEBUG("Trying pmm_alloc gives us page at 0x%p", pmm_alloc());
 }
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