initramfs: Provide a common initrd reserve function

Some architectures(eg, ARM and riscv) have similar logic to
check and reserve the memory of initrd, let's provide a common
function reserve_initrd_mem() to reduce duplicated code.
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

include/linux/initrd.h

@@ -18,6 +18,12 @@ extern int initrd_below_start_ok;
 extern unsigned long initrd_start, initrd_end;
 extern void free_initrd_mem(unsigned long, unsigned long);
 
+#ifdef CONFIG_BLK_DEV_INITRD
+extern void __init reserve_initrd_mem(void);
+#else
+static inline void __init reserve_initrd_mem(void) {}
+#endif
+
 extern phys_addr_t phys_initrd_start;
 extern unsigned long phys_initrd_size;
 

init/initramfs.c

@@ -535,6 +535,51 @@ extern unsigned long __initramfs_size;
 #include <linux/initrd.h>
 #include <linux/kexec.h>
 
+void __init reserve_initrd_mem(void)
+{
+	phys_addr_t start;
+	unsigned long size;
+
+	/* Ignore the virtul address computed during device tree parsing */
+	initrd_start = initrd_end = 0;
+
+	if (!phys_initrd_size)
+		return;
+	/*
+	 * Round the memory region to page boundaries as per free_initrd_mem()
+	 * This allows us to detect whether the pages overlapping the initrd
+	 * are in use, but more importantly, reserves the entire set of pages
+	 * as we don't want these pages allocated for other purposes.
+	 */
+	start = round_down(phys_initrd_start, PAGE_SIZE);
+	size = phys_initrd_size + (phys_initrd_start - start);
+	size = round_up(size, PAGE_SIZE);
+
+	if (!memblock_is_region_memory(start, size)) {
+		pr_err("INITRD: 0x%08llx+0x%08lx is not a memory region",
+		       (u64)start, size);
+		goto disable;
+	}
+
+	if (memblock_is_region_reserved(start, size)) {
+		pr_err("INITRD: 0x%08llx+0x%08lx overlaps in-use memory region\n",
+		       (u64)start, size);
+		goto disable;
+	}
+
+	memblock_reserve(start, size);
+	/* Now convert initrd to virtual addresses */
+	initrd_start = (unsigned long)__va(phys_initrd_start);
+	initrd_end = initrd_start + phys_initrd_size;
+	initrd_below_start_ok = 1;
+
+	return;
+disable:
+	pr_cont(" - disabling initrd\n");
+	initrd_start = 0;
+	initrd_end = 0;
+}
+
 void __weak __init free_initrd_mem(unsigned long start, unsigned long end)
 {
 #ifdef CONFIG_ARCH_KEEP_MEMBLOCK