[RFC v2 PATCH] reserve_mem: add support for static memory

[Shyam Saini <shyamsaini@linux.microsoft.com>]

reserve_mem relies on dynamic memory allocation, this limits the
usecase where memory is required to be preserved across the boots.
Eg: ramoops memory reservation on ACPI platforms

So add support to pass a pre-determined static address and reserve
memory at a specified location. This enables use case like ramoops
on ACPI platforms to reliably access ramoops region with previous
boot logs.

Also skip the parsing of <align> when static address is passed.

Example syntax for static address
 reserve_mem=4M@0x1E0000000:oops

Signed-off-by: Shyam Saini <shyamsaini@linux.microsoft.com>
---
v1: https://lore.kernel.org/lkml/0eaf3be2-5121-48b7-aeed-196405c0a480@infradead.org/
v2: Fix code logic and incorporate Randy's suggestion
---
 .../admin-guide/kernel-parameters.txt         | 15 ++++++
 mm/memblock.c                                 | 47 +++++++++++++------
 2 files changed, 47 insertions(+), 15 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index b5493a7f8f228..7e0baca564b97 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6563,6 +6563,21 @@ Kernel parameters
 
 			reserve_mem=12M:4096:oops ramoops.mem_name=oops
 
+	reserve_mem=	[RAM]
+			Format: nn[KMG]:<@offset>:<label>
+			Reserve physical memory at predetermined location and label it with
+			a name that other subsystems can use to access it. This is typically
+			used for systems that do not wipe the RAM, and this command
+			line will try to reserve the same physical memory on
+			soft reboots. Note, it is guaranteed to be the same
+			location unless some other early allocation, e.g.: crashkernel=256M
+                        (without static address) is reserved or overlaps this region.
+
+			The format is size:offset:label for example, to request
+			4 megabytes for ramoops at 0x1E0000000:
+
+			reserve_mem=4M@0x1E0000000:oops ramoops.mem_name=oops
+
 	reservetop=	[X86-32,EARLY]
 			Format: nn[KMG]
 			Reserves a hole at the top of the kernel virtual
diff --git a/mm/memblock.c b/mm/memblock.c
index 6349c48154f4b..c76cefa0a8a83 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -2721,6 +2721,7 @@ static int __init reserve_mem(char *p)
 	char *name;
 	char *oldp;
 	int len;
+	bool addr_is_static = false;
 
 	if (!p)
 		goto err_param;
@@ -2736,19 +2737,27 @@ static int __init reserve_mem(char *p)
 	if (!size || p == oldp)
 		goto err_param;
 
-	if (*p != ':')
-		goto err_param;
+	/* parse the static memory address */
+	if (*p == '@') {
+		start = memparse(p+1, &p);
+		addr_is_static = true;
+	}
 
-	align = memparse(p+1, &p);
 	if (*p != ':')
 		goto err_param;
 
-	/*
-	 * memblock_phys_alloc() doesn't like a zero size align,
-	 * but it is OK for this command to have it.
-	 */
-	if (align < SMP_CACHE_BYTES)
-		align = SMP_CACHE_BYTES;
+	if (!addr_is_static) {
+		align = memparse(p+1, &p);
+		if (*p != ':')
+			goto err_param;
+
+		/*
+		 * memblock_phys_alloc() doesn't like a zero size align,
+		 * but it is OK for this command to have it.
+		 */
+		if (align < SMP_CACHE_BYTES)
+			align = SMP_CACHE_BYTES;
+	}
 
 	name = p + 1;
 	len = strlen(name);
@@ -2772,14 +2781,22 @@ static int __init reserve_mem(char *p)
 	}
 
 	/* Pick previous allocations up from KHO if available */
-	if (reserve_mem_kho_revive(name, size, align))
+	if (!addr_is_static && reserve_mem_kho_revive(name, size, align))
 		return 1;
 
-	/* TODO: Allocation must be outside of scratch region */
-	start = memblock_phys_alloc(size, align);
-	if (!start) {
-		pr_err("reserve_mem: memblock allocation failed\n");
-		return -ENOMEM;
+	if (addr_is_static) {
+		if (memblock_reserve(start, size)) {
+			pr_err("reserve_mem: memblock reservation failed\n");
+			return -ENOMEM;
+		}
+
+	} else {
+		/* TODO: Allocation must be outside of scratch region */
+		start = memblock_phys_alloc(size, align);
+		if (!start) {
+			pr_err("reserve_mem: memblock allocation failed\n");
+			return -ENOMEM;
+		}
 	}
 
 	reserved_mem_add(start, size, name);
-- 
2.43.0