[RFC/PATCH 3/5] memremap: Add support for read-only memory mappings

[Stephen Boyd]

Sometimes we have memories that are supposed to be read-only, but when
we map these regions the best we can do is map them as write-back with
MEMREMAP_WB. Introduce a read-only memory mapping (MEMREMAP_RO) that
allows us to map reserved memory regions as read-only. This way, we're
less likely to see these special memory regions become corrupted by
stray writes to them.

Cc: Evan Green <evgreen@xxxxxxxxxxxx>
Cc: Rob Herring <robh+dt@xxxxxxxxxx>
Cc: Bjorn Andersson <bjorn.andersson@xxxxxxxxxx>
Cc: Andy Gross <agross@xxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Signed-off-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
---
 include/linux/io.h |  1 +
 kernel/iomem.c     | 15 +++++++++++++--
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/include/linux/io.h b/include/linux/io.h
index 32e30e8fb9db..16c7f4498869 100644
--- a/include/linux/io.h
+++ b/include/linux/io.h
@@ -159,6 +159,7 @@ enum {
 	MEMREMAP_WC = 1 << 2,
 	MEMREMAP_ENC = 1 << 3,
 	MEMREMAP_DEC = 1 << 4,
+	MEMREMAP_RO = 1 << 5,
 };
 
 void *memremap(resource_size_t offset, size_t size, unsigned long flags);
diff --git a/kernel/iomem.c b/kernel/iomem.c
index f7525e14ebc6..8d3cf74a32cb 100644
--- a/kernel/iomem.c
+++ b/kernel/iomem.c
@@ -19,6 +19,13 @@ static void *arch_memremap_wb(resource_size_t offset, unsigned long size)
 }
 #endif
 
+#ifndef arch_memremap_ro
+static void *arch_memremap_ro(resource_size_t offset, unsigned long size)
+{
+	return NULL;
+}
+#endif
+
 #ifndef arch_memremap_can_ram_remap
 static bool arch_memremap_can_ram_remap(resource_size_t offset, size_t size,
 					unsigned long flags)
@@ -84,7 +91,10 @@ void *memremap(resource_size_t offset, size_t size, unsigned long flags)
 	}
 
 	/* Try all mapping types requested until one returns non-NULL */
-	if (flags & MEMREMAP_WB) {
+	if ((flags & MEMREMAP_RO) && is_ram != REGION_INTERSECTS)
+		addr = arch_memremap_ro(offset, size);
+
+	if (!addr && (flags & MEMREMAP_WB)) {
 		/*
 		 * MEMREMAP_WB is special in that it can be satisifed
 		 * from the direct map.  Some archs depend on the
@@ -103,7 +113,8 @@ void *memremap(resource_size_t offset, size_t size, unsigned long flags)
 	 * address mapping.  Enforce that this mapping is not aliasing
 	 * System RAM.
 	 */
-	if (!addr && is_ram == REGION_INTERSECTS && flags != MEMREMAP_WB) {
+	if (!addr && is_ram == REGION_INTERSECTS &&
+	    (flags != MEMREMAP_WB || flags != MEMREMAP_RO)) {
 		WARN_ONCE(1, "memremap attempted on ram %pa size: %#lx\n",
 				&offset, (unsigned long) size);
 		return NULL;
-- 
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