Subject: [PATCH v3 3/5] memremap: Add support for read-only memory mappings

From: Stephen Boyd <[email-masked]>
Date: Tue, 10 Sep 2019 09:09:01 -0700

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 <[email-masked]>
Cc: Rob Herring <[email-masked]>
Cc: Bjorn Andersson <[email-masked]>
Cc: Andy Gross <[email-masked]>
Cc: Will Deacon <[email-masked]>
Cc: Catalin Marinas <[email-masked]>
Cc: Dan Williams <[email-masked]>
Reviewed-by: Bjorn Andersson <[email-masked]>
Signed-off-by: Stephen Boyd <[email-masked]>
---
 include/linux/io.h |  1 +
 kernel/iomem.c     | 20 +++++++++++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/include/linux/io.h b/include/linux/io.h
index accac822336a..15a63efcd153 100644
--- a/include/linux/io.h
+++ b/include/linux/io.h
@@ -148,6 +148,7 @@ 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 62c92e43aa0d..6d76b7398714 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)
@@ -45,7 +52,7 @@ static void *try_ram_remap(resource_size_t offset, size_t size,
  * @offset: iomem resource start address
  * @size: size of remap
  * @flags: any of MEMREMAP_WB, MEMREMAP_WT, MEMREMAP_WC,
- *		  MEMREMAP_ENC, MEMREMAP_DEC
+ *		  MEMREMAP_ENC, MEMREMAP_DEC, MEMREMAP_RO
  *
  * memremap() is "ioremap" for cases where it is known that the resource
  * being mapped does not have i/o side effects and the __iomem
@@ -53,6 +60,9 @@ static void *try_ram_remap(resource_size_t offset, size_t size,
  * mapping types will be attempted in the order listed below until one of
  * them succeeds.
  *
+ * MEMREMAP_RO - establish a mapping whereby writes are ignored/rejected.
+ * Attempts to map System RAM with this mapping type will fail.
+ *
  * MEMREMAP_WB - matches the default mapping for System RAM on
  * the architecture.  This is usually a read-allocate write-back cache.
  * Moreover, if MEMREMAP_WB is specified and the requested remap region is RAM
@@ -84,7 +94,10 @@ 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 satisfied
 		 * from the direct map.  Some archs depend on the
@@ -103,7 +116,8 @@ 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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