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Subject[RFC/PATCH 3/5] memremap: Add support for read-only memory mappings
Date
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@chromium.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Andy Gross <agross@kernel.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
---
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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