[PATCH v2 18/19] drivers: mem: Create file to access second half of 64-bit memory

[Andrey Smirnov]

In order to allow access to second half of address space on 64-bit
machines, add code that creates /dev/highmem dedicated for that.

Note that due to maximum file size being limited to MAX_LFS_FILESIZE
or 0x7fff_ffff_ffff_ffff bytes at addresses 0x7fff_ffff_ffff_ffff and
0xffff_ffff_ffff_ffff cannot be access through /dev/mem and
/dev/hightmem correspondingly.

Signed-off-by: Andrey Smirnov <andrew.smir...@gmail.com>
---
 drivers/misc/mem.c | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/drivers/misc/mem.c b/drivers/misc/mem.c
index 2f3316307..e3fb0d04a 100644
--- a/drivers/misc/mem.c
+++ b/drivers/misc/mem.c
@@ -7,6 +7,10 @@
 #include <driver.h>
 #include <init.h>
 
+#include <linux/pagemap.h>
+#include <linux/fs.h>
+
+
 static struct cdev_operations memops = {
        .read  = mem_read_nofail,
        .write = mem_write,
@@ -21,8 +25,7 @@ static int mem_probe(struct device_d *dev)
        dev->priv = cdev;
 
        cdev->name = (char*)dev->resource[0].name;
-       cdev->size = min_t(unsigned long long, resource_size(&dev->resource[0]),
-                          S64_MAX);
+       cdev->size = resource_size(&dev->resource[0]);
        cdev->ops = &memops;
        cdev->dev = dev;
 
@@ -38,7 +41,21 @@ static struct driver_d mem_drv = {
 
 static int mem_init(void)
 {
-       add_mem_device("mem", 0, ~0, IORESOURCE_MEM_WRITEABLE);
+       const resource_size_t size = min_t(unsigned long long,
+                                          MAX_LFS_FILESIZE,
+                                          (resource_size_t)~0);
+       add_mem_device("mem", 0, size, IORESOURCE_MEM_WRITEABLE);
+       /*
+        * We can't fit all of 64-bit address space into a single
+        * file, so we create a separate one to allow acces to the
+        * second half
+        */
+       if (IS_ENABLED(CONFIG_CPU_64))
+               add_mem_device("highmem",
+                              (resource_size_t)MAX_LFS_FILESIZE + 1,
+                              size,
+                              IORESOURCE_MEM_WRITEABLE);
+
        return platform_driver_register(&mem_drv);
 }
 device_initcall(mem_init);
-- 
2.20.1


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