devmem: map two pages only if it is necessary

function                                             old     new   delta
devmem_main                                          463     469      +6

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 14 insertions, 7 deletions

diff --git a/miscutils/devmem.c b/miscutils/devmem.c
index e13dedc0a..39b580840 100644
--- a/miscutils/devmem.c
+++ b/miscutils/devmem.c
@@ -13,9 +13,9 @@ int devmem_main(int argc UNUSED_PARAM, char **argv)
         uint64_t read_result;
         uint64_t writeval = writeval; /* for compiler */
         off_t target;
-        unsigned page_size = getpagesize();
+        unsigned page_size, mapped_size, offset_in_page;
         int fd;
-        int width = 8 * sizeof(int);
+        unsigned width = 8 * sizeof(int);
 
         /* devmem ADDRESS [WIDTH [VALUE]] */
 // TODO: options?
@@ -50,15 +50,22 @@ int devmem_main(int argc UNUSED_PARAM, char **argv)
                 if (argv[3])
                         writeval = bb_strtoull(argv[3], NULL, 0);
         } else { /* argv[2] == NULL */
-                /* make argv[3] to be a valid thing to use */
+                /* make argv[3] to be a valid thing to fetch */
                 argv--;
         }
         if (errno)
-                bb_show_usage(); /* bb_strtouXX failed */
+                bb_show_usage(); /* one of bb_strtouXX failed */
 
         fd = xopen("/dev/mem", argv[3] ? (O_RDWR | O_SYNC) : (O_RDONLY | O_SYNC));
+        mapped_size = page_size = getpagesize();
+        offset_in_page = (unsigned)target & (page_size - 1);
+        if (offset_in_page + width > page_size) {
+                /* This access spans pages.
+                 * Must map two pages to make it possible: */
+                mapped_size *= 2;
+        }
         map_base = mmap(NULL,
-                        page_size * 2 /* in case value spans page */,
+                        mapped_size,
                         argv[3] ? (PROT_READ | PROT_WRITE) : PROT_READ,
                         MAP_SHARED,
                         fd,
@@ -68,7 +75,7 @@ int devmem_main(int argc UNUSED_PARAM, char **argv)
 
 //      printf("Memory mapped at address %p.\n", map_base);
 
-        virt_addr = (char*)map_base + (target & (page_size - 1));
+        virt_addr = (char*)map_base + offset_in_page;
 
         if (!argv[3]) {
                 switch (width) {
@@ -119,7 +126,7 @@ int devmem_main(int argc UNUSED_PARAM, char **argv)
         }
 
         if (ENABLE_FEATURE_CLEAN_UP) {
-                if (munmap(map_base, page_size * 2) == -1)
+                if (munmap(map_base, mapped_size) == -1)
                         bb_perror_msg_and_die("munmap");
                 close(fd);
         }