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>

diff --git a/miscutils/devmem.c b/miscutils/devmem.c
index e13dedc0a5dccc8347eef0272b02a778dd62081b..39b58084013339c861efa1527dbbe74a6928da3d 100644 (file)
--- 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);
        }