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 e13dedc..39b5808 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);
 	}