yeah, devmem already exists with almost same name and correct :-)

buildroot_config_fragment

@@ -45,3 +45,21 @@ BR2_PACKAGE_LINUX_TOOLS_GPIO=y
 BR2_PACKAGE_DTC=y
 BR2_PACKAGE_DTC_PROGRAMS=y
 BR2_PACKAGE_HOST_DTC=y
+
+# On ARM, with our lkmc_platform_device:
+#
+#     devmem2 0x101e9000 w 0x12345678
+#
+# Then on QEMU monitor, notice that the registers don't actually change value:
+#
+#     xp/4 0x101e9000
+#
+# Uses /dev/mem:
+#
+# - https://superuser.com/questions/71389/what-is-dev-mem/1214662#1214662
+# - https://unix.stackexchange.com/questions/4948/shell-command-to-read-device-registers
+#
+# TODO: why with mmap MAP_PRIVATE (used in my previous custom naive version),
+# the entire register page is read?;
+#
+BR2_PACKAGE_DEVMEM2=y

kernel_module/user/README.md

@@ -11,7 +11,6 @@ These programs can also be compiled and used on host.
 1.  Standalone
     1.  [myinsmod](myinsmod.c)
     1.  [myrmmod](myrmmod.c)
-    1.  [devmem](devmem.c)
     1.  [init_hello](init_hello.c)
 1.  Module tests
     1.  [anonymous_inode](anonymous_inode.c)

kernel_module/user/devmem.c

@@ -1,49 +0,0 @@
-/*
-https://stackoverflow.com/questions/12040303/accessing-physical-address-from-user-space
-
-Sample call:
-
-	/phys.out 0 16
-
-Confirm memory from QEMU monitor with:
-
-	xp/0 16
-*/
-
-#include <fcntl.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <sys/mman.h>
-#include <unistd.h>
-
-int main(int argc, char *argv[]) {
-	if (argc < 3) {
-		printf("Usage: %s <phys_addr> <len>\n", argv[0]);
-		return 0;
-	}
-	off_t offset = strtoul(argv[1], NULL, 0);
-	size_t len = strtoul(argv[2], NULL, 0);
-
-	size_t pagesize = sysconf(_SC_PAGE_SIZE);
-	off_t page_base = (offset / pagesize) * pagesize;
-	off_t page_offset = offset - page_base;
-	int fd = open("/dev/mem", O_SYNC);
-	size_t reallen = page_offset + len;
-	unsigned char *mem = mmap(NULL, reallen, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, page_base);
-	if (mem == MAP_FAILED) {
-		/* TODO why does it fail for some addreses but not for others? */
-		perror("mmap");
-		exit(EXIT_FAILURE);
-	}
-	for (size_t i = 0; i < len; ++i) {
-		printf("%02x ", (unsigned int)mem[page_offset + i]);
-		/* TODO can't edit memory? */
-		mem[page_offset + i] = i % 256;
-	}
-	puts("");
-	if (munmap(mem, reallen) == -1) {
-		perror("munmap");
-		exit(EXIT_FAILURE);
-	}
-	return EXIT_SUCCESS;
-}