[PATCH i-g-t v2] lib/igt_sysfs: make sure to write empty strings

[Lucas De Marchi <lucas.demarchi@intel.com>]

	echo -n "" > /sys/module/<modulename>/parameters/<param>

doesn't really work as it will just create a open() + close() expecting
the file to be truncated. The same issue happens with igt as it will
stop writing when there are 0 chars to write. Special case the empty
string so it always write a '\0' and make sure igt_sysfs_set() accounts
for the extra null char.

Shell example:
	# echo -n "/foo" > /sys/module/firmware_class/parameters/path
	# cat /sys/module/firmware_class/parameters/path
	/foo
	# echo -n "" > /sys/module/firmware_class/parameters/path
	/foo
	# # make sure to actually write a \0:
	echo -ne "\0" > /sys/module/firmware_class/parameters/path
	# cat /sys/module/firmware_class/parameters/path
	/foo

Same thing happens when testing igt_sysfs_set():
       int dirfd = open("/sys/module/firmware_class/parameters", O_RDONLY);
       igt_sysfs_set(dirfd, "path", "");

Previously it was not really setting the param.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
---
 lib/igt_sysfs.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/lib/igt_sysfs.c b/lib/igt_sysfs.c
index 567b4f6d5..64c2aaabb 100644
--- a/lib/igt_sysfs.c
+++ b/lib/igt_sysfs.c
@@ -406,7 +406,7 @@ int igt_sysfs_read(int dir, const char *attr, void *data, int len)
  */
 bool igt_sysfs_set(int dir, const char *attr, const char *value)
 {
-	int len = strlen(value);
+	int len = strlen(value) + 1;
 	return igt_sysfs_write(dir, attr, value, len) == len;
 }
 
@@ -513,8 +513,16 @@ int igt_sysfs_vprintf(int dir, const char *attr, const char *fmt, va_list ap)
 		return -errno;
 
 	va_copy(tmp, ap);
-	ret = vsnprintf(buf, sizeof(stack), fmt, tmp);
+	ret = vsnprintf(stack, sizeof(stack), fmt, tmp);
 	va_end(tmp);
+
+	/*
+	 * make sure to always issue a write() syscall, even if writing an empty string,
+	 * otherwise values in sysfs like module parameters don't really get overwritten
+	 */
+	if (igt_debug_on(ret == 0))
+		return igt_writen(fd, "", 1);
+
 	if (igt_debug_on(ret < 0))
 		return -EINVAL;
 
-- 
2.43.0