From: Bruno Prémont on
When dumping /sys/kernel/debug/hid/$device/events '\0' characters show up
(invisible if cat to console but shown by less or while looking at a dump
These are due to hid_debug_event() adding strlen()+1 bytes to the ring
buffer (e.g. including the trailing '\0').
Any roll-over causes a '\0' as well as hid_debug_event() handles the ring
buffers with HID_DEBUG_BUFSIZE-1 size while hid_debug_events_read() handles
it with full HID_DEBUG_BUFSIZE size.

Signed-off-by: Bruno Prémont <bonbons(a)>
The ring buffer overflow case (when tail crosses head) seems to be
suboptimal at best.
Would there be a good way to mark those cases so reader can know where
data got lost. (though this might not be easy keeping the lockless

diff --git a/drivers/hid/hid-debug.c b/drivers/hid/hid-debug.c
index 6abd036..2602695 100644
--- a/drivers/hid/hid-debug.c
+++ b/drivers/hid/hid-debug.c
@@ -564,10 +564,10 @@ void hid_debug_event(struct hid_device *hdev, char *buf)
struct hid_debug_list *list;

list_for_each_entry(list, &hdev->debug_list, node) {
- for (i = 0; i <= strlen(buf); i++)
- list->hid_debug_buf[(list->tail + i) % (HID_DEBUG_BUFSIZE - 1)] =
+ for (i = 0; i < strlen(buf); i++)
+ list->hid_debug_buf[(list->tail + i) % HID_DEBUG_BUFSIZE] =
- list->tail = (list->tail + i) % (HID_DEBUG_BUFSIZE - 1);
+ list->tail = (list->tail + i) % HID_DEBUG_BUFSIZE;
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