linux-kernel - [PATCH 3.16 09/16] HID: debug: improve hid_debug

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Date:   Fri, 22 Mar 2019 05:20:18 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Jiri Kosina" <jkosina@...e.cz>,
        "Rasmus Villemoes" <linux@...musvillemoes.dk>
Subject: [PATCH 3.16 09/16] HID: debug: improve hid_debug_event()

3.16.64-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Rasmus Villemoes <linux@...musvillemoes.dk>

commit 92529623d242cea4440958d7bcebdf291f4ab15e upstream.

The code in hid_debug_event() causes horrible code generation. First,
we do a strlen() call for every byte we copy (we're doing a store to
global memory, so gcc has no way of proving that strlen(buf) doesn't
change). Second, since both i, list->tail and HID_DEBUG_BUFSIZE have
signed type, the modulo computation has to take into account the
possibility that list->tail+i is negative, so it's not just a simple
and.

Fix the former by simply not doing strlen() at all (we have to load
buf[i] anyway, so testing it is almost free) and the latter by
changing i to unsigned. This cuts 29% (69 bytes) of the size of the
function.

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
Signed-off-by: Jiri Kosina <jkosina@...e.cz>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/hid/hid-debug.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/hid/hid-debug.c
+++ b/drivers/hid/hid-debug.c
@@ -658,13 +658,13 @@ EXPORT_SYMBOL_GPL(hid_dump_device);
 /* enqueue string to 'events' ring buffer */
 void hid_debug_event(struct hid_device *hdev, char *buf)
 {
-	int i;
+	unsigned i;
 	struct hid_debug_list *list;
 	unsigned long flags;

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