linux-kernel - [PATCH] media: usb: ttusb-dec: reduce the number of memory reads in ttusb_dec_handle

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Date:   Thu,  7 May 2020 18:25:21 +0800
From:   Jia-Ju Bai <baijiaju1990@...il.com>
To:     mchehab@...nel.org, gregkh@...uxfoundation.org,
        allison@...utok.net, tglx@...utronix.de, rfontana@...hat.com,
        tomasbortoli@...il.com, sean@...s.org
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jia-Ju Bai <baijiaju1990@...il.com>
Subject: [PATCH] media: usb: ttusb-dec: reduce the number of memory reads in ttusb_dec_handle_irq()

In ttusb_dec_handle_irq(), buffer[4] is continuously read from memory 
three times, without being modified.
To reduce the number of memory reads, buffer[4] is first assigned to a
local variable index, and then index is used to replace buffer[4].

Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
---
 drivers/media/usb/ttusb-dec/ttusb_dec.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/media/usb/ttusb-dec/ttusb_dec.c b/drivers/media/usb/ttusb-dec/ttusb_dec.c
index 3198f9624b7c..8543c552515b 100644
--- a/drivers/media/usb/ttusb-dec/ttusb_dec.c
+++ b/drivers/media/usb/ttusb-dec/ttusb_dec.c
@@ -250,6 +250,7 @@ static void ttusb_dec_handle_irq( struct urb *urb)
 	struct ttusb_dec *dec = urb->context;
 	char *buffer = dec->irq_buffer;
 	int retval;
+	int index = buffer[4];
 
 	switch(urb->status) {
 		case 0: /*success*/
@@ -281,11 +282,11 @@ static void ttusb_dec_handle_irq( struct urb *urb)
 		 * this should/could be added later ...
 		 * for now lets report each signal as a key down and up
 		 */
-		if (buffer[4] - 1 < ARRAY_SIZE(rc_keys)) {
-			dprintk("%s:rc signal:%d\n", __func__, buffer[4]);
-			input_report_key(dec->rc_input_dev, rc_keys[buffer[4] - 1], 1);
+		if (index - 1 < ARRAY_SIZE(rc_keys)) {
+			dprintk("%s:rc signal:%d\n", __func__, index);
+			input_report_key(dec->rc_input_dev, rc_keys[index - 1], 1);
 			input_sync(dec->rc_input_dev);
-			input_report_key(dec->rc_input_dev, rc_keys[buffer[4] - 1], 0);
+			input_report_key(dec->rc_input_dev, rc_keys[index - 1], 0);
 			input_sync(dec->rc_input_dev);
 		}
 	}
-- 
2.17.1