linux-kernel - [PATCH 1/2] usbip: fix off-by-one frame number calculation

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Message-Id: <20171107104010.2884193-1-arnd@arndb.de>
Date:   Tue,  7 Nov 2017 11:39:56 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Valentina Manea <valentina.manea.m@...il.com>,
        Shuah Khan <shuah@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Krzysztof Opasiak <k.opasiak@...sung.com>,
        Johan Hovold <johan@...nel.org>, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] usbip: fix off-by-one frame number calculation

vgadget_get_frame returns a frame number from 0 to 2046, which
may require an expensive division operation to wrap at one lower
than the usual number.

I can't see any reason for this, and all other drivers wrap at
a power-of-two number. My best explanation is that it was a simple
typo, so I'm changing the % modulo operator into a cheaper bitmask
that the other drivers use, to make it wrap after 0x7ff rather than
before it.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 drivers/usb/usbip/vudc_dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/usbip/vudc_dev.c b/drivers/usb/usbip/vudc_dev.c
index 0c07348820ea..16fb4f85a6f6 100644
--- a/drivers/usb/usbip/vudc_dev.c
+++ b/drivers/usb/usbip/vudc_dev.c
@@ -145,7 +145,7 @@ static int vgadget_get_frame(struct usb_gadget *_gadget)
 	do_gettimeofday(&now);
 	return ((now.tv_sec - udc->start_time.tv_sec) * 1000 +
 			(now.tv_usec - udc->start_time.tv_usec) / 1000)
-			% 0x7FF;
+			& 0x7FF;
 }
 
 static int vgadget_set_selfpowered(struct usb_gadget *_gadget, int value)
-- 
2.9.0