From: Paul Mackerras <paulus@samba.org>
To: dbrownell@users.sourceforge.net, greg@kroah.com
Cc: linux-kernel@vger.kernel.org
Subject: unnecessary use of set_bit
Date: Sat, 27 Apr 2002 22:26:02 +1000 (EST) [thread overview]
Message-ID: <15562.39130.683869.175699@argo.ozlabs.ibm.com> (raw)
The ohci_hub_status_data() procedure in drivers/usb/host/ohci-hub.c in
2.5.11 is broken in a couple of ways: it uses set_bit on a char *
address and it assumes little-endian byte order in the bitmap.
Here is a patch to fix both problems. As a bonus, the file ends up
one line shorter. :)
Please, people, don't use set_bit when you don't need an atomic
operation. set_bit(&x, n) is slower than x |= 1 << n on a lot of
platforms, including SMP x86 I believe. (If the bitmap is more than
one word you can use __set_bit if you don't require atomicity.)
Paul.
diff -urN linux-2.5/drivers/usb/host/ohci-hub.c pmac-2.5/drivers/usb/host/ohci-hub.c
--- linux-2.5/drivers/usb/host/ohci-hub.c Sat Apr 27 20:51:31 2002
+++ pmac-2.5/drivers/usb/host/ohci-hub.c Sat Apr 27 15:03:06 2002
@@ -67,7 +67,7 @@
ohci_hub_status_data (struct usb_hcd *hcd, char *buf)
{
struct ohci_hcd *ohci = hcd_to_ohci (hcd);
- int ports, i, changed = 0, length = 1;
+ int ports, i, mask = 0, length = 1;
ports = roothub_a (ohci) & RH_A_NDP;
if (ports > MAX_ROOT_PORTS) {
@@ -80,13 +80,7 @@
/* init status */
if (roothub_status (ohci) & (RH_HS_LPSC | RH_HS_OCIC))
- buf [0] = changed = 1;
- else
- buf [0] = 0;
- if (ports > 7) {
- buf [1] = 0;
- length++;
- }
+ mask = 1;
/* look at each port */
for (i = 0; i < ports; i++) {
@@ -94,12 +88,17 @@
status &= RH_PS_CSC | RH_PS_PESC | RH_PS_PSSC
| RH_PS_OCIC | RH_PS_PRSC;
- if (status) {
- changed = 1;
- set_bit (i + 1, buf);
- }
+ if (status)
+ mask |= 1 << (i + 1);
+ }
+ if (!mask)
+ return 0;
+ buf[0] = mask;
+ if (ports > 7) {
+ length++;
+ buf[1] = mask >> 8;
}
- return changed ? length : 0;
+ return length;
}
/*-------------------------------------------------------------------------*/
next reply other threads:[~2002-04-27 12:27 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-04-27 12:26 Paul Mackerras [this message]
2002-04-27 14:19 ` unnecessary use of set_bit David Brownell
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