On Mon, Aug 17, 2020 at 04:49:32PM +0300, Felipe Balbi wrote:

Hi,

rentao.bupt@gmail.com writes:

quoted

From: Tao Ren <redacted>

This is a follow-on patch for commit a23be4ed8f48 ("usb: gadget: aspeed:
improve vhub port irq handling"): for_each_set_bit() is replaced with
simple for() loop because for() loop runs faster on ASPEED BMC.

Signed-off-by: Tao Ren <redacted>
---
 drivers/usb/gadget/udc/aspeed-vhub/core.c | 10 +++-------
 drivers/usb/gadget/udc/aspeed-vhub/vhub.h |  3 +++
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/drivers/usb/gadget/udc/aspeed-vhub/core.c b/drivers/usb/gadget/udc/aspeed-vhub/core.c
index cdf96911e4b1..be7bb64e3594 100644
--- a/drivers/usb/gadget/udc/aspeed-vhub/core.c
+++ b/drivers/usb/gadget/udc/aspeed-vhub/core.c

@@ -135,13 +135,9 @@ static irqreturn_t ast_vhub_irq(int irq, void *data)
 
 	/* Handle device interrupts */
 	if (istat & vhub->port_irq_mask) {
-		unsigned long bitmap = istat;
-		int offset = VHUB_IRQ_DEV1_BIT;
-		int size = VHUB_IRQ_DEV1_BIT + vhub->max_ports;
-
-		for_each_set_bit_from(offset, &bitmap, size) {
-			i = offset - VHUB_IRQ_DEV1_BIT;
-			ast_vhub_dev_irq(&vhub->ports[i].dev);
+		for (i = 0; i < vhub->max_ports; i++) {
+			if (istat & VHUB_DEV_IRQ(i))
+				ast_vhub_dev_irq(&vhub->ports[i].dev);

how have you measured your statement above? for_each_set_bit() does
exactly what you did. Unless your architecture has an instruction which
helps finds the next set bit (like cls on ARM), which, then, makes it
much faster.

I did some testing and result shows for() loop runs faster than
for_each_set_bit() loop. Please refer to details below (discussion with
Benjamin in the original patch) and kindly let me know your suggestions.

On Mon, 2020-04-06 at 23:02 -0700, Tao Ren wrote:

quoted

I ran some testing on my ast2400 and ast2500 BMC and looks like the
for() loop runs faster than for_each_set_bit_from() loop in my
environment. I'm not sure if something needs to be revised in my test
code, but please kindly share your suggestions:

I use get_cycles() to calculate execution time of 2 different loops, and
ast_vhub_dev_irq() is replaced with barrier() to avoid "noise"; below
are the results:

  - when downstream port number is 5 and only 1 irq bit is set, it takes
    ~30 cycles to finish for_each_set_bit() loop, and 20-25 cycles to
    finish the for() loop.

  - if downstream port number is 5 and all 5 bits are set, then
    for_each_set_bit() loop takes ~50 cycles and for() loop takes ~25
    cycles.

  - when I increase downsteam port number to 16 and set 1 irq bit, the
    for_each_set_bit() loop takes ~30 cycles and for() loop takes 25
    cycles. It's a little surprise to me because I thought for() loop
    would cost 60+ cycles (3 times of the value when port number is 5).

  - if downstream port number is 16 and all irq status bits are set,
    then for_each_set_bit() loop takes 60-70 cycles and for() loop takes
    30+ cycles.

Cheers,

Tao