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From:   Felipe Balbi <balbi@kernel.org>
To:     Tao Ren <rentao.bupt@gmail.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Joel Stanley <joel@jms.id.au>,
        Andrew Jeffery <andrew@aj.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Chunfeng Yun <chunfeng.yun@mediatek.com>,
        Stephen Boyd <swboyd@chromium.org>, linux-usb@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-aspeed@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        openbmc@lists.ozlabs.org, taoren@fb.com
Subject: Re: [PATCH] usb: gadget: aspeed: fixup vhub port irq handling
In-Reply-To: <20200817225603.GA21228@taoren-ubuntu-R90MNF91>
References: <20200528011154.30355-1-rentao.bupt@gmail.com>
 <875z9hz8k3.fsf@kernel.org>
 <20200817225603.GA21228@taoren-ubuntu-R90MNF91>
Date:   Mon, 31 Aug 2020 12:56:22 +0300
Message-ID: <87v9gz9m0p.fsf@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


Hi,

Tao Ren <rentao.bupt@gmail.com> writes:
>> > 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.

no strong feelings, just surprised you're already worried about 20~40
cycles of cpu time ;-)

patch queued for next merge window

-- 
balbi