linux-kernel - Re: [PATCH] usb: gadget: aspeed: fixup vhub port irq handling

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Date:   Mon, 31 Aug 2020 16:26:08 -0700
From:   Tao Ren <rentao.bupt@...il.com>
To:     Felipe Balbi <balbi@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joel Stanley <joel@....id.au>,
        Andrew Jeffery <andrew@...id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Chunfeng Yun <chunfeng.yun@...iatek.com>,
        Stephen Boyd <swboyd@...omium.org>, linux-usb@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-aspeed@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        openbmc@...ts.ozlabs.org, taoren@...com
Subject: Re: [PATCH] usb: gadget: aspeed: fixup vhub port irq handling

On Mon, Aug 31, 2020 at 12:54:57PM +0300, Felipe Balbi wrote:
> 
> Hi,
> 
> Tao Ren <rentao.bupt@...il.com> writes:
> > On Mon, Aug 17, 2020 at 04:49:32PM +0300, Felipe Balbi wrote:
> >> 
> >> Hi,
> >> 
> >> rentao.bupt@...il.com writes:
> >> > From: Tao Ren <rentao.bupt@...il.com>
> >> >
> >> > 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 <rentao.bupt@...il.com>
> >> > ---
> >> >  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.
> 
> no strong feelings, just surprised that you're already worried about
> 20~40 cycles of cpu time ;-)
> 
> Patch applied for next merge window

Thanks Felipe. Ben had some concerns about interrupt handling cost on
AST2400 BMC (ARM9), hence we did the comparison and noticed the small
difference :)


Cheers,

Tao