Re: [PATCH 4/4] usb: atmel_usba_udc: mask status with enabled irqs

[Boris Brezillon <boris.brezillon-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>]

Hi David,

On Mon, 15 Dec 2014 13:34:56 +0000
David Laight <David.Laight-ZS65k/vG3HxXrIkS9f7CXA@public.gmane.org> wrote:

> From: Sergei Shtylyov
> > Hello.
> > 
> > On 12/15/2014 4:03 PM, Boris Brezillon wrote:
> > 
> > > Avoid interpreting useless status flags when we're not waiting for such
> > > events by masking the status variable with the interrupt enabled register
> > > value.
> > 
> > > Reported-by: Patrice VILCHEZ <patrice.vilchez-AIFe0yeh4nAAvxtiuMwx3w@public.gmane.org>
> > > Signed-off-by: Boris Brezillon <boris.brezillon-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>
> > > ---
> > >   drivers/usb/gadget/udc/atmel_usba_udc.c | 6 +++++-
> > >   1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > > diff --git a/drivers/usb/gadget/udc/atmel_usba_udc.c b/drivers/usb/gadget/udc/atmel_usba_udc.c
> > > index 55c8dde..bc3a532 100644
> > > --- a/drivers/usb/gadget/udc/atmel_usba_udc.c
> > > +++ b/drivers/usb/gadget/udc/atmel_usba_udc.c
> > > @@ -1612,12 +1612,14 @@ static irqreturn_t usba_udc_irq(int irq, void *devid)
> > >
> > >   	spin_lock(&udc->lock);
> > >
> > > -	status = usba_readl(udc, INT_STA);
> > > +	status = usba_readl(udc, INT_STA) & usba_readl(udc, INT_ENB);
> > >   	DBG(DBG_INT, "irq, status=%#08x\n", status);
> > >
> > >   	if (status & USBA_DET_SUSPEND) {
> > >   		toggle_bias(udc, 0);
> > >   		usba_writel(udc, INT_CLR, USBA_DET_SUSPEND);
> > > +		usba_writel(udc, INT_ENB,
> > > +			    usba_readl(udc, INT_ENB) | USBA_WAKE_UP);
> > >   		udc->bias_pulse_needed = true;
> > >   		DBG(DBG_BUS, "Suspend detected\n");
> > >   		if (udc->gadget.speed != USB_SPEED_UNKNOWN
> > > @@ -1631,6 +1633,8 @@ static irqreturn_t usba_udc_irq(int irq, void *devid)
> > >   	if (status & USBA_WAKE_UP) {
> > >   		toggle_bias(udc, 1);
> > >   		usba_writel(udc, INT_CLR, USBA_WAKE_UP);
> > > +		usba_writel(udc, INT_ENB,
> > > +			    usba_readl(udc, INT_ENB) & ~USBA_WAKE_UP);
> > >   		DBG(DBG_BUS, "Wake Up CPU detected\n");
> > >   	}
> > 
> >     Looks like t make sense to read the INT_ENB register into a separate
> > variable, to save on extra reads?
> 
> 
> Better still remember the written value in one of the structures so
> that it doesn't have to be read at all.

Hmm, I'm getting back to this suggestion.
While I definitely understand why I should use a local variable to
store INT_ENB value in usba_udc_irq, I don't see the point of mirroring
INT_EN status in an udc struct field (after all, INT_EN will always
contain the value we previously set).
Is this a performance concern ?

Regards,

Boris


-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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