linux-kernel - RE: [PATCH V2 1/1] X86: Probe for PIC and set legacy

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Date:	Mon, 14 Apr 2014 15:46:18 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>
Subject: RE: [PATCH V2 1/1] X86: Probe for PIC and set legacy_pic
 appropriately



> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@...e.com]
> Sent: Monday, April 14, 2014 1:15 AM
> To: KY Srinivasan
> Cc: olaf@...fle.de; apw@...onical.com; x86@...nel.org;
> tglx@...utronix.de; devel@...uxdriverproject.org;
> gregkh@...uxfoundation.org; jasowang@...hat.com; linux-
> kernel@...r.kernel.org; hpa@...or.com
> Subject: Re: [PATCH V2 1/1] X86: Probe for PIC and set legacy_pic
> appropriately
> 
> >>> On 12.04.14 at 07:56, <kys@...rosoft.com> wrote:
> > --- a/arch/x86/kernel/i8259.c
> > +++ b/arch/x86/kernel/i8259.c
> > @@ -299,11 +299,30 @@ static void unmask_8259A(void)  static void
> > init_8259A(int auto_eoi)  {
> >  	unsigned long flags;
> > +	unsigned char probe_val = ~(1 << PIC_CASCADE_IR);
> > +	unsigned char new_val;
> >
> >  	i8259A_auto_eoi = auto_eoi;
> >
> >  	raw_spin_lock_irqsave(&i8259A_lock, flags);
> >
> > +	/*
> > +	 * Check to see if we have a PIC.
> > +	 * Mask all except the cascade and read
> > +	 * back the value we just wrote. If we don't
> > +	 * have a PIC, we will read 0xff as opposed to the
> > +	 * value we wrote.
> > +	 */
> > +	outb(0xff, PIC_SLAVE_IMR);	/* mask all of 8259A-2 */
> > +	outb(probe_val, PIC_MASTER_IMR);
> > +	new_val = inb(PIC_MASTER_IMR);
> > +	if (probe_val != new_val) {
> > +		printk(KERN_INFO "Using NULL legacy PIC\n");
> > +		legacy_pic = &null_legacy_pic;
> > +		raw_spin_unlock_irqrestore(&i8259A_lock, flags);
> > +		return;
> > +	}
> > +
> >  	outb(0xff, PIC_MASTER_IMR);	/* mask all of 8259A-1 */
> >  	outb(0xff, PIC_SLAVE_IMR);	/* mask all of 8259A-2 */
> 
> And I guess you should delete this last line now that this is already being
> done slightly earlier - having it done twice is possibly going to confuse future
> readers (in that they might ask themselves or others whether this really
> needs to be done twice).

Good point! I will fix this and resend.

K. Y
> 
> Jan

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