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

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Date:	Fri, 11 Apr 2014 22:49:45 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"JBeulich@...e.com" <JBeulich@...e.com>,
	"bp@...en8.de" <bp@...en8.de>
CC:	Thomas Gleixner <tglx@...utronix.de>
Subject: RE: [PATCH 1/1] X86: Probe for PIC and set legacy_pic appropriately



> -----Original Message-----
> From: H. Peter Anvin [mailto:hpa@...or.com]
> Sent: Friday, April 11, 2014 3:36 PM
> To: KY Srinivasan; x86@...nel.org; gregkh@...uxfoundation.org; linux-
> kernel@...r.kernel.org; devel@...uxdriverproject.org; olaf@...fle.de;
> apw@...onical.com; jasowang@...hat.com; tglx@...utronix.de;
> JBeulich@...e.com; bp@...en8.de
> Cc: Thomas Gleixner
> Subject: Re: [PATCH 1/1] X86: Probe for PIC and set legacy_pic appropriately
> 
> On 04/11/2014 04:02 PM, K. Y. Srinivasan wrote:
> >
> > diff --git a/arch/x86/kernel/i8259.c b/arch/x86/kernel/i8259.c index
> > 2e977b5..0a57a19 100644
> > --- a/arch/x86/kernel/i8259.c
> > +++ b/arch/x86/kernel/i8259.c
> > @@ -299,11 +299,22 @@ static void unmask_8259A(void)  static void
> > init_8259A(int auto_eoi)  {
> >  	unsigned long flags;
> > +	unsigned char probe_val = 0xfb;
> >
> >  	i8259A_auto_eoi = auto_eoi;
> >
> >  	raw_spin_lock_irqsave(&i8259A_lock, flags);
> >
> > +	/* Check to see if we have a PIC */
> > +	outb(probe_val, PIC_MASTER_IMR);
> > +	probe_val = inb(PIC_MASTER_IMR);
> > +	if (probe_val == 0xff) {
> > +		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 */
> >
> 
> I think it is important to document what this actually means here.  0xfb is
> "mask all except for the cascade."
> 
> Arguably, we should do this after masking the slave PIC.
> 
> On a whole other subject... the whole __byte() logic is a just plain horrific
> piece of crap.  Clearly whomever wrote it had never heard of a union.
> 
> I'm still horrifically confused, though, why we mask IRQ 2 (the cascade
> interrupt) in i8259.c, but we explicitly do *not* mask IRQ 2 in boot/pm.c (nor
> in the assembly code on which boot/pm.c was based.)  In fact, it looks like we
> never unmask it.  I'm guessing that the masking status of the cascade input
> simply doesn't matter.
> 
> At the very least, though, we shouldn't have 0xfb as a magic number, but use
> ~(1U << PIC_CASCADE_IR).

I will get rid of this magic number as you have suggested.

K. Y