> -----Original Message-----
> From: H. Peter Anvin [mailto:h...@zytor.com]
> Sent: Friday, April 11, 2014 3:36 PM
> To: KY Srinivasan; x...@kernel.org; gre...@linuxfoundation.org; linux-
> ker...@vger.kernel.org; de...@linuxdriverproject.org; o...@aepfle.de;
> a...@canonical.com; jasow...@redhat.com; t...@linutronix.de;
> jbeul...@suse.com; b...@alien8.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
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