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

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Date:	Mon, 14 Apr 2014 09:15:14 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"K. Y. Srinivasan" <kys@...rosoft.com>
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).

Jan

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