linux-kernel - RE: [PATCH] x86/i8259: Work around buggy legacy PIC

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Message-ID: <87tun54gg3.ffs@nanos.tec.linutronix.de>
Date:   Fri, 14 May 2021 15:01:48 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     David Laight <David.Laight@...LAB.COM>,
        'Maximilian Luz' <luzmaximilian@...il.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Cc:     "H. Peter Anvin" <hpa@...or.com>, Sachi King <nakato@...ato.io>,
        "x86\@kernel.org" <x86@...nel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable\@vger.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH] x86/i8259: Work around buggy legacy PIC

David,

On Thu, May 13 2021 at 10:36, David Laight wrote:

>> -----Original Message-----
>> From: Maximilian Luz <luzmaximilian@...il.com>
>> Sent: 13 May 2021 11:12
>> To: David Laight <David.Laight@...LAB.COM>; Thomas Gleixner <tglx@...utronix.de>; Ingo Molnar
>> <mingo@...hat.com>; Borislav Petkov <bp@...en8.de>
>> Cc: H. Peter Anvin <hpa@...or.com>; Sachi King <nakato@...ato.io>; x86@...nel.org; linux-
>> kernel@...r.kernel.org; stable@...r.kernel.org
>> Subject: Re: [PATCH] x86/i8259: Work around buggy legacy PIC

can you please fix your mail client and spare us the useless header
duplication in the reply?

> It is also worth noting that the probe code is spectacularly crap.
> It writes 0xff and then checks that 0xff is read back.
> Almost anything (including a failed PCIe read to the ISA bridge)
> will return 0xff and make the test pass.

        unsigned char probe_val = ~(1 << PIC_CASCADE_IR);

	outb(probe_val, PIC_MASTER_IMR);
	new_val = inb(PIC_MASTER_IMR);

How is that writing 0xFF?

Thanks,

        tglx