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

Thomas Gleixner
2021-05-14T13:01:54Z

David,

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

>> -----Original Message-----
>> From: Maximilian Luz <luzmaximilian@gmail.com>
>> Sent: 13 May 2021 11:12
>> To: David Laight <David.Laight@ACULAB.COM>; Thomas Gleixner <tglx@linutronix.de>; Ingo Molnar
>> <mingo@redhat.com>; Borislav Petkov <bp@alien8.de>
>> Cc: H. Peter Anvin <hpa@zytor.com>; Sachi King <nakato@nakato.io>; x86@kernel.org; linux-
>> kernel@vger.kernel.org; stable@vger.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