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

[Thomas Gleixner]

David,

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

>> -----Original Message-----
>> From: Maximilian Luz <luzmaximilian@xxxxxxxxx>
>> Sent: 13 May 2021 11:12
>> To: David Laight <David.Laight@xxxxxxxxxx>; Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Ingo Molnar
>> <mingo@xxxxxxxxxx>; Borislav Petkov <bp@xxxxxxxxx>
>> Cc: H. Peter Anvin <hpa@xxxxxxxxx>; Sachi King <nakato@xxxxxxxxx>; x86@xxxxxxxxxx; linux-
>> kernel@xxxxxxxxxxxxxxx; stable@xxxxxxxxxxxxxxx
>> 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