Subject: Re: [PATCH] 2.6.18-rt7: PowerPC: fix breakage in threaded fasteoi type IRQ handlers

From: Sergei Shtylyov <[email-masked]>
Date: Sun, 19 Nov 2006 23:40:01 +0300

Hello.

Benjamin Herrenschmidt wrote:

>>    I must not that this whole ack() vs eoi() stuff is misleading. For example,
>>in 8259 driver, mask_ack() method actually sends EOI to PIC, not ACK's an IRQ 
>>-- the actual ACK is implicit on x86 and is used to read the interrupt vector 
>>form 8259 on PPC. So, IMO, there probably should only have been either ack() 
>>or eoi() method in the first place. Though I'm not familiar with ARM from 
>>which genirq stuff originated...

> They are different concepts. Ack clears the event on the PIC, it's
> tyically necessary for resetting the edge detection logic for edge
> interrupts and has to happen before the handler is called.

    I know 8259. :-)
    It also resets the corresponding IRQ bit in IRR, and sets it in ISR where 
it's then cleared on EOI command.

> On MPIC or XICS, this is implicit by reading the vector. On some more
> dumb controllers, this has to be done explicitely.

    This is not implicit -- CPU has to read INTACK reg. on OpenPIC. Really 
implicit method is in action on x86 where CPU issues dual ACK bus cycle to get 
the vector form 8259...

> EOI is a more "high level" thing that some "intelligent" PICs that
> automatically raise the priority do to restore the priority to what it
> was before the interrupt occured.

    Thank you, I know. Even 8259 has the notion of priority and EOI works the 
same way here.

> Ben.

WBR, Sergei