Subject: Re: Question regarding Interrupt "delivery" to user mode process

From: Tolunay Orkun <[email-masked]>
Date: Fri, 25 Mar 2005 13:42:58 -0600

Eugene Surovegin wrote:

>On Fri, Mar 25, 2005 at 11:40:16AM -0600, Tolunay Orkun wrote:
>
>[snip]
>
>  
>
>>There is a quirk for PPC405 however: Linux (2.4) calls ack_irq() before 
>>branching to the IRQ handler. However, if irq is level triggered and 
>>external interrupt source has not yet deasserted, the interrupt status 
>>bit in interrupt status register will remain set! To avoid spurious 
>>interrupt it is necessary to call ack_irq() again before enabling the 
>>interrupts again. I had discussed this in the old linuxppc-embedded list 
>>while I was doing this driver.
>>    
>>
>
>This isn't 405 specific. This problem will exist on any system with 
>level-sensitive IRQ source which wasn't ACK'ed. ACK'ed here means 
>acknowledgment in device itself, not in PIC.
>
>This is why this user-space IRQ handling is a bad idea, IMHO. You have 
>to ACK IRQ (in device itself) in kernel-IRQ handler.
>
>--
>Eugene
>  
>
Well, ACK'ing the IRQ in the kernel IRQ handler was impractical for us 
because you have to communicate using I2C (sloooow) and multiple devices 
of the same types is hooked to the same IRQ so we need to poll them to 
see which one has actually generated the the IRQ. That means many Nx I2C 
reads and 1x I2C write. Furthermore, N is a variable as I2C devices are 
hot plugged or removed from the bus so when nobody claims ownership we 
need to probe for new instance of device. Ugly but much better than 
purely polled operation...

Best regards,
Tolunay