Subject: Re: [RFC] 85XX: Allow 8259 cascade to share an MPIC interrupt line.

From: Randy Vinson <[email-masked]>
Date: Thu, 07 Jun 2007 12:25:16 -0700

Sergei Shtylyov wrote:
> Hello.
Hi.

> 
> Randy Vinson wrote:
> 
>> The Freescale MPC8555CDS and MPC8548CDS reference hardware has a legacy
>> 8259 interrupt controller pair contained within a VIA VT82C686B
>> Southbridge
>> on the main carrier board. The processor complex plugs into the carrier
>> card using a PCI slot which limits the available interrupts to the
>> INTA-INTD PCI interrupts. The output of the 8259 cascade pair is routed
>> through a gate array and connected to the PCI INTA interrupt line.
>> The normal interrupt chaining hook (set_irq_chained_handler) does
>> not allow sharing of the chained interrupt which prevents the
>> use of PCI INTA by PCI devices. This patch allows the 8259 cascade
>> pair to share their interrupt line with PCI devices.
> 
>    Hmm, I see you've come up with an interesting solution. :-)
Well, I'm not sure how interesting it is, but it works :)

> 
>> Signed-off-by: Randy Vinson <[email-masked]>
>> ---
>> Note that there may very well be a better way of accomplishing this.
>> If someone
>> has a better alternative, I'm open to it. This was just the simplest
>> way I could
>> get this to work.
>>
>> Also, the addition of the .end routine for the MPIC is not strictly
>> necessary for
>> this patch. It's there so this code will run from within the threaded
>> interrupt
>> context used by the Real Time 
> 
>    Hmmm, why would you need that? Where RT is different from the
> vanilla? :-O
The threaded interrupt system has threaded routines for the 4 standard
interrupts types used by the generic IRQ system (fasteoi, edge, level
and simple), plus a handler for non-generic interrupts (the ones that
still use __do_IRQ.) When interrupts are being threaded and a
non-cascaded MPIC interrupt occurs, desc->handler normally points to
handle_fasteoi_irq which masks the interrupt source, schedules the
corresponding IRQ thread, issues an EOI and returns to the interrupted
context. At a later time, the scheduler dispatches the IRQ thread which
calls a threaded version of the fasteoi handler. The threaded fasteoi
routine processes the action chain and calls .unmask to re-enable the
interrupt before returning to the calling thread. Once the interrupt has
been unmasked, the entire process is free to repeat as needed.

After processing a possible 8259 interrupt, the 8259 cascade handler
calls handle_fasteoi_irq to perform the actions noted above. However,
with the 8259 cascade handler hooked to the interrupt, the threaded IRQ
handler doesn't recognize it as one of the 4 standard generic IRQ types
and calls the threaded version of do_irq instead of the threaded fasteoi
handler. Once the threaded do_irq handler completes the action list, it
uses the .end routine to restart the interrupt flow. Pointing .end to
the standard MPIC unmask routine allows it to balance the interrupt mask
operation performed by handle_fasteoi_irq before it scheduled the IRQ
thread.

> 
>>  arch/powerpc/platforms/85xx/mpc85xx_cds.c |   32
>> +++++++++++++++++++++++++---
>>  arch/powerpc/sysdev/mpic.c                |    1 +
>>  2 files changed, 29 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/powerpc/platforms/85xx/mpc85xx_cds.c
>> b/arch/powerpc/platforms/85xx/mpc85xx_cds.c
>> index 1490eb3..431aaa2 100644
>> --- a/arch/powerpc/platforms/85xx/mpc85xx_cds.c
>> +++ b/arch/powerpc/platforms/85xx/mpc85xx_cds.c
>> @@ -127,16 +127,30 @@ static void __init mpc85xx_cds_pcibios_fixup(void)
>>  }
>>  
>>  #ifdef CONFIG_PPC_I8259
>> -#warning The i8259 PIC support is currently broken
>> -static void mpc85xx_8259_cascade(unsigned int irq, struct irq_desc
>> *desc)
>> +static void mpc85xx_8259_cascade_handler(unsigned int irq,
>> +                     struct irq_desc *desc)
>>  {
>>      unsigned int cascade_irq = i8259_irq();
>>  
>>      if (cascade_irq != NO_IRQ)
>> +        /* handle an interrupt from the 8259 */
>>          generic_handle_irq(cascade_irq);
>>  
>> -    desc->chip->eoi(irq);
>> +    /* check for any interrupts from the shared IRQ line */
>> +    handle_fasteoi_irq(irq, desc);
>>  }
>> +
>> +static irqreturn_t mpc85xx_8259_cascade_action(int irq, void *dev_id)
>> +{
>> +    return IRQ_HANDLED;
>> +}
> 
>    Well, mpc85xx_8259_intr() would probably be more in line with the
> code elsewhere... and you could keep the mpc85xx_8259_cascade() name then.
I chose the names to show that the two routines run at different levels
within the interrupt subsystem. The cascade handler always runs in
interrupt context while the cascade action can run in interrupt context
or in threaded context. While I was researching this, I kept getting
confused between the desc->handler and action->handler fields so I named
them different to make it easier for me to keep the differences clear in
my head. I can change the names it if you think it's important.

Randy V.