linux-kernel - Re: parport_pc: irq= parameter with limited usefulness, kernel hang on PPC/OldWorldMac

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Message-ID: <4C9264DD.5050903@math.tu-berlin.de>
Date:	Thu, 16 Sep 2010 20:41:33 +0200
From:	Thomas Richter <thor@...h.tu-berlin.de>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	linux-kernel@...r.kernel.org, benh@...nel.crashing.org
Subject: Re: parport_pc: irq= parameter with limited usefulness, kernel hang
 on PPC/OldWorldMac

Thanks Alan, hi Ben,

>> *) Under at least 2.6.31 and up, the kernel assigns IRQ 25 to the card. 
>> Under 2.6.28 and below, the card
>> was run in polling mode (i.e. irq was -1, not 25).
>>
>> *) Furthermore, the above report suggests that the system wants to 
>> handle an edge-triggered IRQ, though
>> according to /proc/interrupts, IRQ 25 is level-triggered.
>>     
>
> The parport_pc code just asks for the IRQ so any level/edge mismatch
> would appear to be coming from the firmware or architecture code. The
> older kernel didn't know to use the IRQ on PCI devices which stopped some
> things working and hurt performance. That would also have masked the bug
> you see - so the report makes total sense.
>   

I see, thanks.


>> *) parport_pc should take irq=none seriously and should not silently 
>> override it,
>>     
>
> parport_pc could do with a way to specify whether to use the IRQ or DMA
> on PCI ports but really this shouldn't be needed if the rest of the
> kernel logic is right.
>
>   
>> *) parport_pc should at least not crash the system completely in case 
>> the IRQ is enabled.
>>
>> The latter *might* be an issue with the G3 powermac hardware, which is 
>> of course weird and has a couple of hardware
>>     
>
> The latter looks like a bug in the PPC side support, perhaps misreporting
> an IRQ, or mislabelling it in some way. I think the first step is to find
> a PPC hacker who can figure out why the apparently sane IRQ request from
> the parport layer is exploding.
>
> Even if there is some technical reason the IRQ can't be used then the arch
> code shouldn't be reporting an IRQ for it, and the parport_pc could ought
> to just work.
>
> Adding Ben to the Cc: so we it can get hunted down.
Ben, anything I can do to find this bug? The current state is
"workaround that works ok for me", but that's not very satisfying.

Greetings,
    Thomas


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