Subject: Re: powerpc/cell/axon-msi: fix MSI after kexec

From: Michael Ellerman <[email-masked]>
Date: Mon, 15 Dec 2008 15:30:26 +1100

On Fri, 2008-12-12 at 20:19 +0100, Arnd Bergmann wrote:
> Commit d015fe995 'powerpc/cell/axon-msi: Retry on missing interrupt'
> has turned a rare failure to kexec on QS22 into a reproducible
> error, which we have now analysed.
> 
> The problem is that after a kexec, the MSIC hardware still points
> into the middle of the old ring buffer. We set up the ring buffer
> during reboot, but not the offset into it. On older kernels, this
> would cause a storm of thousands of spurious interrupts after a
> kexec, which would most of the time get dropped silently.
> 
> With the new code, we time out on each interrupt, waiting for
> it to become valid. If more interrupts come in that we time
> out on, this goes on indefinitely, which eventually leads to
> a hard crash.
> 
> The solution in this patch is to read the current offset from
> the MSIC when reinitializing it. This now works correctly, as
> expected.
> 
> Reported-by: Dirk Herrendoerfer <[email-masked]>
> Signed-off-by: Arnd Bergmann <[email-masked]>
> ---
> 
> Please apply when Dirk and Michael have given their Ack.
> Should we have it in 2.6.28? Not sure if going from 'works sometimes'
> to 'works never' counts as a regression. Most users won't be impacted,
> because they don't use kexec on QS22.

I think it does count, it's a pretty small fix.

> diff --git a/arch/powerpc/platforms/cell/axon_msi.c b/arch/powerpc/platforms/cell/axon_msi.c
> index 442cf36..548fa4e 100644
> --- a/arch/powerpc/platforms/cell/axon_msi.c
> +++ b/arch/powerpc/platforms/cell/axon_msi.c
> @@ -413,6 +422,9 @@ static int axon_msi_probe(struct of_device *device,
>  			MSIC_CTRL_IRQ_ENABLE | MSIC_CTRL_ENABLE |
>  			MSIC_CTRL_FIFO_SIZE);
>  
> +	msic->read_offset = dcr_read(msic->dcr_host, MSIC_WRITE_OFFSET_REG)
> +				& MSIC_FIFO_SIZE_MASK;
> +

Acked-by: Michael Ellerman <[email-masked]>

cheers

-- 
Michael Ellerman
OzLabs, IBM Australia Development Lab

wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)

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