[PATCH] powerpc/85xx: workaround for chips with MSI hardware errata

[Scott Wood]

On 03/13/2013 12:04:03 AM, Michael Ellerman wrote:
> On Tue, Mar 12, 2013 at 03:48:02PM +0800, Jia Hongtao wrote:
> > The MPIC chip with version 2.0 has a MSI errata (errata PIC1 of  
> mpc8544),
> > It causes that neither MSI nor MSI-X can work fine. This is a  
> workaround
> > to allow MSI-X to function properly.
> 
> You say "neither MSI nor MSI-X can work fine", which I take to mean
> "both MSI and MSI-X do not work".
> 
> But then you say this is a workaround to allow MSI-X to work.
> 
> So what I think you mean is, the erratum prevents both MSI and MSI-X
> from working. This is a workaround that allows MSI-X to work, and in  
> addition
> the patch prevents MSI from being used on chips with the erratum -
> because there is no workaround for MSI.

There actually is a workaround for MSI, but it's more complicated and  
not implemented by this patch.

We could also possibly get away with allowing exactly one MSI  
(byteswapping doesn't matter if the value is zero) -- not sure how hard  
that would be.

> > +		list_for_each_entry(msi, &msi_head, list)
> > +			if (msi->feature & MSI_HW_ERRATA_ENDIAN)
> > +				return -EINVAL;
> 
> I take it you're happy preventing MSI for all devices even if only a
> single chip in the machine has the erratum? In practice you probably
> have all or none with the erratum right?

Yes, it's all integrated onto one chip (the SoC itself).  In fact there  
should only be one MSI block on these chips.

-Scott