Subject: [PATCH] of/irq: Fix msi-map calculation for nonzero rid-base

From: [email-masked] (Mark Rutland)
Date: Tue, 9 Feb 2016 16:08:12 +0000

On Tue, Feb 09, 2016 at 03:56:55PM +0000, Stuart Yoder wrote:
> 
> > -----Original Message-----
> > From: Marc Zyngier [mailto:marc.zyngier at arm.com]
> > Sent: Tuesday, February 09, 2016 6:06 AM
> > To: Robin Murphy <[email-masked]>; robh+dt at kernel.org; frowand.list at gmail.com;
> > grant.likely at linaro.org; devicetree at vger.kernel.org
> > Cc: mark.rutland at arm.com; david.daney at cavium.com; Stuart Yoder <[email-masked]>;
> > linux-arm-kernel at lists.infradead.org; linux-kernel at vger.kernel.org;
> > stable at vger.kernel.org
> > Subject: Re: [PATCH] of/irq: Fix msi-map calculation for nonzero rid-base
> > 
> > Hi Robin,
> > 
> > On 09/02/16 11:04, Robin Murphy wrote:
> > > The existing msi-map code is fine for shifting the entire RID space
> > > upwards, but attempting finer-grained remapping reveals a bug. It turns
> > > out that we are mistakenly treating the msi-base part as an offset, not
> > > as a new base to remap onto, so things get squiffy when rid-base is
> > > nonzero. Fix this, and at the same time add a sanity check against
> > > having msi-map-mask clash with a nonzero rid-base, as that's another
> > > thing one can easily get wrong.
> > >
> > > CC: <[email-masked]>
> > > Signed-off-by: Robin Murphy <[email-masked]>
> > 
> > Looks like Stuart and you both found the same bug at the same time:
> > https://lkml.org/lkml/2016/2/8/1066
> > 
> > but yours seem more correct to me (the rid_base masking in Stuart's
> > version seems odd).
> > 
> > > ---
> > >  drivers/of/irq.c | 9 ++++++++-
> > >  1 file changed, 8 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/of/irq.c b/drivers/of/irq.c
> > > index 7ee21ae..e7bfc17 100644
> > > --- a/drivers/of/irq.c
> > > +++ b/drivers/of/irq.c
> > > @@ -635,6 +635,13 @@ static u32 __of_msi_map_rid(struct device *dev, struct
> > device_node **np,
> > >  		msi_base = be32_to_cpup(msi_map + 2);
> > >  		rid_len = be32_to_cpup(msi_map + 3);
> > >
> > > +		if (rid_base & ~map_mask) {
> > > +			dev_err(parent_dev,
> > > +				"Invalid msi-map translation - msi-map-mask (0x%x) ignores rid-
> > base (0x%x)\n",
> > > +				map_mask, rid_base);
> > > +			return rid_out;
> > > +		}
> > > +
> > >  		msi_controller_node = of_find_node_by_phandle(phandle);
> > >
> > >  		matched = (masked_rid >= rid_base &&
> > > @@ -654,7 +661,7 @@ static u32 __of_msi_map_rid(struct device *dev, struct device_node
> > **np,
> > >  	if (!matched)
> > >  		return rid_out;
> > >
> > > -	rid_out = masked_rid + msi_base;
> > > +	rid_out = masked_rid - rid_base + msi_base;
> > >  	dev_dbg(dev,
> > >  		"msi-map at: %s, using mask %08x, rid-base: %08x, msi-base: %08x, length:
> > %08x, rid: %08x -> %08x\n",
> > >  		dev_name(parent_dev), map_mask, rid_base, msi_base,
> > >
> 
> This computation:  masked_rid - rid_base
> 
> ...doesn't seem right to me.  We are taking a rid that
> has been already masked and subtracting a rid base that has
> not been masked.

The binding only mentions that the input RID is masked, not the base, so
that seems correct to me.

> I don't see how you can combine masked and unmasked values in the same
> calculation.
> 
> Say I have this msi mapping:
> 
>                    msi-map = <0x0100 &its 0x11 0x1>;
>                    msi-map-mask = <0xff>;
> 

I'd say that this is an inconsistent set of properties, and it's
probably worth warning if we encounter this. There is no possible way
that rid-base can be encountered.

> masked_rid = 0x0
> rid_base = 0x0100
> msi_base = 0x11
> 
> masked_rid - rid_base is 0x0 - 0x0100...which does not
> give the msi index/offset we want.
> 
> Correct final answer should be 0x11.

You can unambiguously describe this with:

	msi-map = <0x00 &its 0x11 0x1>;
	msi-map-mask = <0xff>;

This is exactly the pattern we follow in example 2 in the binding
document.

> In my patch I masked the rid_base so it can be subtracted
> from the masked_rid.
> 
> masked_rid_base = 0x00
> 
>    msi_base + (masked_rid - masked_rid_base) = 0x11

As above, I think that this is an inconsistent DT, and we should
warn/fail in that case.

Thanks,
Mark.