Re: [PATCH 2/9] iommu/arm-smmu: Calculate SMMU_CB_BASE from smmu register values

[Andreas Herrmann <andreas.herrmann-bsGFqQB8/DxBDgjK7y7TUQ@public.gmane.org>]

On Fri, Sep 27, 2013 at 05:51:57AM -0400, Will Deacon wrote:
> Hi Andreas,
> 
> On Thu, Sep 26, 2013 at 11:36:14PM +0100, Andreas Herrmann wrote:
> > Currently it is derived from smmu resource size.  In case of a
> > mismatchin between the two calculations trust DT more than register
> > values and overwrite cb_base.
> 
> I thought the driver already favoured the DT?
> 
> > @@ -1702,12 +1704,23 @@ static int arm_smmu_device_cfg_probe(struct arm_smmu_device *smmu)
> >  
> >  	/* Check that we ioremapped enough */
> >  	size = 1 << (((id >> ID1_NUMPAGENDXB_SHIFT) & ID1_NUMPAGENDXB_MASK) + 1);
> > -	size *= (smmu->pagesize << 1);
> > +	size *= smmu->pagesize;
> > +	smmu->cb_base = smmu->base + size;
> > +	size *= 2;
> > +
> >  	if (smmu->size < size)
> >  		dev_warn(smmu->dev,
> >  			 "device is 0x%lx bytes but only mapped 0x%lx!\n",
> >  			 size, smmu->size);
> >  
> > +	t = (unsigned long) smmu->base + (smmu->size >> 1);
> > +	if ((unsigned long)smmu->cb_base != t) {
> > +		dev_warn(smmu->dev, "address space mismatch, "
> > +			"overwriting cb_base (old: 0x%lx, new: 0x%lx)\n",
> > +			(unsigned long) smmu->cb_base, t);
> > +		smmu->cb_base = (void *) t;
> > +	}
> > +
> 
> I expect I'm just being slow here (only one coffee in), but I can't see what
> this gets us over the current use of resource_size (which goes and uses the
> DT).

On balance it adds a warning if there is an inconsistency between the
resource size and the relevant registers describing the SMMU address
space.

If there is no such warning you might use a broken DT (with wrong
resource size) causing weird malfunction (because cb_base might be
wrong) and it will take some effort to root cause this.

But if you see such a warning it's clear what to check first.

I admit the commit message is misleading (somehow inherited from the
previous patch version) and maybe this kind of consistency check
should not be combined with cb_base calculation but moved into a
separate function called from arm_smmu_device_cfg_probe.


Andreas