Re: [PATCH v2 2/2] iommu/arm-smmu: Do not access non-existing SMR registers

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

Hi Olav,

On Wed, Aug 06, 2014 at 05:44:38PM +0100, Olav Haugan wrote:
> On 8/6/2014 3:19 AM, Will Deacon wrote:
> > On Mon, Aug 04, 2014 at 07:01:03PM +0100, Olav Haugan wrote:
> >> The SMR registers do not exist when stream matching is not
> >> supported by the hardware. Avoid writing to this register if not needed.
> >>
> >> Signed-off-by: Olav Haugan <ohaugan-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>
> >> ---
> >>  drivers/iommu/arm-smmu.c | 5 ++++-
> >>  1 file changed, 4 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
> >> index c16431f..1f3a5b3 100644
> >> --- a/drivers/iommu/arm-smmu.c
> >> +++ b/drivers/iommu/arm-smmu.c
> >> @@ -1731,7 +1731,10 @@ static void arm_smmu_device_reset(struct arm_smmu_device *smmu)
> >>  
> >>  	/* Mark all SMRn as invalid and all S2CRn as bypass */
> >>  	for (i = 0; i < smmu->num_mapping_groups; ++i) {
> >> -		writel_relaxed(~SMR_VALID, gr0_base + ARM_SMMU_GR0_SMR(i));
> >> +		if (smmu->features & ARM_SMMU_FEAT_STREAM_MATCH) {
> >> +			writel_relaxed(~SMR_VALID,
> >> +					gr0_base + ARM_SMMU_GR0_SMR(i));
> >> +		}
> >>  		writel_relaxed(S2CR_TYPE_BYPASS,
> >>  			gr0_base + ARM_SMMU_GR0_S2CR(i));
> > 
> > smmu->num_mapping_groups should be zero for an SMMU that doesn't include
> > the SMR registers, so I don't think this change is needed. Are you seeing
> > problems with real hardware?
> 
> Yes, you are correct. However, since that is the case we wouldn't be
> setting the S2CR registers to bypass then? Seems like
> num_mappings_groups should be initialized regardless whether stream
> matching or stream indexing is used. SMMU_IDR0.NUMSMRG specifies the
> number of stream mapping register groups (Section 2.5.2 of the ARM
> SMMUv1-v2 spec). So with stream indexing support this register should
> still tell us how many S2CR registers exist?

Hmm, I'm checking this with the architects because the TRMs aren't exactly
clear. The NUMSMRG works for stream-indexing (i.e. reports the number of
S2CRs), then all we have to do is change the above ~SMR_VALID to 0x0, since
those registers will be SBZP.

> As far as I can tell there are no other register telling us how many S2CR
> registers exist. That also brings up another point that there is no check
> in the code to ensure we are not trying to program more than the available
> S2CR registers when we use stream indexing.

On an SMMU using stream-indexing, if the StreamID goes off the end of the
S2CRs that's a fairly serious hardware configuration issue which I don't
think Linux is in a position to handle. I agree that a warning wouldn't
hurt on device add/attach though.

Will