Subject: Re: [PATCH v4 2/3] iommu/arm-smmu-qcom: Read back stream mappings

From: Bjorn Andersson <[email-masked]>
Date: Mon, 19 Oct 2020 10:31:59 -0500

On Mon 19 Oct 09:03 CDT 2020, Robin Murphy wrote:

> On 2020-10-17 05:39, Bjorn Andersson wrote:
> > The Qualcomm boot loader configures stream mapping for the peripherals
> > that it accesses and in particular it sets up the stream mapping for the
> > display controller to be allowed to scan out a splash screen or EFI
> > framebuffer.
> > 
> > Read back the stream mappings during initialization and make the
> > arm-smmu driver maintain the streams in bypass mode.
> > 
> > Signed-off-by: Bjorn Andersson <[email-masked]>
> > ---
> > 
> > Changes since v3:
> > - Extracted from different patch in v3.
> > - Now configures the stream as BYPASS, rather than translate, which should work
> >    for platforms with working S2CR handling as well.
> > 
> >   drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c | 24 ++++++++++++++++++++++
> >   1 file changed, 24 insertions(+)
> > 
> > diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> > index be4318044f96..0089048342dd 100644
> > --- a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> > +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> > @@ -23,6 +23,29 @@ static const struct of_device_id qcom_smmu_client_of_match[] __maybe_unused = {
> >   	{ }
> >   };
> > +static int qcom_smmu_cfg_probe(struct arm_smmu_device *smmu)
> > +{
> > +	u32 smr;
> > +	int i;
> > +
> > +	for (i = 0; i < smmu->num_mapping_groups; i++) {
> > +		smr = arm_smmu_gr0_read(smmu, ARM_SMMU_GR0_SMR(i));
> > +
> > +		if (FIELD_GET(ARM_SMMU_SMR_VALID, smr)) {
> > +			smmu->smrs[i].id = FIELD_GET(ARM_SMMU_SMR_ID, smr);
> > +			smmu->smrs[i].mask = FIELD_GET(ARM_SMMU_SMR_MASK, smr);
> > +			smmu->smrs[i].valid = true;
> > +
> > +			smmu->s2crs[i].type = S2CR_TYPE_BYPASS;
> > +			smmu->s2crs[i].privcfg = S2CR_PRIVCFG_DEFAULT;
> > +			smmu->s2crs[i].cbndx = 0xff;
> > +			smmu->s2crs[i].count++;
> 
> FWIW I don't think you actually need to adjust the count here - the SMR
> being valid should already prevent the whole SME from being reassigned until
> the display probes, at which point it should "take over" the SMR based on
> matching values and claim the "initial" refcount. After that you're back
> into the standard flow. It might be a little unintuitive to have something
> in a valid but "unused" state, but arguably it's entirely accurate in terms
> of the software abstraction here.
> 
> Otherwise, you end up making boot-time SMRs - so potentially all SMRs after
> a kexec - effectively immutable, since even after Linux has taken control of
> the whole system such that they *could* be reassigned safely, there's still
> this undroppable refcount hanging around preventing it.
> 

I did increment the count here to make sure the stream mapping do
survive a probe deferral of the display controller (which is rather
common when you have some bridge chip hanging off it).

But after digging through the code further I've convinced myself that
the sme won't be freed while the device is pending probe deferral.

So I will drop this.

> That said, for a mobile SoC use-case if you have enough SMRs for all your
> stream IDs and don't have any kind of device hotplug, that restriction
> shouldn't make much difference in practice, so I'm not too concerned either
> way. Otherwise this is as nice and tidy as I'd hoped :)

I agree, I'm quite happy with where we are now!

Thanks,
Bjorn

> 
> Robin.
> 
> > +		}
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> >   static int qcom_smmu_def_domain_type(struct device *dev)
> >   {
> >   	const struct of_device_id *match =
> > @@ -61,6 +84,7 @@ static int qcom_smmu500_reset(struct arm_smmu_device *smmu)
> >   }
> >   static const struct arm_smmu_impl qcom_smmu_impl = {
> > +	.cfg_probe = qcom_smmu_cfg_probe,
> >   	.def_domain_type = qcom_smmu_def_domain_type,
> >   	.reset = qcom_smmu500_reset,
> >   };
> > 

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