Subject: Re: [PATCH v4 3/3] iommu/arm-smmu-qcom: Implement S2CR quirk

From: Bjorn Andersson <[email-masked]>
Date: Mon, 19 Oct 2020 13:12:25 -0500

On Mon 19 Oct 09:04 CDT 2020, Robin Murphy wrote:

> On 2020-10-17 05:39, Bjorn Andersson wrote:
> > The firmware found in some Qualcomm platforms intercepts writes to S2CR
> > in order to replace bypass type streams with fault; and ignore S2CR
> > updates of type fault.
> > 
> > Detect this behavior and implement a custom write_s2cr function in order
> > to trick the firmware into supporting bypass streams by the means of
> > configuring the stream for translation using a reserved and disabled
> > context bank.
> > 
> > Also circumvent the problem of configuring faulting streams by
> > configuring the stream as bypass.
> > 
> > Signed-off-by: Bjorn Andersson <[email-masked]>
> > ---
> > 
> > Changes since v3:
> > - Move the reservation of the "identity context bank" to the Qualcomm specific
> >    implementation.
> > - Implement the S2CR quirk with the newly introduced write_s2cr callback.
> > 
> >   drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c | 68 ++++++++++++++++++++++
> >   1 file changed, 68 insertions(+)
> > 
> > diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> > index 0089048342dd..c0f42d6a6e01 100644
> > --- a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> > +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> > @@ -10,8 +10,14 @@
> >   struct qcom_smmu {
> >   	struct arm_smmu_device smmu;
> > +	bool bypass_cbndx;
> 
> Nit: variables named "*ndx" usually hold an actual index value. If it's just
> a flag then maybe name it something like "use_bypass_context"?
> 
> >   };
> > +static struct qcom_smmu *to_qcom_smmu(struct arm_smmu_device *smmu)
> > +{
> > +	return container_of(smmu, struct qcom_smmu, smmu);
> > +}
> > +
> >   static const struct of_device_id qcom_smmu_client_of_match[] __maybe_unused = {
> >   	{ .compatible = "qcom,adreno" },
> >   	{ .compatible = "qcom,mdp4" },
> > @@ -25,9 +31,32 @@ static const struct of_device_id qcom_smmu_client_of_match[] __maybe_unused = {
> >   static int qcom_smmu_cfg_probe(struct arm_smmu_device *smmu)
> >   {
> > +	unsigned int last_s2cr = ARM_SMMU_GR0_S2CR(smmu->num_mapping_groups - 1);
> > +	struct qcom_smmu *qsmmu = to_qcom_smmu(smmu);
> > +	u32 reg;
> >   	u32 smr;
> >   	int i;
> > +	/*
> > +	 * With some firmware versions writes to S2CR of type FAULT are
> > +	 * ignored, and writing BYPASS will end up written as FAULT in the
> > +	 * register. Perform a write to S2CR to detect if this is the case and
> > +	 * if so reserve a context bank to emulate bypass streams.
> > +	 */
> > +	reg = FIELD_PREP(ARM_SMMU_S2CR_TYPE, S2CR_TYPE_BYPASS) |
> > +	      FIELD_PREP(ARM_SMMU_S2CR_CBNDX, 0xff) |
> > +	      FIELD_PREP(ARM_SMMU_S2CR_PRIVCFG, S2CR_PRIVCFG_DEFAULT);
> > +	arm_smmu_gr0_write(smmu, last_s2cr, reg);
> > +	reg = arm_smmu_gr0_read(smmu, last_s2cr);
> > +	if (FIELD_GET(ARM_SMMU_S2CR_TYPE, reg) != S2CR_TYPE_BYPASS) {
> > +		qsmmu->bypass_cbndx = smmu->num_context_banks - 1;
> 
> Oh, so maybe the name is in fact OK but the type is wrong :/
> 
> I guess this does happens to work out, but for the wrong reason...
> 

Odd, but "it works on my machine"... Sorry about that.

> > +
> > +		set_bit(qsmmu->bypass_cbndx, smmu->context_map);
> > +
> > +		reg = FIELD_PREP(ARM_SMMU_CBAR_TYPE, CBAR_TYPE_S1_TRANS_S2_BYPASS);
> > +		arm_smmu_gr1_write(smmu, ARM_SMMU_GR1_CBAR(qsmmu->bypass_cbndx), reg);
> > +	}
> > +
> >   	for (i = 0; i < smmu->num_mapping_groups; i++) {
> >   		smr = arm_smmu_gr0_read(smmu, ARM_SMMU_GR0_SMR(i));
> > @@ -46,6 +75,44 @@ static int qcom_smmu_cfg_probe(struct arm_smmu_device *smmu)
> >   	return 0;
> >   }
> > +static void qcom_smmu_write_s2cr(struct arm_smmu_device *smmu, int idx)
> > +{
> > +	struct arm_smmu_s2cr *s2cr = smmu->s2crs + idx;
> > +	struct qcom_smmu *qsmmu = to_qcom_smmu(smmu);
> > +	u32 cbndx = s2cr->cbndx;
> > +	u32 type = s2cr->type;
> > +	u32 reg;
> > +
> > +	if (qsmmu->bypass_cbndx) {
> 
> Note that if we are talking indices here then 0 would be perfectly valid in
> general. This works out OK in practice given that we're always reserving the
> last implemented context above, and if we ever *did* only have one such that
> index 0 is the last then we're going to have a bad time either way, but it's
> not necessarily the most obvious.
> 

Right. In the event that we have a SMMU instance with a single context
bank hitting this quirk would probably be bad regardless, as the
cfg_probe would have just stolen the only available context bank for
bypass purposes :)

But I've updated this to keep track of the need for bypass separate from
the index. We don't have a lot of SMMU controllers, so it's not a big
waste.

> > +		if (type == S2CR_TYPE_BYPASS) {
> > +			/*
> > +			 * Firmware with quirky S2CR handling will substitute
> > +			 * BYPASS writes with FAULT, so point the stream to the
> > +			 * reserved context bank and ask for translation on the
> > +			 * stream
> > +			 */
> > +			type = S2CR_TYPE_TRANS;
> > +			cbndx = qsmmu->bypass_cbndx;
> > +		} else if (type == S2CR_TYPE_FAULT) {
> > +			/*
> > +			 * Firmware with quirky S2CR handling will ignore FAULT
> > +			 * writes, so trick it to write FAULT by asking for a
> > +			 * BYPASS.
> > +			 */
> > +			type = S2CR_TYPE_BYPASS;
> 
> Ha, that's brilliant :)
> 
> > +			cbndx = 0xff;
> > +		}
> > +	}
> > +
> > +	reg = FIELD_PREP(ARM_SMMU_S2CR_TYPE, type) |
> > +	      FIELD_PREP(ARM_SMMU_S2CR_CBNDX, cbndx) |
> > +	      FIELD_PREP(ARM_SMMU_S2CR_PRIVCFG, s2cr->privcfg);
> > +
> > +	if (smmu->features & ARM_SMMU_FEAT_EXIDS && smmu->smrs && smmu->smrs[idx].valid)
> > +		reg |= ARM_SMMU_S2CR_EXIDVALID;
> 
> Does any of your hardware actually have EXIDS implemented? No big deal if
> you only want this here "just in case", I'm just curious as I was under then
> impression that it was essentially a ThunderX special.
> 

I tested a couple of platforms and I don't think we do. Dropping it
makes the code slightly better to read, so let's do that.

> Other than sorting out bypass_cbndx one way or the other, overall this is
> now looking about as nice as it ever could - thanks for persevering!
> 

Thank you,
Bjorn

> Robin.
> 
> > +	arm_smmu_gr0_write(smmu, ARM_SMMU_GR0_S2CR(idx), reg);
> > +}
> > +
> >   static int qcom_smmu_def_domain_type(struct device *dev)
> >   {
> >   	const struct of_device_id *match =
> > @@ -87,6 +154,7 @@ 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,
> > +	.write_s2cr = qcom_smmu_write_s2cr,
> >   };
> >   struct arm_smmu_device *qcom_smmu_impl_init(struct arm_smmu_device *smmu)
> > 

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